parts of a research paper introduction

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How to Write a Research Paper Introduction (with Examples)

The research paper introduction section, along with the Title and Abstract, can be considered the face of any research paper. The following article is intended to guide you in organizing and writing the research paper introduction for a quality academic article or dissertation.

The research paper introduction aims to present the topic to the reader. A study will only be accepted for publishing if you can ascertain that the available literature cannot answer your research question. So it is important to ensure that you have read important studies on that particular topic, especially those within the last five to ten years, and that they are properly referenced in this section. 1 What should be included in the research paper introduction is decided by what you want to tell readers about the reason behind the research and how you plan to fill the knowledge gap. The best research paper introduction provides a systemic review of existing work and demonstrates additional work that needs to be done. It needs to be brief, captivating, and well-referenced; a well-drafted research paper introduction will help the researcher win half the battle.

The introduction for a research paper is where you set up your topic and approach for the reader. It has several key goals:

• Present your research topic
• Capture reader interest
• Summarize existing research
• Position your own approach
• Define your specific research problem and problem statement
• Highlight the novelty and contributions of the study
• Give an overview of the paper’s structure

The research paper introduction can vary in size and structure depending on whether your paper presents the results of original empirical research or is a review paper. Some research paper introduction examples are only half a page while others are a few pages long. In many cases, the introduction will be shorter than all of the other sections of your paper; its length depends on the size of your paper as a whole.

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Table of Contents

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The introduction in a research paper is placed at the beginning to guide the reader from a broad subject area to the specific topic that your research addresses. They present the following information to the reader

• Scope: The topic covered in the research paper
• Context: Background of your topic
• Importance: Why your research matters in that particular area of research and the industry problem that can be targeted

The research paper introduction conveys a lot of information and can be considered an essential roadmap for the rest of your paper. A good introduction for a research paper is important for the following reasons:

• It stimulates your reader’s interest: A good introduction section can make your readers want to read your paper by capturing their interest. It informs the reader what they are going to learn and helps determine if the topic is of interest to them.
• It helps the reader understand the research background: Without a clear introduction, your readers may feel confused and even struggle when reading your paper. A good research paper introduction will prepare them for the in-depth research to come. It provides you the opportunity to engage with the readers and demonstrate your knowledge and authority on the specific topic.
• It explains why your research paper is worth reading: Your introduction can convey a lot of information to your readers. It introduces the topic, why the topic is important, and how you plan to proceed with your research.
• It helps guide the reader through the rest of the paper: The research paper introduction gives the reader a sense of the nature of the information that will support your arguments and the general organization of the paragraphs that will follow. It offers an overview of what to expect when reading the main body of your paper.

What are the parts of introduction in the research?

A good research paper introduction section should comprise three main elements: 2

• What is known: This sets the stage for your research. It informs the readers of what is known on the subject.
• What is lacking: This is aimed at justifying the reason for carrying out your research. This could involve investigating a new concept or method or building upon previous research.
• What you aim to do: This part briefly states the objectives of your research and its major contributions. Your detailed hypothesis will also form a part of this section.

How to write a research paper introduction?

The first step in writing the research paper introduction is to inform the reader what your topic is and why it’s interesting or important. This is generally accomplished with a strong opening statement. The second step involves establishing the kinds of research that have been done and ending with limitations or gaps in the research that you intend to address. Finally, the research paper introduction clarifies how your own research fits in and what problem it addresses. If your research involved testing hypotheses, these should be stated along with your research question. The hypothesis should be presented in the past tense since it will have been tested by the time you are writing the research paper introduction.

The following key points, with examples, can guide you when writing the research paper introduction section:

• Highlight the importance of the research field or topic
• Describe the background of the topic
• Present an overview of current research on the topic

Example: The inclusion of experiential and competency-based learning has benefitted electronics engineering education. Industry partnerships provide an excellent alternative for students wanting to engage in solving real-world challenges. Industry-academia participation has grown in recent years due to the need for skilled engineers with practical training and specialized expertise. However, from the educational perspective, many activities are needed to incorporate sustainable development goals into the university curricula and consolidate learning innovation in universities.

• Reveal a gap in existing research or oppose an existing assumption
• Formulate the research question

Example: There have been plausible efforts to integrate educational activities in higher education electronics engineering programs. However, very few studies have considered using educational research methods for performance evaluation of competency-based higher engineering education, with a focus on technical and or transversal skills. To remedy the current need for evaluating competencies in STEM fields and providing sustainable development goals in engineering education, in this study, a comparison was drawn between study groups without and with industry partners.

• State the purpose of your study
• Highlight the key characteristics of your study
• Describe important results
• Highlight the novelty of the study.
• Offer a brief overview of the structure of the paper.

Example: The study evaluates the main competency needed in the applied electronics course, which is a fundamental core subject for many electronics engineering undergraduate programs. We compared two groups, without and with an industrial partner, that offered real-world projects to solve during the semester. This comparison can help determine significant differences in both groups in terms of developing subject competency and achieving sustainable development goals.

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How to use Paperpal to write the Introduction section

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Step 3: Fill in the specifics, such as your field of study, brief description or details you want to include, which will help the AI generate the outline for your Introduction.

Step 4: Use this outline and sentence suggestions to develop your content, adding citations where needed and modifying it to align with your specific research focus.

Step 5: Turn to Paperpal’s granular language checks to refine your content, tailor it to reflect your personal writing style, and ensure it effectively conveys your message.

You can use the same process to develop each section of your article, and finally your research paper in half the time and without any of the stress.

The purpose of the research paper introduction is to introduce the reader to the problem definition, justify the need for the study, and describe the main theme of the study. The aim is to gain the reader’s attention by providing them with necessary background information and establishing the main purpose and direction of the research.

The length of the research paper introduction can vary across journals and disciplines. While there are no strict word limits for writing the research paper introduction, an ideal length would be one page, with a maximum of 400 words over 1-4 paragraphs. Generally, it is one of the shorter sections of the paper as the reader is assumed to have at least a reasonable knowledge about the topic. 2 For example, for a study evaluating the role of building design in ensuring fire safety, there is no need to discuss definitions and nature of fire in the introduction; you could start by commenting upon the existing practices for fire safety and how your study will add to the existing knowledge and practice.

When deciding what to include in the research paper introduction, the rest of the paper should also be considered. The aim is to introduce the reader smoothly to the topic and facilitate an easy read without much dependency on external sources. 3 Below is a list of elements you can include to prepare a research paper introduction outline and follow it when you are writing the research paper introduction. Topic introduction: This can include key definitions and a brief history of the topic. Research context and background: Offer the readers some general information and then narrow it down to specific aspects. Details of the research you conducted: A brief literature review can be included to support your arguments or line of thought. Rationale for the study: This establishes the relevance of your study and establishes its importance. Importance of your research: The main contributions are highlighted to help establish the novelty of your study Research hypothesis: Introduce your research question and propose an expected outcome. Organization of the paper: Include a short paragraph of 3-4 sentences that highlights your plan for the entire paper

Cite only works that are most relevant to your topic; as a general rule, you can include one to three. Note that readers want to see evidence of original thinking. So it is better to avoid using too many references as it does not leave much room for your personal standpoint to shine through. Citations in your research paper introduction support the key points, and the number of citations depend on the subject matter and the point discussed. If the research paper introduction is too long or overflowing with citations, it is better to cite a few review articles rather than the individual articles summarized in the review. A good point to remember when citing research papers in the introduction section is to include at least one-third of the references in the introduction.

The literature review plays a significant role in the research paper introduction section. A good literature review accomplishes the following: Introduces the topic – Establishes the study’s significance – Provides an overview of the relevant literature – Provides context for the study using literature – Identifies knowledge gaps However, remember to avoid making the following mistakes when writing a research paper introduction: Do not use studies from the literature review to aggressively support your research Avoid direct quoting Do not allow literature review to be the focus of this section. Instead, the literature review should only aid in setting a foundation for the manuscript.

Remember the following key points for writing a good research paper introduction: 4

• Avoid stuffing too much general information: Avoid including what an average reader would know and include only that information related to the problem being addressed in the research paper introduction. For example, when describing a comparative study of non-traditional methods for mechanical design optimization, information related to the traditional methods and differences between traditional and non-traditional methods would not be relevant. In this case, the introduction for the research paper should begin with the state-of-the-art non-traditional methods and methods to evaluate the efficiency of newly developed algorithms.
• Avoid packing too many references: Cite only the required works in your research paper introduction. The other works can be included in the discussion section to strengthen your findings.
• Avoid extensive criticism of previous studies: Avoid being overly critical of earlier studies while setting the rationale for your study. A better place for this would be the Discussion section, where you can highlight the advantages of your method.
• Avoid describing conclusions of the study: When writing a research paper introduction remember not to include the findings of your study. The aim is to let the readers know what question is being answered. The actual answer should only be given in the Results and Discussion section.

To summarize, the research paper introduction section should be brief yet informative. It should convince the reader the need to conduct the study and motivate him to read further. If you’re feeling stuck or unsure, choose trusted AI academic writing assistants like Paperpal to effortlessly craft your research paper introduction and other sections of your research article.

1. Jawaid, S. A., & Jawaid, M. (2019). How to write introduction and discussion. Saudi Journal of Anaesthesia, 13(Suppl 1), S18.

2. Dewan, P., & Gupta, P. (2016). Writing the title, abstract and introduction: Looks matter!. Indian pediatrics, 53, 235-241.

3. Cetin, S., & Hackam, D. J. (2005). An approach to the writing of a scientific Manuscript1. Journal of Surgical Research, 128(2), 165-167.

4. Bavdekar, S. B. (2015). Writing introduction: Laying the foundations of a research paper. Journal of the Association of Physicians of India, 63(7), 44-6.

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Home » Research Paper – Structure, Examples and Writing Guide

Research Paper – Structure, Examples and Writing Guide

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Research Paper

Definition:

Research Paper is a written document that presents the author’s original research, analysis, and interpretation of a specific topic or issue.

It is typically based on Empirical Evidence, and may involve qualitative or quantitative research methods, or a combination of both. The purpose of a research paper is to contribute new knowledge or insights to a particular field of study, and to demonstrate the author’s understanding of the existing literature and theories related to the topic.

Structure of Research Paper

The structure of a research paper typically follows a standard format, consisting of several sections that convey specific information about the research study. The following is a detailed explanation of the structure of a research paper:

The title page contains the title of the paper, the name(s) of the author(s), and the affiliation(s) of the author(s). It also includes the date of submission and possibly, the name of the journal or conference where the paper is to be published.

The abstract is a brief summary of the research paper, typically ranging from 100 to 250 words. It should include the research question, the methods used, the key findings, and the implications of the results. The abstract should be written in a concise and clear manner to allow readers to quickly grasp the essence of the research.

Introduction

The introduction section of a research paper provides background information about the research problem, the research question, and the research objectives. It also outlines the significance of the research, the research gap that it aims to fill, and the approach taken to address the research question. Finally, the introduction section ends with a clear statement of the research hypothesis or research question.

Literature Review

The literature review section of a research paper provides an overview of the existing literature on the topic of study. It includes a critical analysis and synthesis of the literature, highlighting the key concepts, themes, and debates. The literature review should also demonstrate the research gap and how the current study seeks to address it.

The methods section of a research paper describes the research design, the sample selection, the data collection and analysis procedures, and the statistical methods used to analyze the data. This section should provide sufficient detail for other researchers to replicate the study.

The results section presents the findings of the research, using tables, graphs, and figures to illustrate the data. The findings should be presented in a clear and concise manner, with reference to the research question and hypothesis.

The discussion section of a research paper interprets the findings and discusses their implications for the research question, the literature review, and the field of study. It should also address the limitations of the study and suggest future research directions.

The conclusion section summarizes the main findings of the study, restates the research question and hypothesis, and provides a final reflection on the significance of the research.

The references section provides a list of all the sources cited in the paper, following a specific citation style such as APA, MLA or Chicago.

How to Write Research Paper

You can write Research Paper by the following guide:

• Choose a Topic: The first step is to select a topic that interests you and is relevant to your field of study. Brainstorm ideas and narrow down to a research question that is specific and researchable.
• Conduct a Literature Review: The literature review helps you identify the gap in the existing research and provides a basis for your research question. It also helps you to develop a theoretical framework and research hypothesis.
• Develop a Thesis Statement : The thesis statement is the main argument of your research paper. It should be clear, concise and specific to your research question.
• Plan your Research: Develop a research plan that outlines the methods, data sources, and data analysis procedures. This will help you to collect and analyze data effectively.
• Collect and Analyze Data: Collect data using various methods such as surveys, interviews, observations, or experiments. Analyze data using statistical tools or other qualitative methods.
• Organize your Paper : Organize your paper into sections such as Introduction, Literature Review, Methods, Results, Discussion, and Conclusion. Ensure that each section is coherent and follows a logical flow.
• Write your Paper : Start by writing the introduction, followed by the literature review, methods, results, discussion, and conclusion. Ensure that your writing is clear, concise, and follows the required formatting and citation styles.
• Edit and Proofread your Paper: Review your paper for grammar and spelling errors, and ensure that it is well-structured and easy to read. Ask someone else to review your paper to get feedback and suggestions for improvement.
• Cite your Sources: Ensure that you properly cite all sources used in your research paper. This is essential for giving credit to the original authors and avoiding plagiarism.

Research Paper Example

Note : The below example research paper is for illustrative purposes only and is not an actual research paper. Actual research papers may have different structures, contents, and formats depending on the field of study, research question, data collection and analysis methods, and other factors. Students should always consult with their professors or supervisors for specific guidelines and expectations for their research papers.

Research Paper Example sample for Students:

Title: The Impact of Social Media on Mental Health among Young Adults

Abstract: This study aims to investigate the impact of social media use on the mental health of young adults. A literature review was conducted to examine the existing research on the topic. A survey was then administered to 200 university students to collect data on their social media use, mental health status, and perceived impact of social media on their mental health. The results showed that social media use is positively associated with depression, anxiety, and stress. The study also found that social comparison, cyberbullying, and FOMO (Fear of Missing Out) are significant predictors of mental health problems among young adults.

Introduction: Social media has become an integral part of modern life, particularly among young adults. While social media has many benefits, including increased communication and social connectivity, it has also been associated with negative outcomes, such as addiction, cyberbullying, and mental health problems. This study aims to investigate the impact of social media use on the mental health of young adults.

Literature Review: The literature review highlights the existing research on the impact of social media use on mental health. The review shows that social media use is associated with depression, anxiety, stress, and other mental health problems. The review also identifies the factors that contribute to the negative impact of social media, including social comparison, cyberbullying, and FOMO.

Methods : A survey was administered to 200 university students to collect data on their social media use, mental health status, and perceived impact of social media on their mental health. The survey included questions on social media use, mental health status (measured using the DASS-21), and perceived impact of social media on their mental health. Data were analyzed using descriptive statistics and regression analysis.

Results : The results showed that social media use is positively associated with depression, anxiety, and stress. The study also found that social comparison, cyberbullying, and FOMO are significant predictors of mental health problems among young adults.

Discussion : The study’s findings suggest that social media use has a negative impact on the mental health of young adults. The study highlights the need for interventions that address the factors contributing to the negative impact of social media, such as social comparison, cyberbullying, and FOMO.

Conclusion : In conclusion, social media use has a significant impact on the mental health of young adults. The study’s findings underscore the need for interventions that promote healthy social media use and address the negative outcomes associated with social media use. Future research can explore the effectiveness of interventions aimed at reducing the negative impact of social media on mental health. Additionally, longitudinal studies can investigate the long-term effects of social media use on mental health.

Limitations : The study has some limitations, including the use of self-report measures and a cross-sectional design. The use of self-report measures may result in biased responses, and a cross-sectional design limits the ability to establish causality.

Implications: The study’s findings have implications for mental health professionals, educators, and policymakers. Mental health professionals can use the findings to develop interventions that address the negative impact of social media use on mental health. Educators can incorporate social media literacy into their curriculum to promote healthy social media use among young adults. Policymakers can use the findings to develop policies that protect young adults from the negative outcomes associated with social media use.

References :

• Twenge, J. M., & Campbell, W. K. (2019). Associations between screen time and lower psychological well-being among children and adolescents: Evidence from a population-based study. Preventive medicine reports, 15, 100918.
• Primack, B. A., Shensa, A., Escobar-Viera, C. G., Barrett, E. L., Sidani, J. E., Colditz, J. B., … & James, A. E. (2017). Use of multiple social media platforms and symptoms of depression and anxiety: A nationally-representative study among US young adults. Computers in Human Behavior, 69, 1-9.
• Van der Meer, T. G., & Verhoeven, J. W. (2017). Social media and its impact on academic performance of students. Journal of Information Technology Education: Research, 16, 383-398.

Appendix : The survey used in this study is provided below.

Social Media and Mental Health Survey

• How often do you use social media per day?
• Less than 30 minutes
• 30 minutes to 1 hour
• 1 to 2 hours
• 2 to 4 hours
• More than 4 hours
• Which social media platforms do you use?
• Others (Please specify)
• How often do you experience the following on social media?
• Social comparison (comparing yourself to others)
• Cyberbullying
• Fear of Missing Out (FOMO)
• Have you ever experienced any of the following mental health problems in the past month?
• Do you think social media use has a positive or negative impact on your mental health?
• Very positive
• Somewhat positive
• Somewhat negative
• Very negative
• In your opinion, which factors contribute to the negative impact of social media on mental health?
• Social comparison
• In your opinion, what interventions could be effective in reducing the negative impact of social media on mental health?
• Education on healthy social media use
• Counseling for mental health problems caused by social media
• Social media detox programs
• Regulation of social media use

Thank you for your participation!

Applications of Research Paper

Research papers have several applications in various fields, including:

• Advancing knowledge: Research papers contribute to the advancement of knowledge by generating new insights, theories, and findings that can inform future research and practice. They help to answer important questions, clarify existing knowledge, and identify areas that require further investigation.
• Informing policy: Research papers can inform policy decisions by providing evidence-based recommendations for policymakers. They can help to identify gaps in current policies, evaluate the effectiveness of interventions, and inform the development of new policies and regulations.
• Improving practice: Research papers can improve practice by providing evidence-based guidance for professionals in various fields, including medicine, education, business, and psychology. They can inform the development of best practices, guidelines, and standards of care that can improve outcomes for individuals and organizations.
• Educating students : Research papers are often used as teaching tools in universities and colleges to educate students about research methods, data analysis, and academic writing. They help students to develop critical thinking skills, research skills, and communication skills that are essential for success in many careers.
• Fostering collaboration: Research papers can foster collaboration among researchers, practitioners, and policymakers by providing a platform for sharing knowledge and ideas. They can facilitate interdisciplinary collaborations and partnerships that can lead to innovative solutions to complex problems.

When to Write Research Paper

Research papers are typically written when a person has completed a research project or when they have conducted a study and have obtained data or findings that they want to share with the academic or professional community. Research papers are usually written in academic settings, such as universities, but they can also be written in professional settings, such as research organizations, government agencies, or private companies.

Here are some common situations where a person might need to write a research paper:

• For academic purposes: Students in universities and colleges are often required to write research papers as part of their coursework, particularly in the social sciences, natural sciences, and humanities. Writing research papers helps students to develop research skills, critical thinking skills, and academic writing skills.
• For publication: Researchers often write research papers to publish their findings in academic journals or to present their work at academic conferences. Publishing research papers is an important way to disseminate research findings to the academic community and to establish oneself as an expert in a particular field.
• To inform policy or practice : Researchers may write research papers to inform policy decisions or to improve practice in various fields. Research findings can be used to inform the development of policies, guidelines, and best practices that can improve outcomes for individuals and organizations.
• To share new insights or ideas: Researchers may write research papers to share new insights or ideas with the academic or professional community. They may present new theories, propose new research methods, or challenge existing paradigms in their field.

Purpose of Research Paper

The purpose of a research paper is to present the results of a study or investigation in a clear, concise, and structured manner. Research papers are written to communicate new knowledge, ideas, or findings to a specific audience, such as researchers, scholars, practitioners, or policymakers. The primary purposes of a research paper are:

• To contribute to the body of knowledge : Research papers aim to add new knowledge or insights to a particular field or discipline. They do this by reporting the results of empirical studies, reviewing and synthesizing existing literature, proposing new theories, or providing new perspectives on a topic.
• To inform or persuade: Research papers are written to inform or persuade the reader about a particular issue, topic, or phenomenon. They present evidence and arguments to support their claims and seek to persuade the reader of the validity of their findings or recommendations.
• To advance the field: Research papers seek to advance the field or discipline by identifying gaps in knowledge, proposing new research questions or approaches, or challenging existing assumptions or paradigms. They aim to contribute to ongoing debates and discussions within a field and to stimulate further research and inquiry.
• To demonstrate research skills: Research papers demonstrate the author’s research skills, including their ability to design and conduct a study, collect and analyze data, and interpret and communicate findings. They also demonstrate the author’s ability to critically evaluate existing literature, synthesize information from multiple sources, and write in a clear and structured manner.

Characteristics of Research Paper

Research papers have several characteristics that distinguish them from other forms of academic or professional writing. Here are some common characteristics of research papers:

• Evidence-based: Research papers are based on empirical evidence, which is collected through rigorous research methods such as experiments, surveys, observations, or interviews. They rely on objective data and facts to support their claims and conclusions.
• Structured and organized: Research papers have a clear and logical structure, with sections such as introduction, literature review, methods, results, discussion, and conclusion. They are organized in a way that helps the reader to follow the argument and understand the findings.
• Formal and objective: Research papers are written in a formal and objective tone, with an emphasis on clarity, precision, and accuracy. They avoid subjective language or personal opinions and instead rely on objective data and analysis to support their arguments.
• Citations and references: Research papers include citations and references to acknowledge the sources of information and ideas used in the paper. They use a specific citation style, such as APA, MLA, or Chicago, to ensure consistency and accuracy.
• Peer-reviewed: Research papers are often peer-reviewed, which means they are evaluated by other experts in the field before they are published. Peer-review ensures that the research is of high quality, meets ethical standards, and contributes to the advancement of knowledge in the field.
• Objective and unbiased: Research papers strive to be objective and unbiased in their presentation of the findings. They avoid personal biases or preconceptions and instead rely on the data and analysis to draw conclusions.

Advantages of Research Paper

Research papers have many advantages, both for the individual researcher and for the broader academic and professional community. Here are some advantages of research papers:

• Contribution to knowledge: Research papers contribute to the body of knowledge in a particular field or discipline. They add new information, insights, and perspectives to existing literature and help advance the understanding of a particular phenomenon or issue.
• Opportunity for intellectual growth: Research papers provide an opportunity for intellectual growth for the researcher. They require critical thinking, problem-solving, and creativity, which can help develop the researcher’s skills and knowledge.
• Career advancement: Research papers can help advance the researcher’s career by demonstrating their expertise and contributions to the field. They can also lead to new research opportunities, collaborations, and funding.
• Academic recognition: Research papers can lead to academic recognition in the form of awards, grants, or invitations to speak at conferences or events. They can also contribute to the researcher’s reputation and standing in the field.
• Impact on policy and practice: Research papers can have a significant impact on policy and practice. They can inform policy decisions, guide practice, and lead to changes in laws, regulations, or procedures.
• Advancement of society: Research papers can contribute to the advancement of society by addressing important issues, identifying solutions to problems, and promoting social justice and equality.

Limitations of Research Paper

Research papers also have some limitations that should be considered when interpreting their findings or implications. Here are some common limitations of research papers:

• Limited generalizability: Research findings may not be generalizable to other populations, settings, or contexts. Studies often use specific samples or conditions that may not reflect the broader population or real-world situations.
• Potential for bias : Research papers may be biased due to factors such as sample selection, measurement errors, or researcher biases. It is important to evaluate the quality of the research design and methods used to ensure that the findings are valid and reliable.
• Ethical concerns: Research papers may raise ethical concerns, such as the use of vulnerable populations or invasive procedures. Researchers must adhere to ethical guidelines and obtain informed consent from participants to ensure that the research is conducted in a responsible and respectful manner.
• Limitations of methodology: Research papers may be limited by the methodology used to collect and analyze data. For example, certain research methods may not capture the complexity or nuance of a particular phenomenon, or may not be appropriate for certain research questions.
• Publication bias: Research papers may be subject to publication bias, where positive or significant findings are more likely to be published than negative or non-significant findings. This can skew the overall findings of a particular area of research.
• Time and resource constraints: Research papers may be limited by time and resource constraints, which can affect the quality and scope of the research. Researchers may not have access to certain data or resources, or may be unable to conduct long-term studies due to practical limitations.

About the author

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Researcher, Academic Writer, Web developer

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How to Write a Research Paper: Parts of the Paper

• Choosing Your Topic
• Citation & Style Guides This link opens in a new window
• Critical Thinking
• Evaluating Information
• Parts of the Paper
• Writing Tips from UNC-Chapel Hill
• Librarian Contact

Parts of the Research Paper Papers should have a beginning, a middle, and an end. Your introductory paragraph should grab the reader's attention, state your main idea, and indicate how you will support it. The body of the paper should expand on what you have stated in the introduction. Finally, the conclusion restates the paper's thesis and should explain what you have learned, giving a wrap up of your main ideas.

1. The Title The title should be specific and indicate the theme of the research and what ideas it addresses. Use keywords that help explain your paper's topic to the reader. Try to avoid abbreviations and jargon. Think about keywords that people would use to search for your paper and include them in your title.

2. The Abstract The abstract is used by readers to get a quick overview of your paper. Typically, they are about 200 words in length (120 words minimum to  250 words maximum). The abstract should introduce the topic and thesis, and should provide a general statement about what you have found in your research. The abstract allows you to mention each major aspect of your topic and helps readers decide whether they want to read the rest of the paper. Because it is a summary of the entire research paper, it is often written last. 

3. The Introduction The introduction should be designed to attract the reader's attention and explain the focus of the research. You will introduce your overview of the topic,  your main points of information, and why this subject is important. You can introduce the current understanding and background information about the topic. Toward the end of the introduction, you add your thesis statement, and explain how you will provide information to support your research questions. This provides the purpose and focus for the rest of the paper.

4. Thesis Statement Most papers will have a thesis statement or main idea and supporting facts/ideas/arguments. State your main idea (something of interest or something to be proven or argued for or against) as your thesis statement, and then provide your supporting facts and arguments. A thesis statement is a declarative sentence that asserts the position a paper will be taking. It also points toward the paper's development. This statement should be both specific and arguable. Generally, the thesis statement will be placed at the end of the first paragraph of your paper. The remainder of your paper will support this thesis.

Students often learn to write a thesis as a first step in the writing process, but often, after research, a writer's viewpoint may change. Therefore a thesis statement may be one of the final steps in writing. 

Examples of Thesis Statements from Purdue OWL

5. The Literature Review The purpose of the literature review is to describe past important research and how it specifically relates to the research thesis. It should be a synthesis of the previous literature and the new idea being researched. The review should examine the major theories related to the topic to date and their contributors. It should include all relevant findings from credible sources, such as academic books and peer-reviewed journal articles. You will want  to:

• Explain how the literature helps the researcher understand the topic.
• Try to show connections and any disparities between the literature.
• Identify new ways to interpret prior research.
• Reveal any gaps that exist in the literature.

More about writing a literature review. . .

6. The Discussion ​The purpose of the discussion is to interpret and describe what you have learned from your research. Make the reader understand why your topic is important. The discussion should always demonstrate what you have learned from your readings (and viewings) and how that learning has made the topic evolve, especially from the short description of main points in the introduction.Explain any new understanding or insights you have had after reading your articles and/or books. Paragraphs should use transitioning sentences to develop how one paragraph idea leads to the next. The discussion will always connect to the introduction, your thesis statement, and the literature you reviewed, but it does not simply repeat or rearrange the introduction. You want to: 

• Demonstrate critical thinking, not just reporting back facts that you gathered.
• If possible, tell how the topic has evolved over the past and give it's implications for the future.
• Fully explain your main ideas with supporting information.
• Explain why your thesis is correct giving arguments to counter points.

7. The Conclusion A concluding paragraph is a brief summary of your main ideas and restates the paper's main thesis, giving the reader the sense that the stated goal of the paper has been accomplished. What have you learned by doing this research that you didn't know before? What conclusions have you drawn? You may also want to suggest further areas of study, improvement of research possibilities, etc. to demonstrate your critical thinking regarding your research.

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Organizing Your Social Sciences Research Paper

• 4. The Introduction
• Purpose of Guide
• Design Flaws to Avoid
• Independent and Dependent Variables
• Glossary of Research Terms
• Reading Research Effectively
• Narrowing a Topic Idea
• Broadening a Topic Idea
• Extending the Timeliness of a Topic Idea
• Academic Writing Style
• Applying Critical Thinking
• Choosing a Title
• Making an Outline
• Paragraph Development
• Research Process Video Series
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• The C.A.R.S. Model
• Background Information
• The Research Problem/Question
• Theoretical Framework
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The introduction leads the reader from a general subject area to a particular topic of inquiry. It establishes the scope, context, and significance of the research being conducted by summarizing current understanding and background information about the topic, stating the purpose of the work in the form of the research problem supported by a hypothesis or a set of questions, explaining briefly the methodological approach used to examine the research problem, highlighting the potential outcomes your study can reveal, and outlining the remaining structure and organization of the paper.

Key Elements of the Research Proposal. Prepared under the direction of the Superintendent and by the 2010 Curriculum Design and Writing Team. Baltimore County Public Schools.

Importance of a Good Introduction

Think of the introduction as a mental road map that must answer for the reader these four questions:

• What was I studying?
• Why was this topic important to investigate?
• What did we know about this topic before I did this study?
• How will this study advance new knowledge or new ways of understanding?

According to Reyes, there are three overarching goals of a good introduction: 1) ensure that you summarize prior studies about the topic in a manner that lays a foundation for understanding the research problem; 2) explain how your study specifically addresses gaps in the literature, insufficient consideration of the topic, or other deficiency in the literature; and, 3) note the broader theoretical, empirical, and/or policy contributions and implications of your research.

A well-written introduction is important because, quite simply, you never get a second chance to make a good first impression. The opening paragraphs of your paper will provide your readers with their initial impressions about the logic of your argument, your writing style, the overall quality of your research, and, ultimately, the validity of your findings and conclusions. A vague, disorganized, or error-filled introduction will create a negative impression, whereas, a concise, engaging, and well-written introduction will lead your readers to think highly of your analytical skills, your writing style, and your research approach. All introductions should conclude with a brief paragraph that describes the organization of the rest of the paper.

Hirano, Eliana. “Research Article Introductions in English for Specific Purposes: A Comparison between Brazilian, Portuguese, and English.” English for Specific Purposes 28 (October 2009): 240-250; Samraj, B. “Introductions in Research Articles: Variations Across Disciplines.” English for Specific Purposes 21 (2002): 1–17; Introductions. The Writing Center. University of North Carolina; “Writing Introductions.” In Good Essay Writing: A Social Sciences Guide. Peter Redman. 4th edition. (London: Sage, 2011), pp. 63-70; Reyes, Victoria. Demystifying the Journal Article. Inside Higher Education.

Structure and Writing Style

I.  Structure and Approach

The introduction is the broad beginning of the paper that answers three important questions for the reader:

• What is this?
• Why should I read it?
• What do you want me to think about / consider doing / react to?

Think of the structure of the introduction as an inverted triangle of information that lays a foundation for understanding the research problem. Organize the information so as to present the more general aspects of the topic early in the introduction, then narrow your analysis to more specific topical information that provides context, finally arriving at your research problem and the rationale for studying it [often written as a series of key questions to be addressed or framed as a hypothesis or set of assumptions to be tested] and, whenever possible, a description of the potential outcomes your study can reveal.

These are general phases associated with writing an introduction: 1.  Establish an area to research by:

• Highlighting the importance of the topic, and/or
• Making general statements about the topic, and/or
• Presenting an overview on current research on the subject.

2.  Identify a research niche by:

• Opposing an existing assumption, and/or
• Revealing a gap in existing research, and/or
• Formulating a research question or problem, and/or
• Continuing a disciplinary tradition.

3.  Place your research within the research niche by:

• Stating the intent of your study,
• Outlining the key characteristics of your study,
• Describing important results, and
• Giving a brief overview of the structure of the paper.

NOTE:   It is often useful to review the introduction late in the writing process. This is appropriate because outcomes are unknown until you've completed the study. After you complete writing the body of the paper, go back and review introductory descriptions of the structure of the paper, the method of data gathering, the reporting and analysis of results, and the conclusion. Reviewing and, if necessary, rewriting the introduction ensures that it correctly matches the overall structure of your final paper.

II.  Delimitations of the Study

Delimitations refer to those characteristics that limit the scope and define the conceptual boundaries of your research . This is determined by the conscious exclusionary and inclusionary decisions you make about how to investigate the research problem. In other words, not only should you tell the reader what it is you are studying and why, but you must also acknowledge why you rejected alternative approaches that could have been used to examine the topic.

Obviously, the first limiting step was the choice of research problem itself. However, implicit are other, related problems that could have been chosen but were rejected. These should be noted in the conclusion of your introduction. For example, a delimitating statement could read, "Although many factors can be understood to impact the likelihood young people will vote, this study will focus on socioeconomic factors related to the need to work full-time while in school." The point is not to document every possible delimiting factor, but to highlight why previously researched issues related to the topic were not addressed.

Examples of delimitating choices would be:

• The key aims and objectives of your study,
• The research questions that you address,
• The variables of interest [i.e., the various factors and features of the phenomenon being studied],
• The method(s) of investigation,
• The time period your study covers, and
• Any relevant alternative theoretical frameworks that could have been adopted.

Review each of these decisions. Not only do you clearly establish what you intend to accomplish in your research, but you should also include a declaration of what the study does not intend to cover. In the latter case, your exclusionary decisions should be based upon criteria understood as, "not interesting"; "not directly relevant"; “too problematic because..."; "not feasible," and the like. Make this reasoning explicit!

NOTE:   Delimitations refer to the initial choices made about the broader, overall design of your study and should not be confused with documenting the limitations of your study discovered after the research has been completed.

ANOTHER NOTE: Do not view delimitating statements as admitting to an inherent failing or shortcoming in your research. They are an accepted element of academic writing intended to keep the reader focused on the research problem by explicitly defining the conceptual boundaries and scope of your study. It addresses any critical questions in the reader's mind of, "Why the hell didn't the author examine this?"

III.  The Narrative Flow

Issues to keep in mind that will help the narrative flow in your introduction :

• Your introduction should clearly identify the subject area of interest . A simple strategy to follow is to use key words from your title in the first few sentences of the introduction. This will help focus the introduction on the topic at the appropriate level and ensures that you get to the subject matter quickly without losing focus, or discussing information that is too general.
• Establish context by providing a brief and balanced review of the pertinent published literature that is available on the subject. The key is to summarize for the reader what is known about the specific research problem before you did your analysis. This part of your introduction should not represent a comprehensive literature review--that comes next. It consists of a general review of the important, foundational research literature [with citations] that establishes a foundation for understanding key elements of the research problem. See the drop-down menu under this tab for " Background Information " regarding types of contexts.
• Clearly state the hypothesis that you investigated . When you are first learning to write in this format it is okay, and actually preferable, to use a past statement like, "The purpose of this study was to...." or "We investigated three possible mechanisms to explain the...."
• Why did you choose this kind of research study or design? Provide a clear statement of the rationale for your approach to the problem studied. This will usually follow your statement of purpose in the last paragraph of the introduction.

IV.  Engaging the Reader

A research problem in the social sciences can come across as dry and uninteresting to anyone unfamiliar with the topic . Therefore, one of the goals of your introduction is to make readers want to read your paper. Here are several strategies you can use to grab the reader's attention:

• Open with a compelling story . Almost all research problems in the social sciences, no matter how obscure or esoteric , are really about the lives of people. Telling a story that humanizes an issue can help illuminate the significance of the problem and help the reader empathize with those affected by the condition being studied.
• Include a strong quotation or a vivid, perhaps unexpected, anecdote . During your review of the literature, make note of any quotes or anecdotes that grab your attention because they can used in your introduction to highlight the research problem in a captivating way.
• Pose a provocative or thought-provoking question . Your research problem should be framed by a set of questions to be addressed or hypotheses to be tested. However, a provocative question can be presented in the beginning of your introduction that challenges an existing assumption or compels the reader to consider an alternative viewpoint that helps establish the significance of your study. 
• Describe a puzzling scenario or incongruity . This involves highlighting an interesting quandary concerning the research problem or describing contradictory findings from prior studies about a topic. Posing what is essentially an unresolved intellectual riddle about the problem can engage the reader's interest in the study.
• Cite a stirring example or case study that illustrates why the research problem is important . Draw upon the findings of others to demonstrate the significance of the problem and to describe how your study builds upon or offers alternatives ways of investigating this prior research.

NOTE:   It is important that you choose only one of the suggested strategies for engaging your readers. This avoids giving an impression that your paper is more flash than substance and does not distract from the substance of your study.

Freedman, Leora  and Jerry Plotnick. Introductions and Conclusions. University College Writing Centre. University of Toronto; Introduction. The Structure, Format, Content, and Style of a Journal-Style Scientific Paper. Department of Biology. Bates College; Introductions. The Writing Center. University of North Carolina; Introductions. The Writer’s Handbook. Writing Center. University of Wisconsin, Madison; Introductions, Body Paragraphs, and Conclusions for an Argument Paper. The Writing Lab and The OWL. Purdue University; “Writing Introductions.” In Good Essay Writing: A Social Sciences Guide . Peter Redman. 4th edition. (London: Sage, 2011), pp. 63-70; Resources for Writers: Introduction Strategies. Program in Writing and Humanistic Studies. Massachusetts Institute of Technology; Sharpling, Gerald. Writing an Introduction. Centre for Applied Linguistics, University of Warwick; Samraj, B. “Introductions in Research Articles: Variations Across Disciplines.” English for Specific Purposes 21 (2002): 1–17; Swales, John and Christine B. Feak. Academic Writing for Graduate Students: Essential Skills and Tasks . 2nd edition. Ann Arbor, MI: University of Michigan Press, 2004 ; Writing Your Introduction. Department of English Writing Guide. George Mason University.

Writing Tip

Avoid the "Dictionary" Introduction

Giving the dictionary definition of words related to the research problem may appear appropriate because it is important to define specific terminology that readers may be unfamiliar with. However, anyone can look a word up in the dictionary and a general dictionary is not a particularly authoritative source because it doesn't take into account the context of your topic and doesn't offer particularly detailed information. Also, placed in the context of a particular discipline, a term or concept may have a different meaning than what is found in a general dictionary. If you feel that you must seek out an authoritative definition, use a subject specific dictionary or encyclopedia [e.g., if you are a sociology student, search for dictionaries of sociology]. A good database for obtaining definitive definitions of concepts or terms is Credo Reference .

Saba, Robert. The College Research Paper. Florida International University; Introductions. The Writing Center. University of North Carolina.

Another Writing Tip

When Do I Begin?

A common question asked at the start of any paper is, "Where should I begin?" An equally important question to ask yourself is, "When do I begin?" Research problems in the social sciences rarely rest in isolation from history. Therefore, it is important to lay a foundation for understanding the historical context underpinning the research problem. However, this information should be brief and succinct and begin at a point in time that illustrates the study's overall importance. For example, a study that investigates coffee cultivation and export in West Africa as a key stimulus for local economic growth needs to describe the beginning of exporting coffee in the region and establishing why economic growth is important. You do not need to give a long historical explanation about coffee exports in Africa. If a research problem requires a substantial exploration of the historical context, do this in the literature review section. In your introduction, make note of this as part of the "roadmap" [see below] that you use to describe the organization of your paper.

Introductions. The Writing Center. University of North Carolina; “Writing Introductions.” In Good Essay Writing: A Social Sciences Guide . Peter Redman. 4th edition. (London: Sage, 2011), pp. 63-70.

Yet Another Writing Tip

Always End with a Roadmap

The final paragraph or sentences of your introduction should forecast your main arguments and conclusions and provide a brief description of the rest of the paper [the "roadmap"] that let's the reader know where you are going and what to expect. A roadmap is important because it helps the reader place the research problem within the context of their own perspectives about the topic. In addition, concluding your introduction with an explicit roadmap tells the reader that you have a clear understanding of the structural purpose of your paper. In this way, the roadmap acts as a type of promise to yourself and to your readers that you will follow a consistent and coherent approach to addressing the topic of inquiry. Refer to it often to help keep your writing focused and organized.

Cassuto, Leonard. “On the Dissertation: How to Write the Introduction.” The Chronicle of Higher Education , May 28, 2018; Radich, Michael. A Student's Guide to Writing in East Asian Studies . (Cambridge, MA: Harvard University Writing n. d.), pp. 35-37.

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How to Write a Research Introduction

Last Updated: December 6, 2023 Fact Checked

This article was co-authored by Megan Morgan, PhD . Megan Morgan is a Graduate Program Academic Advisor in the School of Public & International Affairs at the University of Georgia. She earned her PhD in English from the University of Georgia in 2015. There are 7 references cited in this article, which can be found at the bottom of the page. This article has been fact-checked, ensuring the accuracy of any cited facts and confirming the authority of its sources. This article has been viewed 2,654,536 times.

The introduction to a research paper can be the most challenging part of the paper to write. The length of the introduction will vary depending on the type of research paper you are writing. An introduction should announce your topic, provide context and a rationale for your work, before stating your research questions and hypothesis. Well-written introductions set the tone for the paper, catch the reader's interest, and communicate the hypothesis or thesis statement.

Introducing the Topic of the Paper

• In scientific papers this is sometimes known as an "inverted triangle", where you start with the broadest material at the start, before zooming in on the specifics. [2] X Research source
• The sentence "Throughout the 20th century, our views of life on other planets have drastically changed" introduces a topic, but does so in broad terms.
• It provides the reader with an indication of the content of the essay and encourages them to read on.

• For example, if you were writing a paper about the behaviour of mice when exposed to a particular substance, you would include the word "mice", and the scientific name of the relevant compound in the first sentences.
• If you were writing a history paper about the impact of the First World War on gender relations in Britain, you should mention those key words in your first few lines.

• This is especially important if you are attempting to develop a new conceptualization that uses language and terminology your readers may be unfamiliar with.

• If you use an anecdote ensure that is short and highly relevant for your research. It has to function in the same way as an alternative opening, namely to announce the topic of your research paper to your reader.
• For example, if you were writing a sociology paper about re-offending rates among young offenders, you could include a brief story of one person whose story reflects and introduces your topic.
• This kind of approach is generally not appropriate for the introduction to a natural or physical sciences research paper where the writing conventions are different.

Establishing the Context for Your Paper

• It is important to be concise in the introduction, so provide an overview on recent developments in the primary research rather than a lengthy discussion.
• You can follow the "inverted triangle" principle to focus in from the broader themes to those to which you are making a direct contribution with your paper.
• A strong literature review presents important background information to your own research and indicates the importance of the field.

• By making clear reference to existing work you can demonstrate explicitly the specific contribution you are making to move the field forward.
• You can identify a gap in the existing scholarship and explain how you are addressing it and moving understanding forward.

• For example, if you are writing a scientific paper you could stress the merits of the experimental approach or models you have used.
• Stress what is novel in your research and the significance of your new approach, but don't give too much detail in the introduction.
• A stated rationale could be something like: "the study evaluates the previously unknown anti-inflammatory effects of a topical compound in order to evaluate its potential clinical uses".

Specifying Your Research Questions and Hypothesis

• The research question or questions generally come towards the end of the introduction, and should be concise and closely focused.
• The research question might recall some of the key words established in the first few sentences and the title of your paper.
• An example of a research question could be "what were the consequences of the North American Free Trade Agreement on the Mexican export economy?"
• This could be honed further to be specific by referring to a particular element of the Free Trade Agreement and the impact on a particular industry in Mexico, such as clothing manufacture.
• A good research question should shape a problem into a testable hypothesis.

• If possible try to avoid using the word "hypothesis" and rather make this implicit in your writing. This can make your writing appear less formulaic.
• In a scientific paper, giving a clear one-sentence overview of your results and their relation to your hypothesis makes the information clear and accessible. [10] X Trustworthy Source PubMed Central Journal archive from the U.S. National Institutes of Health Go to source
• An example of a hypothesis could be "mice deprived of food for the duration of the study were expected to become more lethargic than those fed normally".

• This is not always necessary and you should pay attention to the writing conventions in your discipline.
• In a natural sciences paper, for example, there is a fairly rigid structure which you will be following.
• A humanities or social science paper will most likely present more opportunities to deviate in how you structure your paper.

Research Introduction Help

Community Q&A

• Use your research papers' outline to help you decide what information to include when writing an introduction. Thanks Helpful 0 Not Helpful 1
• Consider drafting your introduction after you have already completed the rest of your research paper. Writing introductions last can help ensure that you don't leave out any major points. Thanks Helpful 0 Not Helpful 0

• Avoid emotional or sensational introductions; these can create distrust in the reader. Thanks Helpful 50 Not Helpful 12
• Generally avoid using personal pronouns in your introduction, such as "I," "me," "we," "us," "my," "mine," or "our." Thanks Helpful 31 Not Helpful 7
• Don't overwhelm the reader with an over-abundance of information. Keep the introduction as concise as possible by saving specific details for the body of your paper. Thanks Helpful 24 Not Helpful 14

You Might Also Like

• ↑ https://library.sacredheart.edu/c.php?g=29803&p=185916
• ↑ https://www.aresearchguide.com/inverted-pyramid-structure-in-writing.html
• ↑ https://libguides.usc.edu/writingguide/introduction
• ↑ https://writing.wisc.edu/Handbook/PlanResearchPaper.html
• ↑ https://dept.writing.wisc.edu/wac/writing-an-introduction-for-a-scientific-paper/
• ↑ https://writing.wisc.edu/handbook/assignments/planresearchpaper/
• ↑ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3178846/

About This Article

To introduce your research paper, use the first 1-2 sentences to describe your general topic, such as “women in World War I.” Include and define keywords, such as “gender relations,” to show your reader where you’re going. Mention previous research into the topic with a phrase like, “Others have studied…”, then transition into what your contribution will be and why it’s necessary. Finally, state the questions that your paper will address and propose your “answer” to them as your thesis statement. For more information from our English Ph.D. co-author about how to craft a strong hypothesis and thesis, keep reading! Did this summary help you? Yes No

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How to write an introduction for a research paper

Writing an introduction for a research paper can be one of the hardest parts of the writing process. How do you get started? In this post, we discuss the components of an introduction and explore strategies for writing one successfully.

What is an introduction?

The introduction to a research paper provides background information or context on the topic. It also includes the thesis statement and signposts that let the reader know what you will cover in the rest of the paper.

Depending on the type of research paper that you’re writing, you may also include a brief state of the field in your introduction. You might also put that in a separate section, called a literature review. Before you tackle writing your introduction, be sure to consult the assignment guidelines for your paper.

How to write an introduction

An introduction provides an overview of your topic and any background information that your readers need to know in order to understand the context. It generally concludes with an explicit statement of your position on the topic, which is known as your thesis statement.

The opening section

Many papers begin with a hook: a short anecdote or scenario that draws the reader in and gives a hint of what the paper will cover. A hook allows you to capture your reader’s attention and provides an anchor for the context that you will provide in the bulk of the introduction.

Most of your introduction should be taken up with background information, but this doesn’t mean that you should fill your opening section with overly general statements. Instead, provide key pieces of information (like statistics) that a reader would need to know in order to understand your main argument.

The thesis statement

Towards the end of the introduction, you should state your thesis, preferably in the form of "I argue that..." or "This paper argues that..." or a similar phrase. Although it’s called a “thesis statement,” your thesis can be more than one sentence.

Finally, an introduction contains a brief outline or "signposts" of what the rest of the article will cover (also known as forecasting statements). You can use language like, “in what follows,” or “in the rest of the paper,” to signal that you are describing what you’ll do in the remainder of the paper.

Tips for writing an introduction

1. don’t rely on generalizations.

An introduction is not simply filler. It has a very specific function in a research paper: to provide context that leads up to a thesis statement.

You may be tempted to start your paper with generalizations like, “many people believe that...” or, “in our society...,” or a general dictionary definition, because you’re not sure what kind of context to provide. Instead, use specific facts like statistics or historical anecdotes to open your paper.

2. State your thesis directly

Once you’ve provided the appropriate, and specific, background information on your topic, you can move on to stating your thesis. As a rule of thumb, state your thesis as directly as possible. Use phrases like “I argue that..” to indicate that you are laying out your main argument.

3. Include signposts

A strong introduction includes clear signposts that outline what you will cover in the rest of the paper. You can signal this by using words like, “in what follows,” and by describing the steps that you will take to build your argument.

4. Situate your argument within the scholarly conversation

Some types of research papers require a separate literature review in which you explore what others have written about your topic.

Even if you’re not required to have a formal literature review, you should still include at least a paragraph in which you engage with the scholarly debate on your chosen subject. Be sure to include direct quotes from your sources . You can use BibGuru’s citation generator to create accurate in-text citations for your quotes.

This section can come directly before your thesis statement or directly after it. In the former case, your state of the field will function as additional context for your thesis.

Frequently Asked Questions about how to write an introduction for a research paper

A good introduction provides specific background information on your topic, sets up your thesis statement, and includes signposts for what you’ll cover in the rest of the paper.

An introduction should include context, a thesis statement, and signposts.

Do not include generalizations, apologies for not being an expert, or dictionary definitions in your introduction.

The length of your introduction depends on the overall length of your paper. For instance, an introduction for an 8-10 page paper will likely be anywhere from 1-3 pages.

You can choose to start an introduction with a hook, an important statistic, an historical anecdote, or another specific piece of background information.

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Writing Research Papers

• Research Paper Structure

Whether you are writing a B.S. Degree Research Paper or completing a research report for a Psychology course, it is highly likely that you will need to organize your research paper in accordance with American Psychological Association (APA) guidelines.  Here we discuss the structure of research papers according to APA style.

Major Sections of a Research Paper in APA Style

A complete research paper in APA style that is reporting on experimental research will typically contain a Title page, Abstract, Introduction, Methods, Results, Discussion, and References sections. 1  Many will also contain Figures and Tables and some will have an Appendix or Appendices.  These sections are detailed as follows (for a more in-depth guide, please refer to " How to Write a Research Paper in APA Style ”, a comprehensive guide developed by Prof. Emma Geller). 2

What is this paper called and who wrote it? – the first page of the paper; this includes the name of the paper, a “running head”, authors, and institutional affiliation of the authors.  The institutional affiliation is usually listed in an Author Note that is placed towards the bottom of the title page.  In some cases, the Author Note also contains an acknowledgment of any funding support and of any individuals that assisted with the research project.

One-paragraph summary of the entire study – typically no more than 250 words in length (and in many cases it is well shorter than that), the Abstract provides an overview of the study.

Introduction

What is the topic and why is it worth studying? – the first major section of text in the paper, the Introduction commonly describes the topic under investigation, summarizes or discusses relevant prior research (for related details, please see the Writing Literature Reviews section of this website), identifies unresolved issues that the current research will address, and provides an overview of the research that is to be described in greater detail in the sections to follow.

What did you do? – a section which details how the research was performed.  It typically features a description of the participants/subjects that were involved, the study design, the materials that were used, and the study procedure.  If there were multiple experiments, then each experiment may require a separate Methods section.  A rule of thumb is that the Methods section should be sufficiently detailed for another researcher to duplicate your research.

What did you find? – a section which describes the data that was collected and the results of any statistical tests that were performed.  It may also be prefaced by a description of the analysis procedure that was used. If there were multiple experiments, then each experiment may require a separate Results section.

What is the significance of your results? – the final major section of text in the paper.  The Discussion commonly features a summary of the results that were obtained in the study, describes how those results address the topic under investigation and/or the issues that the research was designed to address, and may expand upon the implications of those findings.  Limitations and directions for future research are also commonly addressed.

List of articles and any books cited – an alphabetized list of the sources that are cited in the paper (by last name of the first author of each source).  Each reference should follow specific APA guidelines regarding author names, dates, article titles, journal titles, journal volume numbers, page numbers, book publishers, publisher locations, websites, and so on (for more information, please see the Citing References in APA Style page of this website).

Tables and Figures

Graphs and data (optional in some cases) – depending on the type of research being performed, there may be Tables and/or Figures (however, in some cases, there may be neither).  In APA style, each Table and each Figure is placed on a separate page and all Tables and Figures are included after the References.   Tables are included first, followed by Figures.   However, for some journals and undergraduate research papers (such as the B.S. Research Paper or Honors Thesis), Tables and Figures may be embedded in the text (depending on the instructor’s or editor’s policies; for more details, see "Deviations from APA Style" below).

Supplementary information (optional) – in some cases, additional information that is not critical to understanding the research paper, such as a list of experiment stimuli, details of a secondary analysis, or programming code, is provided.  This is often placed in an Appendix.

Variations of Research Papers in APA Style

Although the major sections described above are common to most research papers written in APA style, there are variations on that pattern.  These variations include: 

• Literature reviews – when a paper is reviewing prior published research and not presenting new empirical research itself (such as in a review article, and particularly a qualitative review), then the authors may forgo any Methods and Results sections. Instead, there is a different structure such as an Introduction section followed by sections for each of the different aspects of the body of research being reviewed, and then perhaps a Discussion section. 
• Multi-experiment papers – when there are multiple experiments, it is common to follow the Introduction with an Experiment 1 section, itself containing Methods, Results, and Discussion subsections. Then there is an Experiment 2 section with a similar structure, an Experiment 3 section with a similar structure, and so on until all experiments are covered.  Towards the end of the paper there is a General Discussion section followed by References.  Additionally, in multi-experiment papers, it is common for the Results and Discussion subsections for individual experiments to be combined into single “Results and Discussion” sections.

Departures from APA Style

In some cases, official APA style might not be followed (however, be sure to check with your editor, instructor, or other sources before deviating from standards of the Publication Manual of the American Psychological Association).  Such deviations may include:

• Placement of Tables and Figures  – in some cases, to make reading through the paper easier, Tables and/or Figures are embedded in the text (for example, having a bar graph placed in the relevant Results section). The embedding of Tables and/or Figures in the text is one of the most common deviations from APA style (and is commonly allowed in B.S. Degree Research Papers and Honors Theses; however you should check with your instructor, supervisor, or editor first). 
• Incomplete research – sometimes a B.S. Degree Research Paper in this department is written about research that is currently being planned or is in progress. In those circumstances, sometimes only an Introduction and Methods section, followed by References, is included (that is, in cases where the research itself has not formally begun).  In other cases, preliminary results are presented and noted as such in the Results section (such as in cases where the study is underway but not complete), and the Discussion section includes caveats about the in-progress nature of the research.  Again, you should check with your instructor, supervisor, or editor first.
• Class assignments – in some classes in this department, an assignment must be written in APA style but is not exactly a traditional research paper (for instance, a student asked to write about an article that they read, and to write that report in APA style). In that case, the structure of the paper might approximate the typical sections of a research paper in APA style, but not entirely.  You should check with your instructor for further guidelines.

Workshops and Downloadable Resources

• For in-person discussion of the process of writing research papers, please consider attending this department’s “Writing Research Papers” workshop (for dates and times, please check the undergraduate workshops calendar).

Downloadable Resources

• How to Write APA Style Research Papers (a comprehensive guide) [ PDF ]
• Tips for Writing APA Style Research Papers (a brief summary) [ PDF ]
• Example APA Style Research Paper (for B.S. Degree – empirical research) [ PDF ]
• Example APA Style Research Paper (for B.S. Degree – literature review) [ PDF ]

Further Resources

How-To Videos     

• Writing Research Paper Videos

APA Journal Article Reporting Guidelines

• Appelbaum, M., Cooper, H., Kline, R. B., Mayo-Wilson, E., Nezu, A. M., & Rao, S. M. (2018). Journal article reporting standards for quantitative research in psychology: The APA Publications and Communications Board task force report . American Psychologist , 73 (1), 3.
• Levitt, H. M., Bamberg, M., Creswell, J. W., Frost, D. M., Josselson, R., & Suárez-Orozco, C. (2018). Journal article reporting standards for qualitative primary, qualitative meta-analytic, and mixed methods research in psychology: The APA Publications and Communications Board task force report . American Psychologist , 73 (1), 26.  

External Resources

• Formatting APA Style Papers in Microsoft Word
• How to Write an APA Style Research Paper from Hamilton University
• WikiHow Guide to Writing APA Research Papers
• Sample APA Formatted Paper with Comments
• Sample APA Formatted Paper
• Tips for Writing a Paper in APA Style

1 VandenBos, G. R. (Ed). (2010). Publication manual of the American Psychological Association (6th ed.) (pp. 41-60).  Washington, DC: American Psychological Association.

2 geller, e. (2018).  how to write an apa-style research report . [instructional materials]. , prepared by s. c. pan for ucsd psychology.

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• What Types of References Are Appropriate?
• Evaluating References and Taking Notes
• Citing References
• Writing a Literature Review
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• Academic Integrity and Avoiding Plagiarism
• Writing Research Papers Videos

How to Write an Introduction For a Research Paper

Learn how to write a strong and efficient research paper introduction by following the suitable structure and avoiding typical errors.

An introduction to any type of paper is sometimes misunderstood as the beginning; yet, an introduction is actually intended to present your chosen subject to the audience in a way that makes it more appealing and leaves your readers thirsty for more information. After the title and abstract, your audience will read the introduction, thus it’s critical to get off to a solid start.  

This article includes instructions on how to write an introduction for a research paper that engages the reader in your research. You can produce a strong opening for your research paper if you stick to the format and a few basic principles.

What is An Introduction To a Research Paper?

An introduction is the opening section of a research paper and the section that a reader is likely to read first, in which the objective and goals of the subsequent writing are stated. 

The introduction serves numerous purposes. It provides context for your research, explains your topic and objectives, and provides an outline of the work. A solid introduction will establish the tone for the remainder of your paper, enticing readers to continue reading through the methodology, findings, and discussion. 

Even though introductions are generally presented at the beginning of a document, we must distinguish an introduction from the beginning of your research. An introduction, as the name implies, is supposed to introduce your subject without extending it. All relevant information and facts should be placed in the body and conclusion, not the introduction.

Structure Of An Introduction

Before explaining how to write an introduction for a research paper , it’s necessary to comprehend a structure that will make your introduction stronger and more straightforward.

A Good Hook

A hook is one of the most effective research introduction openers. A hook’s objective is to stimulate the reader’s interest to read the research paper.  There are various approaches you may take to generate a strong hook:  startling facts, a question, a brief overview, or even a quotation. 

Broad Overview

Following an excellent hook, you should present a wide overview of your major issue and some background information on your research. If you’re unsure about how to begin an essay introduction, the best approach is to offer a basic explanation of your topic before delving into specific issues. Simply said, you should begin with general information and then narrow it down to your relevant topics.

After offering some background information regarding your research’s main topic, go on to give readers a better understanding of what you’ll be covering throughout your research. In this section of your introduction, you should swiftly clarify your important topics in the sequence in which they will be addressed later, gradually introducing your thesis statement. You can use some  The following are some critical questions to address in this section of your introduction: Who? What? Where? When? How? And why is that?

Thesis Statement

The thesis statement, which must be stated in the beginning clause of your research since your entire research revolves around it, is the most important component of your research.

A thesis statement presents your audience with a quick overview of the research’s main assertion. In the body section of your work, your key argument is what you will expose or debate about it. An excellent thesis statement is usually very succinct, accurate, explicit, clear, and focused. Typically, your thesis should be at the conclusion of your introductory paragraph/section.

Tips for Writing a Strong Introduction

Aside from the good structure, here are a few tips to make your introduction strong and accurate:

• Keep in mind the aim of your research and make sure your introduction supports it.
• Use an appealing and relevant hook that catches the reader’s attention right away.
• Make it obvious to your readers what your stance is.
• Demonstrate your knowledge of your subject.
• Provide your readers with a road map to help them understand what you will address throughout the research.
• Be succinct – it is advised that your opening introduction consists of around 8-9 percent of the overall amount of words in your article (for example, 160 words for a 2000 words essay). 
• Make a strong and unambiguous thesis statement.
• Explain why the article is significant in 1-2 sentences.
• Remember to keep it interesting.

Mistakes to Avoid in Your Introduction

Check out what not to do and what to avoid now that you know the structure and how to write an introduction for a research paper .

• Lacking a feeling of direction or purpose.
• Giving out too much.
• Creating lengthy paragraphs.
• Excessive or insufficient background, literature, and theory.
• Including material that should be placed in the body and conclusion.
• Not writing enough or writing excessively.
• Using too many quotes.

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Parts of a Research Paper

One of the most important aspects of science is ensuring that you get all the parts of the written research paper in the right order.

This article is a part of the guide:

• Outline Examples
• Example of a Paper
• Write a Hypothesis
• Introduction

Browse Full Outline

• 1 Write a Research Paper
• 2 Writing a Paper
• 3.1 Write an Outline
• 3.2 Outline Examples
• 4.1 Thesis Statement
• 4.2 Write a Hypothesis
• 5.2 Abstract
• 5.3 Introduction
• 5.4 Methods
• 5.5 Results
• 5.6 Discussion
• 5.7 Conclusion
• 5.8 Bibliography
• 6.1 Table of Contents
• 6.2 Acknowledgements
• 6.3 Appendix
• 7.1 In Text Citations
• 7.2 Footnotes
• 7.3.1 Floating Blocks
• 7.4 Example of a Paper
• 7.5 Example of a Paper 2
• 7.6.1 Citations
• 7.7.1 Writing Style
• 7.7.2 Citations
• 8.1.1 Sham Peer Review
• 8.1.2 Advantages
• 8.1.3 Disadvantages
• 8.2 Publication Bias
• 8.3.1 Journal Rejection
• 9.1 Article Writing
• 9.2 Ideas for Topics

You may have finished the best research project on earth but, if you do not write an interesting and well laid out paper, then nobody is going to take your findings seriously.

The main thing to remember with any research paper is that it is based on an hourglass structure. It begins with general information and undertaking a literature review , and becomes more specific as you nail down a research problem and hypothesis .

Finally, it again becomes more general as you try to apply your findings to the world at general.

Whilst there are a few differences between the various disciplines, with some fields placing more emphasis on certain parts than others, there is a basic underlying structure.

These steps are the building blocks of constructing a good research paper. This section outline how to lay out the parts of a research paper, including the various experimental methods and designs.

The principles for literature review and essays of all types follow the same basic principles.

Reference List

For many students, writing the introduction is the first part of the process, setting down the direction of the paper and laying out exactly what the research paper is trying to achieve.

For others, the introduction is the last thing written, acting as a quick summary of the paper. As long as you have planned a good structure for the parts of a research paper, both approaches are acceptable and it is a matter of preference.

A good introduction generally consists of three distinct parts:

• You should first give a general presentation of the research problem.
• You should then lay out exactly what you are trying to achieve with this particular research project.
• You should then state your own position.

Ideally, you should try to give each section its own paragraph, but this will vary given the overall length of the paper.

1) General Presentation

Look at the benefits to be gained by the research or why the problem has not been solved yet. Perhaps nobody has thought about it, or maybe previous research threw up some interesting leads that the previous researchers did not follow up.

Another researcher may have uncovered some interesting trends, but did not manage to reach the significance level , due to experimental error or small sample sizes .

2) Purpose of the Paper

The research problem does not have to be a statement, but must at least imply what you are trying to find.

Many writers prefer to place the thesis statement or hypothesis here, which is perfectly acceptable, but most include it in the last sentences of the introduction, to give the reader a fuller picture.

3) A Statement of Intent From the Writer

The idea is that somebody will be able to gain an overall view of the paper without needing to read the whole thing. Literature reviews are time-consuming enough, so give the reader a concise idea of your intention before they commit to wading through pages of background.

In this section, you look to give a context to the research, including any relevant information learned during your literature review. You are also trying to explain why you chose this area of research, attempting to highlight why it is necessary. The second part should state the purpose of the experiment and should include the research problem. The third part should give the reader a quick summary of the form that the parts of the research paper is going to take and should include a condensed version of the discussion.

This should be the easiest part of the paper to write, as it is a run-down of the exact design and methodology used to perform the research. Obviously, the exact methodology varies depending upon the exact field and type of experiment .

There is a big methodological difference between the apparatus based research of the physical sciences and the methods and observation methods of social sciences. However, the key is to ensure that another researcher would be able to replicate the experiment to match yours as closely as possible, but still keeping the section concise.

You can assume that anybody reading your paper is familiar with the basic methods, so try not to explain every last detail. For example, an organic chemist or biochemist will be familiar with chromatography, so you only need to highlight the type of equipment used rather than explaining the whole process in detail.

In the case of a survey , if you have too many questions to cover in the method, you can always include a copy of the questionnaire in the appendix . In this case, make sure that you refer to it.

This is probably the most variable part of any research paper, and depends on the results and aims of the experiment.

For quantitative research , it is a presentation of the numerical results and data, whereas for qualitative research it should be a broader discussion of trends, without going into too much detail.

For research generating a lot of results , then it is better to include tables or graphs of the analyzed data and leave the raw data in the appendix, so that a researcher can follow up and check your calculations.

A commentary is essential to linking the results together, rather than just displaying isolated and unconnected charts and figures.

It can be quite difficult to find a good balance between the results and the discussion section, because some findings, especially in a quantitative or descriptive experiment , will fall into a grey area. Try to avoid repeating yourself too often.

It is best to try to find a middle path, where you give a general overview of the data and then expand on it in the discussion - you should try to keep your own opinions and interpretations out of the results section, saving that for the discussion later on.

This is where you elaborate on your findings, and explain what you found, adding your own personal interpretations.

Ideally, you should link the discussion back to the introduction, addressing each point individually.

It’s important to make sure that every piece of information in your discussion is directly related to the thesis statement , or you risk cluttering your findings. In keeping with the hourglass principle, you can expand on the topic later in the conclusion .

The conclusion is where you build on your discussion and try to relate your findings to other research and to the world at large.

In a short research paper, it may be a paragraph or two, or even a few lines.

In a dissertation, it may well be the most important part of the entire paper - not only does it describe the results and discussion in detail, it emphasizes the importance of the results in the field, and ties it in with the previous research.

Some research papers require a recommendations section, postulating the further directions of the research, as well as highlighting how any flaws affected the results. In this case, you should suggest any improvements that could be made to the research design .

No paper is complete without a reference list , documenting all the sources that you used for your research. This should be laid out according to APA , MLA or other specified format, allowing any interested researcher to follow up on the research.

One habit that is becoming more common, especially with online papers, is to include a reference to your own paper on the final page. Lay this out in MLA, APA and Chicago format, allowing anybody referencing your paper to copy and paste it.

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Frequently asked questions

What should i include in a research paper introduction.

The introduction of a research paper includes several key elements:

• A hook to catch the reader’s interest
• Relevant background on the topic
• Details of your research problem

and your problem statement

• A thesis statement or research question
• Sometimes an overview of the paper

Frequently asked questions: Writing a research paper

A research project is an academic, scientific, or professional undertaking to answer a research question . Research projects can take many forms, such as qualitative or quantitative , descriptive , longitudinal , experimental , or correlational . What kind of research approach you choose will depend on your topic.

The best way to remember the difference between a research plan and a research proposal is that they have fundamentally different audiences. A research plan helps you, the researcher, organize your thoughts. On the other hand, a dissertation proposal or research proposal aims to convince others (e.g., a supervisor, a funding body, or a dissertation committee) that your research topic is relevant and worthy of being conducted.

Formulating a main research question can be a difficult task. Overall, your question should contribute to solving the problem that you have defined in your problem statement .

However, it should also fulfill criteria in three main areas:

• Researchability
• Feasibility and specificity
• Relevance and originality

Research questions anchor your whole project, so it’s important to spend some time refining them.

In general, they should be:

• Focused and researchable
• Answerable using credible sources
• Complex and arguable
• Feasible and specific
• Relevant and original

All research questions should be:

• Focused on a single problem or issue
• Researchable using primary and/or secondary sources
• Feasible to answer within the timeframe and practical constraints
• Specific enough to answer thoroughly
• Complex enough to develop the answer over the space of a paper or thesis
• Relevant to your field of study and/or society more broadly

A research aim is a broad statement indicating the general purpose of your research project. It should appear in your introduction at the end of your problem statement , before your research objectives.

Research objectives are more specific than your research aim. They indicate the specific ways you’ll address the overarching aim.

Once you’ve decided on your research objectives , you need to explain them in your paper, at the end of your problem statement .

Keep your research objectives clear and concise, and use appropriate verbs to accurately convey the work that you will carry out for each one.

I will compare …

Your research objectives indicate how you’ll try to address your research problem and should be specific:

Research objectives describe what you intend your research project to accomplish.

They summarize the approach and purpose of the project and help to focus your research.

Your objectives should appear in the introduction of your research paper , at the end of your problem statement .

The main guidelines for formatting a paper in Chicago style are to:

• Use a standard font like 12 pt Times New Roman
• Use 1 inch margins or larger
• Apply double line spacing
• Indent every new paragraph ½ inch
• Include a title page
• Place page numbers in the top right or bottom center
• Cite your sources with author-date citations or Chicago footnotes
• Include a bibliography or reference list

To automatically generate accurate Chicago references, you can use Scribbr’s free Chicago reference generator .

The main guidelines for formatting a paper in MLA style are as follows:

• Use an easily readable font like 12 pt Times New Roman
• Set 1 inch page margins
• Include a four-line MLA heading on the first page
• Center the paper’s title
• Use title case capitalization for headings
• Cite your sources with MLA in-text citations
• List all sources cited on a Works Cited page at the end

To format a paper in APA Style , follow these guidelines:

• Use a standard font like 12 pt Times New Roman or 11 pt Arial
• If submitting for publication, insert a running head on every page
• Apply APA heading styles
• Cite your sources with APA in-text citations
• List all sources cited on a reference page at the end

No, it’s not appropriate to present new arguments or evidence in the conclusion . While you might be tempted to save a striking argument for last, research papers follow a more formal structure than this.

All your findings and arguments should be presented in the body of the text (more specifically in the results and discussion sections if you are following a scientific structure). The conclusion is meant to summarize and reflect on the evidence and arguments you have already presented, not introduce new ones.

The conclusion of a research paper has several key elements you should make sure to include:

• A restatement of the research problem
• A summary of your key arguments and/or findings
• A short discussion of the implications of your research

Don’t feel that you have to write the introduction first. The introduction is often one of the last parts of the research paper you’ll write, along with the conclusion.

This is because it can be easier to introduce your paper once you’ve already written the body ; you may not have the clearest idea of your arguments until you’ve written them, and things can change during the writing process .

The way you present your research problem in your introduction varies depending on the nature of your research paper . A research paper that presents a sustained argument will usually encapsulate this argument in a thesis statement .

A research paper designed to present the results of empirical research tends to present a research question that it seeks to answer. It may also include a hypothesis —a prediction that will be confirmed or disproved by your research.

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Structuring the Research Paper

Formal research structure.

These are the primary purposes for formal research:

enter the discourse, or conversation, of other writers and scholars in your field

learn how others in your field use primary and secondary resources

find and understand raw data and information

For the formal academic research assignment, consider an organizational pattern typically used for primary academic research.  The pattern includes the following: introduction, methods, results, discussion, and conclusions/recommendations.

Usually, research papers flow from the general to the specific and back to the general in their organization. The introduction uses a general-to-specific movement in its organization, establishing the thesis and setting the context for the conversation. The methods and results sections are more detailed and specific, providing support for the generalizations made in the introduction. The discussion section moves toward an increasingly more general discussion of the subject, leading to the conclusions and recommendations, which then generalize the conversation again.

Sections of a Formal Structure

The introduction section.

Many students will find that writing a structured  introduction  gets them started and gives them the focus needed to significantly improve their entire paper. 

Introductions usually have three parts:

presentation of the problem statement, the topic, or the research inquiry

purpose and focus of your paper

summary or overview of the writer’s position or arguments

In the first part of the introduction—the presentation of the problem or the research inquiry—state the problem or express it so that the question is implied. Then, sketch the background on the problem and review the literature on it to give your readers a context that shows them how your research inquiry fits into the conversation currently ongoing in your subject area. 

In the second part of the introduction, state your purpose and focus. Here, you may even present your actual thesis. Sometimes your purpose statement can take the place of the thesis by letting your reader know your intentions. 

The third part of the introduction, the summary or overview of the paper, briefly leads readers through the discussion, forecasting the main ideas and giving readers a blueprint for the paper. 

The following example provides a blueprint for a well-organized introduction.

Example of an Introduction

Entrepreneurial Marketing: The Critical Difference

In an article in the Harvard Business Review, John A. Welsh and Jerry F. White remind us that “a small business is not a little big business.” An entrepreneur is not a multinational conglomerate but a profit-seeking individual. To survive, he must have a different outlook and must apply different principles to his endeavors than does the president of a large or even medium-sized corporation. Not only does the scale of small and big businesses differ, but small businesses also suffer from what the Harvard Business Review article calls “resource poverty.” This is a problem and opportunity that requires an entirely different approach to marketing. Where large ad budgets are not necessary or feasible, where expensive ad production squanders limited capital, where every marketing dollar must do the work of two dollars, if not five dollars or even ten, where a person’s company, capital, and material well-being are all on the line—that is, where guerrilla marketing can save the day and secure the bottom line (Levinson, 1984, p. 9).

By reviewing the introductions to research articles in the discipline in which you are writing your research paper, you can get an idea of what is considered the norm for that discipline. Study several of these before you begin your paper so that you know what may be expected. If you are unsure of the kind of introduction your paper needs, ask your professor for more information.  The introduction is normally written in present tense.

THE METHODS SECTION

The methods section of your research paper should describe in detail what methodology and special materials if any, you used to think through or perform your research. You should include any materials you used or designed for yourself, such as questionnaires or interview questions, to generate data or information for your research paper. You want to include any methodologies that are specific to your particular field of study, such as lab procedures for a lab experiment or data-gathering instruments for field research. The methods section is usually written in the past tense.

THE RESULTS SECTION

How you present the results of your research depends on what kind of research you did, your subject matter, and your readers’ expectations. 

Quantitative information —data that can be measured—can be presented systematically and economically in tables, charts, and graphs. Quantitative information includes quantities and comparisons of sets of data. 

Qualitative information , which includes brief descriptions, explanations, or instructions, can also be presented in prose tables. This kind of descriptive or explanatory information, however, is often presented in essay-like prose or even lists.

There are specific conventions for creating tables, charts, and graphs and organizing the information they contain. In general, you should use them only when you are sure they will enlighten your readers rather than confuse them. In the accompanying explanation and discussion, always refer to the graphic by number and explain specifically what you are referring to; you can also provide a caption for the graphic. The rule of thumb for presenting a graphic is first to introduce it by name, show it, and then interpret it. The results section is usually written in the past tense.

THE DISCUSSION SECTION

Your discussion section should generalize what you have learned from your research. One way to generalize is to explain the consequences or meaning of your results and then make your points that support and refer back to the statements you made in your introduction. Your discussion should be organized so that it relates directly to your thesis. You want to avoid introducing new ideas here or discussing tangential issues not directly related to the exploration and discovery of your thesis. The discussion section, along with the introduction, is usually written in the present tense.

THE CONCLUSIONS AND RECOMMENDATIONS SECTION

Your conclusion ties your research to your thesis, binding together all the main ideas in your thinking and writing. By presenting the logical outcome of your research and thinking, your conclusion answers your research inquiry for your reader. Your conclusions should relate directly to the ideas presented in your introduction section and should not present any new ideas.

You may be asked to present your recommendations separately in your research assignment. If so, you will want to add some elements to your conclusion section. For example, you may be asked to recommend a course of action, make a prediction, propose a solution to a problem, offer a judgment, or speculate on the implications and consequences of your ideas. The conclusions and recommendations section is usually written in the present tense.

Key Takeaways

• For the formal academic research assignment, consider an organizational pattern typically used for primary academic research. 
•  The pattern includes the following: introduction, methods, results, discussion, and conclusions/recommendations.

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Table of Contents: Online Guide to Writing

Chapter 1: College Writing

How Does College Writing Differ from Workplace Writing?

What Is College Writing?

Why So Much Emphasis on Writing?

Chapter 2: The Writing Process

Doing Exploratory Research

Getting from Notes to Your Draft

Introduction

Prewriting - Techniques to Get Started - Mining Your Intuition

Prewriting: Targeting Your Audience

Prewriting: Techniques to Get Started

Prewriting: Understanding Your Assignment

Rewriting: Being Your Own Critic

Rewriting: Creating a Revision Strategy

Rewriting: Getting Feedback

Rewriting: The Final Draft

Techniques to Get Started - Outlining

Techniques to Get Started - Using Systematic Techniques

Thesis Statement and Controlling Idea

Writing: Getting from Notes to Your Draft - Freewriting

Writing: Getting from Notes to Your Draft - Summarizing Your Ideas

Writing: Outlining What You Will Write

Chapter 3: Thinking Strategies

A Word About Style, Voice, and Tone

A Word About Style, Voice, and Tone: Style Through Vocabulary and Diction

Critical Strategies and Writing

Critical Strategies and Writing: Analysis

Critical Strategies and Writing: Evaluation

Critical Strategies and Writing: Persuasion

Critical Strategies and Writing: Synthesis

Developing a Paper Using Strategies

Kinds of Assignments You Will Write

Patterns for Presenting Information

Patterns for Presenting Information: Critiques

Patterns for Presenting Information: Discussing Raw Data

Patterns for Presenting Information: General-to-Specific Pattern

Patterns for Presenting Information: Problem-Cause-Solution Pattern

Patterns for Presenting Information: Specific-to-General Pattern

Patterns for Presenting Information: Summaries and Abstracts

Supporting with Research and Examples

Writing Essay Examinations

Writing Essay Examinations: Make Your Answer Relevant and Complete

Writing Essay Examinations: Organize Thinking Before Writing

Writing Essay Examinations: Read and Understand the Question

Chapter 4: The Research Process

Planning and Writing a Research Paper

Planning and Writing a Research Paper: Ask a Research Question

Planning and Writing a Research Paper: Cite Sources

Planning and Writing a Research Paper: Collect Evidence

Planning and Writing a Research Paper: Decide Your Point of View, or Role, for Your Research

Planning and Writing a Research Paper: Draw Conclusions

Planning and Writing a Research Paper: Find a Topic and Get an Overview

Planning and Writing a Research Paper: Manage Your Resources

Planning and Writing a Research Paper: Outline

Planning and Writing a Research Paper: Survey the Literature

Planning and Writing a Research Paper: Work Your Sources into Your Research Writing

Research Resources: Where Are Research Resources Found? - Human Resources

Research Resources: What Are Research Resources?

Research Resources: Where Are Research Resources Found?

Research Resources: Where Are Research Resources Found? - Electronic Resources

Research Resources: Where Are Research Resources Found? - Print Resources

Structuring the Research Paper: Formal Research Structure

Structuring the Research Paper: Informal Research Structure

The Nature of Research

The Research Assignment: How Should Research Sources Be Evaluated?

The Research Assignment: When Is Research Needed?

The Research Assignment: Why Perform Research?

Chapter 5: Academic Integrity

Academic Integrity

Giving Credit to Sources

Giving Credit to Sources: Copyright Laws

Giving Credit to Sources: Documentation

Giving Credit to Sources: Style Guides

Integrating Sources

Practicing Academic Integrity

Practicing Academic Integrity: Keeping Accurate Records

Practicing Academic Integrity: Managing Source Material

Practicing Academic Integrity: Managing Source Material - Paraphrasing Your Source

Practicing Academic Integrity: Managing Source Material - Quoting Your Source

Practicing Academic Integrity: Managing Source Material - Summarizing Your Sources

Types of Documentation

Types of Documentation: Bibliographies and Source Lists

Types of Documentation: Citing World Wide Web Sources

Types of Documentation: In-Text or Parenthetical Citations

Types of Documentation: In-Text or Parenthetical Citations - APA Style

Types of Documentation: In-Text or Parenthetical Citations - CSE/CBE Style

Types of Documentation: In-Text or Parenthetical Citations - Chicago Style

Types of Documentation: In-Text or Parenthetical Citations - MLA Style

Types of Documentation: Note Citations

Chapter 6: Using Library Resources

Finding Library Resources

Chapter 7: Assessing Your Writing

How Is Writing Graded?

How Is Writing Graded?: A General Assessment Tool

The Draft Stage

The Draft Stage: The First Draft

The Draft Stage: The Revision Process and the Final Draft

The Draft Stage: Using Feedback

The Research Stage

Using Assessment to Improve Your Writing

Chapter 8: Other Frequently Assigned Papers

Reviews and Reaction Papers: Article and Book Reviews

Reviews and Reaction Papers: Reaction Papers

Writing Arguments

Writing Arguments: Adapting the Argument Structure

Writing Arguments: Purposes of Argument

Writing Arguments: References to Consult for Writing Arguments

Writing Arguments: Steps to Writing an Argument - Anticipate Active Opposition

Writing Arguments: Steps to Writing an Argument - Determine Your Organization

Writing Arguments: Steps to Writing an Argument - Develop Your Argument

Writing Arguments: Steps to Writing an Argument - Introduce Your Argument

Writing Arguments: Steps to Writing an Argument - State Your Thesis or Proposition

Writing Arguments: Steps to Writing an Argument - Write Your Conclusion

Writing Arguments: Types of Argument

Appendix A: Books to Help Improve Your Writing

Dictionaries

General Style Manuals

Researching on the Internet

Special Style Manuals

Writing Handbooks

Appendix B: Collaborative Writing and Peer Reviewing

Collaborative Writing: Assignments to Accompany the Group Project

Collaborative Writing: Informal Progress Report

Collaborative Writing: Issues to Resolve

Collaborative Writing: Methodology

Collaborative Writing: Peer Evaluation

Collaborative Writing: Tasks of Collaborative Writing Group Members

Collaborative Writing: Writing Plan

General Introduction

Peer Reviewing

Appendix C: Developing an Improvement Plan

Working with Your Instructor’s Comments and Grades

Appendix D: Writing Plan and Project Schedule

Devising a Writing Project Plan and Schedule

Reviewing Your Plan with Others

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Organizing Academic Research Papers: 4. The Introduction

• Purpose of Guide
• Design Flaws to Avoid
• Glossary of Research Terms
• Narrowing a Topic Idea
• Broadening a Topic Idea
• Extending the Timeliness of a Topic Idea
• Academic Writing Style
• Choosing a Title
• Making an Outline
• Paragraph Development
• Executive Summary
• Background Information
• The Research Problem/Question
• Theoretical Framework
• Citation Tracking
• Content Alert Services
• Evaluating Sources
• Primary Sources
• Secondary Sources
• Tertiary Sources
• What Is Scholarly vs. Popular?
• Qualitative Methods
• Quantitative Methods
• Using Non-Textual Elements
• Limitations of the Study
• Common Grammar Mistakes
• Avoiding Plagiarism
• Footnotes or Endnotes?
• Further Readings
• Annotated Bibliography
• Dealing with Nervousness
• Using Visual Aids
• Grading Someone Else's Paper
• How to Manage Group Projects
• Multiple Book Review Essay
• Reviewing Collected Essays
• About Informed Consent
• Writing Field Notes
• Writing a Policy Memo
• Writing a Research Proposal
• Acknowledgements

The introduction serves the purpose of leading the reader from a general subject area to a particular field of research. It establishes the context of the research being conducted by summarizing current understanding and background information about the topic, stating the purpose of the work in the form of the hypothesis, question, or research problem, briefly explaining your rationale, methodological approach, highlighting the potential outcomes your study can reveal, and describing the remaining structure of the paper.

Key Elements of the Research Proposal. Prepared under the direction of the Superintendent and by the 2010 Curriculum Design and Writing Team. Baltimore County Public Schools.

Importance of a Good Introduction

Think of the introduction as a mental road map that must answer for the reader these four questions:

• What was I studying?
• Why was this topic important to investigate?
• What did we know about this topic before I did this study?
• How will this study advance our knowledge?

A well-written introduction is important because, quite simply, you never get a second chance to make a good first impression. The opening paragraph of your paper will provide your readers with their initial impressions about the logic of your argument, your writing style, the overall quality of your research, and, ultimately, the validity of your findings and conclusions. A vague, disorganized, or error-filled introduction will create a negative impression, whereas, a concise, engaging, and well-written introduction will start your readers off thinking highly of your analytical skills, your writing style, and your research approach.

Introductions . The Writing Center. University of North Carolina.

Structure and Writing Style

I. Structure and Approach

The introduction is the broad beginning of the paper that answers three important questions for the reader:

• What is this?
• Why am I reading it?
• What do you want me to think about / consider doing / react to?

Think of the structure of the introduction as an inverted triangle of information. Organize the information so as to present the more general aspects of the topic early in the introduction, then narrow toward the more specific topical information that provides context, finally arriving at your statement of purpose and rationale and, whenever possible, the potential outcomes your study can reveal.

These are general phases associated with writing an introduction:

• Highlighting the importance of the topic, and/or
• Making general statements about the topic, and/or
• Presenting an overview on current research on the subject.
• Opposing an existing assumption, and/or
• Revealing a gap in existing research, and/or
• Formulating a research question or problem, and/or
• Continuing a disciplinary tradition.
• Stating the intent of your study,
• Outlining the key characteristics of your study,
• Describing important results, and
• Giving a brief overview of the structure of the paper.

NOTE: Even though the introduction is the first main section of a research paper, it is often useful to finish the introduction very late in the writing process because the structure of the paper, the reporting and analysis of results, and the conclusion will have been completed and it ensures that your introduction matches the overall structure of your paper.

II.  Delimitations of the Study

Delimitations refer to those characteristics that limit the scope and define the conceptual boundaries of your study . This is determined by the conscious exclusionary and inclusionary decisions you make about how to investigate the research problem. In other words, not only should you tell the reader what it is you are studying and why, but you must also acknowledge why you rejected alternative approaches that could have been used to examine the research problem.

Obviously, the first limiting step was the choice of research problem itself. However, implicit are other, related problems that could have been chosen but were rejected. These should be noted in the conclusion of your introduction.

Examples of delimitating choices would be:

• The key aims and objectives of your study,
• The research questions that you address,
• The variables of interest [i.e., the various factors and features of the phenomenon being studied],
• The method(s) of investigation, and
• Any relevant alternative theoretical frameworks that could have been adopted.

Review each of these decisions. You need to not only clearly establish what you intend to accomplish, but to also include a declaration of what the study does not intend to cover. In the latter case, your exclusionary decisions should be based upon criteria stated as, "not interesting"; "not directly relevant"; “too problematic because..."; "not feasible," and the like. Make this reasoning explicit!

NOTE: Delimitations refer to the initial choices made about the broader, overall design of your study and should not be confused with documenting the limitations of your study discovered after the research has been completed.

III. The Narrative Flow

Issues to keep in mind that will help the narrative flow in your introduction :

• Your introduction should clearly identify the subject area of interest . A simple strategy to follow is to use key words from your title in the first few sentences of the introduction. This will help focus the introduction on the topic at the appropriate level and ensures that you get to the primary subject matter quickly without losing focus, or discussing information that is too general.
• Establish context by providing a brief and balanced review of the pertinent published literature that is available on the subject. The key is to summarize for the reader what is known about the specific research problem before you did your analysis. This part of your introduction should not represent a comprehensive literature review but consists of a general review of the important, foundational research literature (with citations) that lays a foundation for understanding key elements of the research problem. See the drop-down tab for "Background Information" for types of contexts.
• Clearly state the hypothesis that you investigated . When you are first learning to write in this format it is okay, and actually preferable, to use a past statement like, "The purpose of this study was to...." or "We investigated three possible mechanisms to explain the...."
• Why did you choose this kind of research study or design? Provide a clear statement of the rationale for your approach to the problem studied. This will usually follow your statement of purpose in the last paragraph of the introduction.

IV. Engaging the Reader

The overarching goal of your introduction is to make your readers want to read your paper. The introduction should grab your reader's attention. Strategies for doing this can be to:

• Open with a compelling story,
• Include a strong quotation or a vivid, perhaps unexpected anecdote,
• Pose a provocative or thought-provoking question,
• Describe a puzzling scenario or incongruity, or
• Cite a stirring example or case study that illustrates why the research problem is important.

NOTE:   Only choose one strategy for engaging your readers; avoid giving an impression that your paper is more flash than substance.

Freedman, Leora  and Jerry Plotnick. Introductions and Conclusions . University College Writing Centre. University of Toronto; Introduction . The Structure, Format, Content, and Style of a Journal-Style Scientific Paper. Department of Biology. Bates College; Introductions . The Writing Center. University of North Carolina; Introductions . The Writer’s Handbook. Writing Center. University of Wisconsin, Madison; Introductions, Body Paragraphs, and Conclusions for an Argument Paper. The Writing Lab and The OWL. Purdue University; Resources for Writers: Introduction Strategies . Program in Writing and Humanistic Studies. Massachusetts Institute of Technology; Sharpling, Gerald. Writing an Introduction . Centre for Applied Linguistics, University of Warwick; Writing Your Introduction. Department of English Writing Guide. George Mason University.

Writing Tip

Avoid the "Dictionary" Introduction

Giving the dictionary definition of words related to the research problem may appear appropriate because it is important to define specific words or phrases with which readers may be unfamiliar. However, anyone can look a word up in the dictionary and a general dictionary is not a particularly authoritative source. It doesn't take into account the context of your topic and doesn't offer particularly detailed information. Also, placed in the context of a particular discipline, a term may have a different meaning than what is found in a general dictionary. If you feel that you must seek out an authoritative definition, try to find one that is from subject specific dictionaries or encyclopedias [e.g., if you are a sociology student, search for dictionaries of sociology].

Saba, Robert. The College Research Paper . Florida International University; Introductions . The Writing Center. University of North Carolina.

Another Writing Tip

When Do I Begin?

A common question asked at the start of any paper is, "where should I begin?" An equally important question to ask yourself is, "When do I begin?" Research problems in the social sciences rarely rest in isolation from the history of the issue being investigated. It is, therefore, important to lay a foundation for understanding the historical context underpinning the research problem. However, this information should be brief and succinct and begin at a point in time that best informs the reader of study's overall importance. For example, a study about coffee cultivation and export in West Africa as a key stimulus for local economic growth needs to describe the beginning of exporting coffee in the region and establishing why economic growth is important. You do not need to give a long historical explanation about coffee exportation in Africa. If a research problem demands a substantial exploration of historical context, do this in the literature review section; note in the introduction as part of your "roadmap" [see below] that you covering this in the literature review.

Yet Another Writing Tip

Always End with a Roadmap

The final paragraph or sentences of your introduction should forecast your main arguments and conclusions and provide a description of the rest of the paper [a "roadmap"] that let's the reader know where you are going and what to expect.

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Research Writing ~ How to Write a Research Paper

• Choosing A Topic
• Critical Thinking
• Domain Names
• Starting Your Research
• Writing Tips
• Parts of the Paper
• Edit & Rewrite
• Citations This link opens in a new window

Papers should have a beginning, a middle, and an end. Your introductory paragraph should grab the reader's attention, state your main idea and how you will support it. The body of the paper should expand on what you have stated in the introduction. Finally, the conclusion restates the paper's thesis and should explain what you have learned, giving a wrap up of your main ideas.   

1. The Title The title should be specific and indicate the theme of the research and what ideas it addresses. Use keywords that help explain your paper's topic to the reader. Try to avoid  abbreviations  and  jargon.  Think about keywords that people would use to search for your paper and include them in your title. 

2. The Abstract The abstract is used by readers to get a quick overview of your paper. Typically, they are about 200 words in length (120 words minimum to  250 words maximum). The abstract should introduce the topic and thesis, and should provide a general statement about what you have found in your research. The abstract allows you to mention each major aspect of you topic and helps readers decide whether they want to read the rest of the paper. Because it is a summary of the entire research paper, it is often written last. 

3. The Introduction The introduction should be designed to attract the reader's attention and explain the focus of the research. You will introduce your overview of the topic, your main points of information, and why this subject is important. You can introduce the current understanding and background information about the topic. Toward the end of the introduction, you add your thesis statement, and explain how you will provide information to support your research questions. This provides the purpose, focus, and structure for the rest of the paper.

4. Thesis Statement Most papers will have a thesis statement or main idea and supporting facts/ideas/arguments. State your main idea (something of interest or something to be proven or argued for or against) as your thesis statement, and then provide  supporting facts and arguments. A thesis statement is a declarative sentence that asserts the position a paper will be taking. It also points toward the paper's development. This statement should be both specific and arguable. Generally, the thesis statement will be placed at the end of the first paragraph of your paper. The remainder of your paper will support this thesis.

Students often learn to write a thesis as a first step in the writing process, but often, after research, a writers viewpoint may change. Therefore a thesis statement may be one of the final steps in writing. 

Examples of thesis statements from Purdue OWL. . .

5. The Literature Review The purpose of the literature review is to describe past important research and how it specifically relates to the research thesis. It should be a synthesis of the previous literature and the new idea being researched. The review should examine the major theories related to the topic to date and their contributors. It should include all relevant findings from credible sources, such as academic books and peer-reviewed journal articles. You will want  to:

• Explain how the literature helps the researcher understand the topic.
• Try to show connections and any disparities between the literature.
• Identify new ways to interpret prior research.
• Reveal any gaps that exist in the literature.

More about writing a literature review. . .  from The Writing Center at UNC-Chapel Hill More about summarizing. . . from the Center for Writing Studies at the University of Illinois-Urbana Champaign

6. The Discussion ​The purpose of the discussion is to interpret and describe what you have learned from your research. Make the reader understand why your topic is important. The discussion should always demonstrate what you have learned from your readings (and viewings) and how that learning has made the topic evolve, especially from the short description of main points in the introduction. Explain any new understanding or insights you have had after reading your articles and/or books. Paragraphs should use transitioning sentences to develop how one paragraph idea leads to the next. The discussion will always connect to the introduction, your thesis statement, and the literature you reviewed, but it does not simply repeat or rearrange the introduction. You want to: 

• Demonstrate critical thinking, not just reporting back facts that you gathered.
• If possible, tell how the topic has evolved over the past and give it's implications for the future.
• Fully explain your main ideas with supporting information.
• Explain why your thesis is correct giving arguments to counter points.

​7. The Conclusion A concluding paragraph is a brief summary of your main ideas and restates the paper's main thesis, giving the reader the sense that the stated goal of the paper has been accomplished. What have you learned by doing this research that you didn't know before? What conclusions have you drawn? You may also want to suggest further areas of study, improvement of research possibilities, etc. to demonstrate your critical thinking regarding your research.

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BSCI 1510L Literature and Stats Guide: 3.2 Components of a scientific paper

• 1 What is a scientific paper?
• 2 Referencing and accessing papers
• 2.1 Literature Cited
• 2.2 Accessing Scientific Papers
• 2.3 Traversing the web of citations
• 2.4 Keyword Searches
• 3 Style of scientific writing
• 3.1 Specific details regarding scientific writing

3.2 Components of a scientific paper

• 4 Summary of the Writing Guide and Further Information
• Appendix A: Calculation Final Concentrations
• 1 Formulas in Excel
• 2 Basic operations in Excel
• 3 Measurement and Variation
• 3.1 Describing Quantities and Their Variation
• 3.2 Samples Versus Populations
• 3.3 Calculating Descriptive Statistics using Excel
• 4 Variation and differences
• 5 Differences in Experimental Science
• 5.1 Aside: Commuting to Nashville
• 5.2 P and Detecting Differences in Variable Quantities
• 5.3 Statistical significance
• 5.4 A test for differences of sample means: 95% Confidence Intervals
• 5.5 Error bars in figures
• 5.6 Discussing statistics in your scientific writing
• 6 Scatter plot, trendline, and linear regression
• 7 The t-test of Means
• 8 Paired t-test
• 9 Two-Tailed and One-Tailed Tests
• 10 Variation on t-tests: ANOVA
• 11 Reporting the Results of a Statistical Test
• 12 Summary of statistical tests
• 1 Objectives
• 2 Project timeline
• 3 Background
• 4 Previous work in the BSCI 111 class
• 5 General notes about the project
• 6 About the paper
• 7 References

Nearly all journal articles are divided into the following major sections: abstract, introduction, methods, results, discussion, and references or literature cited.   Usually the sections are labeled as such, although often the introduction (and sometimes the abstract) is not labeled.  Sometimes alternative section titles are used.  The abstract is sometimes called the "summary", the methods are sometimes called "materials and methods", and the discussion is sometimes called "conclusions".   Some journals also include the minor sections of "key words" following the abstract, and "acknowledgments" following the discussion.  In some journals, the sections may be divided into subsections that are given descriptive titles.  However, the general division into the six major sections is nearly universal.

3.2.1 Abstract

The abstract is a short summary (150-200 words or less) of the important points of the paper.  It does not generally include background information.  There may be a very brief statement of the rationale for conducting the study.  It describes what was done, but without details.  It also describes the results in a summarized way that usually includes whether or not the statistical tests were significant.  It usually concludes with a brief statement of the importance of the results.  Abstracts do not include references.  When writing a paper, the abstract is always the last part to be written.

The purpose of the abstract is to allow potential readers of a paper to find out the important points of the paper without having to actually read the paper.  It should be a self-contained unit capable of being understood without the benefit of the text of the article . It essentially serves as an "advertisement" for the paper that readers use to determine whether or not they actually want to wade through the entire paper or not.  Abstracts are generally freely available in electronic form and are often presented in the results of an electronic search.  If searchers do not have electronic access to the journal in which the article is published, the abstract is the only means that they have to decide whether to go through the effort (going to the library to look up the paper journal, requesting a reprint from the author, buying a copy of the article from a service, requesting the article by Interlibrary Loan) of acquiring the article.  Therefore it is important that the abstract accurately and succinctly presents the most important information in the article.

3.2.2 Introduction

The introduction section of a paper provides the background information necessary to understand why the described experiment was conducted.  The introduction should describe previous research on the topic that has led to the unanswered questions being addressed by the experiment and should cite important previous papers that form the background for the experiment.  The introduction should also state in an organized fashion the goals of the research, i.e. the particular, specific questions that will be tested in the experiments.  There should be a one-to-one correspondence between questions raised in the introduction and points discussed in the conclusion section of the paper.  In other words, do not raise questions in the introduction unless you are going to have some kind of answer to the question that you intend to discuss at the end of the paper. 

You may have been told that every paper must have a hypothesis that can be clearly stated.  That is often true, but not always.  If your experiment involves a manipulation which tests a specific hypothesis, then you should clearly state that hypothesis.  On the other hand, if your experiment was primarily exploratory, descriptive, or measurative, then you probably did not have an  a priori  hypothesis, so don't pretend that you did and make one up.  (See the discussion in the introduction to Experiment 5 for more on this.)  If you state a hypothesis in the introduction, it should be a general hypothesis and not a null or alternative hypothesis for a statistical test.  If it is necessary to explain how a statistical test will help you evaluate your general hypothesis, explain that in the methods section. 

A good introduction should be fairly heavy with citations.  This indicates to the reader that the authors are informed about previous work on the topic and are not working in a vacuum.  Citations also provide jumping-off points to allow the reader to explore other tangents to the subject that are not directly addressed in the paper.  If the paper supports or refutes previous work, readers can look up the citations and make a comparison for themselves. 

"Do not get lost in reviewing background information. Remember that the Introduction is meant to introduce the reader to your research, not summarize and evaluate all past literature on the subject (which is the purpose of a review paper). Many of the other studies you may be tempted to discuss in your Introduction are better saved for the Discussion, where they become a powerful tool for comparing and interpreting your results. Include only enough background information to allow your reader to understand why you are asking the questions you are and why your hypotheses are reasonable ones. Often, a brief explanation of the theory involved is sufficient.

Write this section in the past or present tense, never in the future. " (Steingraber et al. 1985)

In other words, the introduction section relates what the topic being investigated is, why it is important, what research (if any) has been done prior that is relevant to what you are trying to do, and in what ways you will be looking into this topic.

An example to think about:

This is an example of a student-derived introduction.  Read the paragraph and before you go beyond, think about the paragraph first.

"Hand-washing is one of the most effective and simplest of ways to reduce infection and disease, and thereby causing less death.  When examining the effects of soap on hands, it was the work of Sickbert-Bennett and colleagues (2005) that showed that using soap or an alcohol on the hands during hand-washing was a significant effect in removing bacteria from the human hand.  Based on the work of this, the team led by Larsen (1991) then showed that the use of computer imaging could be a more effective way to compare the amount of bacteria on a hand."

There are several aspects within this "introduction" that could use improvement.  A group of any random 4 of you could easily come up with at 10 different things to reword, revise, expand upon.

In specific, there should be one thing addressed that more than likely you did not catch when you were reading it.

The citations: Not the format, but the logical use of them.

Look again. "...the work of Sickbert-Bennett...(2005)" and then "Based on the work of this, the team led by Larsen (1991)..."

How can someone in 1991 use or base their work on something from 2005?

They cannot.  You can spend an entire hour using spellcheck and reading through and through again to find all the little things to "give it more oomph", but at the core, you still must present a clear and concise and logical thought-process.

3.2.3 Methods (taken mostly verbatim from Steingraber et al. 1985, until the version A, B,C portion)

The function of the methods section is to describe all experimental procedures, including controls.  The description should be complete enough to enable someone else to repeat your work.  If there is more than one part to the experiment, it is a good idea to describe your methods and present your results in the same order in each section. This may not be the same order in which the experiments were performed -it is up to you to decide what order of presentation will make the most sense to your reader.

1.  Explain why each procedure was done, i.e., what variable were you measuring and why? Example:

Difficult to understand :  First, I removed the frog muscle and then I poured Ringer’s solution on it. Next, I attached it to the kymograph.

Improved:   I removed the frog muscle and poured Ringer’s solution on it to prevent it from drying out. I then attached the muscle to the kymograph in order to determine the minimum voltage required for contraction.

Better:   Frog muscle was excised between attachment points to the bone. Ringer's solution was added to the excised section to prevent drying out. The muscle was attached to the kymograph in order to determine the minimum voltage required for contraction.

2.  Experimental procedures and results are narrated in the past tense (what you did, what you found, etc.) whereas conclusions from your results are given in the present tense.

3.  Mathematical equations and statistical tests are considered mathematical methods and should be described in this section along with the actual experimental work. (Show a sample calculation, state the type of test(s) performed and program used)

4.  Use active rather than passive voice when possible.  [Note: see Section 3.1.4 for more about this.]  Always use the singular "I" rather than the plural "we" when you are the only author of the paper (Methods section only).  Throughout the paper, avoid contractions, e.g. did not vs. didn’t.

5.  If any of your methods is fully described in a previous publication (yours or someone else’s), you can cite work that instead of describing the procedure again.

Example:  The chromosomes were counted at meiosis in the anthers with the standard acetocarmine technique of Snow (1955).

Below is a PARTIAL and incomplete version of a "method".  Without getting into the details of why, Version A and B are bad.  A is missing too many details and B is giving some extra details but not giving some important ones, such as the volumes used.  Version C is still not complete, but it is at least a viable method. Notice that C is also not the longest....it is possible to be detailed without being long-winded.

In other words, the methods section is what you did in the experiment and has enough details that someone else can repeat your experiment.  If the methods section has excluded one or more important detail(s) such that the reader of the method does not know what happened, it is a 'poor' methods section.  Similarly, by giving out too many useless details a methods section can be 'poor'.

You may have multiple sub-sections within your methods (i.e., a section for media preparation, a section for where the chemicals came from, a section for the basic physical process that occurred, etc.,).  A methods section is  NEVER  a list of numbered steps.

3.2.4 Results (with excerpts from Steingraber et al. 1985)

The function of this section is to summarize general trends in the data without comment, bias, or interpretation. The results of statistical tests applied to your data are reported in this section although conclusions about your original hypotheses are saved for the Discussion section. In other words, you state "the P-value" in Results and whether below/above 0.05 and thus significant/not significant while in the Discussion you restate the P-value and then formally state what that means beyond "significant/not significant".

Tables and figures  should be used  when they are a more efficient way to convey information than verbal description. They must be independent units, accompanied by explanatory captions that allow them to be understood by someone who has not read the text. Do not repeat in the text the information in tables and figures, but do cite them, with a summary statement when that is appropriate.  Example:

Incorrect:   The results are given in Figure 1.

Correct:   Temperature was directly proportional to metabolic rate (Fig. 1).

Please note that the entire word "Figure" is almost never written in an article.  It is nearly always abbreviated as "Fig." and capitalized.  Tables are cited in the same way, although Table is not abbreviated.

Whenever possible, use a figure instead of a table. Relationships between numbers are more readily grasped when they are presented graphically rather than as columns in a table.

Data may be presented in figures and tables, but this may not substitute for a verbal summary of the findings. The text should be  understandable  by someone who has not seen your figures and tables.

1.  All results should be presented, including those that do not support the hypothesis.

2.  Statements made in the text must be supported by the results contained in figures and tables.

3.  The results of statistical tests can be presented in parentheses following a verbal description.

Example: Fruit size was significantly greater in trees growing alone (t = 3.65, df = 2, p < 0.05).

Simple results of statistical tests may be reported in the text as shown in the preceding example.  The results of multiple tests may be reported in a table if that increases clarity. (See Section 11 of the Statistics Manual for more details about reporting the results of statistical tests.)  It is not necessary to provide a citation for a simple t-test of means, paired t-test, or linear regression.  If you use other more complex (or less well-known) tests, you should cite the text or reference you followed to do the test.  In your materials and methods section, you should report how you did the test (e.g. using the statistical analysis package of Excel). 

It is NEVER appropriate to simply paste the results from statistical software into the results section of your paper.   The output generally reports more information than is required and it is not in an appropriate format for a paper. Similar, do NOT place a screenshot.  

Should you include every data point or not in the paper?  Prior to 2010 or so, most papers would probably not present the actual raw data collected, but rather show the "descriptive statistics" about their data (mean, SD, SE, CI, etc.). Often, people could simply contact the author(s) for the data and go from there.  As many journals have a significant on-line footprint now, it has become increasingly more common that the entire data could be included in the paper.  And realize why the entire raw data may not have been included in a publication. Prior to about 2010, your publication had limited  paper space  to be seen on.  If you have a sample of size of 10 or 50, you probably could show the entire data set easily in one table/figure and it not take up too much printed space. If your sample size was 500 or 5,000 or more, the size of the data alone would take pages of printed text.  Given how much the Internet and on-line publications have improved/increased in storage space, often now there will be either an embedded file to access or the author(s) will place the file on-line somewhere with an address link, such as GitHub.  Videos of the experiment are also shown as well now.

3.2.4.1 Tables

• Do not repeat information in a table that you are depicting in a graph or histogram; include a table only if it presents new information.
• It is easier to compare numbers by reading down a column rather than across a row. Therefore, list sets of data you want your reader to compare in vertical form.
• Provide each table with a number (Table 1, Table 2, etc.) and a title. The numbered title is placed above the table .
• Please see Section 11 of the Excel Reference and Statistics Manual for further information on reporting the results of statistical tests.

3.2.4.2. Figures

• These comprise graphs, histograms, and illustrations, both drawings and photographs. Provide each figure with a number (Fig. 1, Fig. 2, etc.) and a caption (or "legend") that explains what the figure shows. The numbered caption is placed below the figure .  Figure legend = Figure caption.
• Figures submitted for publication must be "photo ready," i.e., they will appear just as you submit them, or photographically reduced. Therefore, when you graduate from student papers to publishable manuscripts, you must learn to prepare figures that will not embarrass you. At the present time, virtually all journals require manuscripts to be submitted electronically and it is generally assumed that all graphs and maps will be created using software rather than being created by hand.  Nearly all journals have specific guidelines for the file types, resolution, and physical widths required for figures.  Only in a few cases (e.g. sketched diagrams) would figures still be created by hand using ink and those figures would be scanned and labeled using graphics software.  Proportions must be the same as those of the page in the journal to which the paper will be submitted. 
• Graphs and Histograms: Both can be used to compare two variables. However, graphs show continuous change, whereas histograms show discrete variables only.  You can compare groups of data by plotting two or even three lines on one graph, but avoid cluttered graphs that are hard to read, and do not plot unrelated trends on the same graph. For both graphs, and histograms, plot the independent variable on the horizontal (x) axis and the dependent variable on the vertical (y) axis. Label both axes, including units of measurement except in the few cases where variables are unitless, such as absorbance.
• Drawings and Photographs: These are used to illustrate organisms, experimental apparatus, models of structures, cellular and subcellular structure, and results of procedures like electrophoresis. Preparing such figures well is a lot of work and can be very expensive, so each figure must add enough to justify its preparation and publication, but good figures can greatly enhance a professional article, as your reading in biological journals has already shown.

3.2.5 Discussion (modified; taken from Steingraber et al. 1985)

The function of this section is to analyze the data and relate them to other studies. To "analyze" means to evaluate the meaning of your results in terms of the original question or hypothesis and point out their biological significance.

1. The Discussion should contain at least:

• the relationship between the results and the original hypothesis, i.e., whether they support the hypothesis, or cause it to be rejected or modified
• an integration of your results with those of previous studies in order to arrive at explanations for the observed phenomena
• possible explanations for unexpected results and observations, phrased as hypotheses that can be tested by realistic experimental procedures, which you should describe

2. Trends that are not statistically significant can still be discussed if they are suggestive or interesting, but cannot be made the basis for conclusions as if they were significant.

3. Avoid redundancy between the Results and the Discussion section. Do not repeat detailed descriptions of the data and results in the Discussion. In some journals, Results and Discussions are joined in a single section, in order to permit a single integrated treatment with minimal repetition. This is more appropriate for short, simple articles than for longer, more complicated ones.

4.  End the Discussion with a summary of the principal points you want the reader to remember. This is also the appropriate place to propose specific further study if that will serve some purpose,  but do not end with the tired cliché  that "this problem needs more study." All problems in biology need more study. Do not close on what you wish you had done, rather finish stating your conclusions and contributions.

5.  Conclusion section.  Primarily dependent upon the complexity and depth of an experiment, there may be a formal conclusion section after the discussion section. In general, the last line or so of the discussion section should be a more or less summary statement of the overall finding of the experiment.  IF the experiment was large enough/complex enough/multiple findings uncovered, a distinct paragraph (or two) may be needed to help clarify the findings.  Again, only if the experiment scale/findings warrant a separate conclusion section.

3.2.6 Title

The title of the paper should be the last thing that you write.  That is because it should distill the essence of the paper even more than the abstract (the next to last thing that you write). 

The title should contain three elements:

1. the name of the organism studied;

2. the particular aspect or system studied;

3. the variable(s) manipulated.

Do not be afraid to be grammatically creative. Here are some variations on a theme, all suitable as titles:

THE EFFECT OF TEMPERATURE ON GERMINATION OF ZEA MAYS

DOES TEMPERATURE AFFECT GERMINATION OF ZEA MAYS?

TEMPERATURE AND ZEA MAYS GERMINATION: IMPLICATIONS FOR AGRICULTURE

Sometimes it is possible to include the principal result or conclusion in the title:

HIGH TEMPERATURES REDUCE GERMINATION OF ZEA MAYS

Note for the BSCI 1510L class: to make your paper look more like a real paper, you can list all of the other group members as co-authors.  However, if you do that, you should list you name first so that we know that you wrote it.

3.2.7 Literature Cited

Please refer to section 2.1 of this guide.

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• Last Updated: Jun 21, 2024 9:40 AM
• URL: https://researchguides.library.vanderbilt.edu/bsci1510L

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Parts of a Research Paper

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Inhaltsverzeichnis

• 1 Parts of a Research Paper: Definition
• 3 Research Paper Structure
• 4 Research Paper Examples
• 5 Research Paper APA Formatting
• 6 In a Nutshell

Parts of a Research Paper: Definition

The point of having specifically defined parts of a research paper is not to make your life as a student harder. In fact, it’s very much the opposite. The different parts of a research paper have been established to provide a structure that can be consistently used to make your research projects easier, as well as helping you follow the proper scientific methodology.

This will help guide your writing process so you can focus on key elements one at a time. It will also provide a valuable outline that you can rely on to effectively structure your assignment. Having a solid structure will make your research paper easier to understand, and it will also prepare you for a possible future as a researcher, since all modern science is created around similar precepts.

Have you been struggling with your academic homework lately, especially where it concerns all the different parts of a research paper? This is actually a very common situation, so we have prepared this article to outline all the key parts of a research paper and explain what you must focus as you go through each one of the various parts of a research paper; read the following sections and you should have a clearer idea of how to tackle your next research paper effectively.

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What are the main parts of a research paper?

There are eight main parts in a research paper :

• Title (cover page)

Introduction

• Literature review
• Research methodology
• Data analysis
• Reference page

If you stick to this structure, your end product will be a concise, well-organized research paper.

Do you have to follow the exact research paper structure?

Yes, and failing to do so will likely impact your grade very negatively. It’s very important to write your research paper according to the structure given on this article. Follow your research paper outline   to avoid a messy structure. Different types of academic papers have very particular structures. For example, the structure required for a literature review is very different to the structure required for a scientific research paper.

What if I'm having trouble with certain parts of a research paper?

If you’re having problems with some parts of a research paper, it will be useful to look at some examples of finished research papers in a similar field of study, so you will have a better idea of the elements you need to include. Read a step-by-step guide for writing a research paper, or take a look at the section towards the end of this article for some research paper examples. Perhaps you’re just lacking inspiration!

Is there a special formatting you need to use when citing sources?

Making adequate citations to back up your research is a key consideration in almost every part of a research paper. There are various formatting conventions and referencing styles that should be followed as specified in your assignment. The most common is APA formatting, but you could also be required to use MLA formatting. Your professor or supervisor should tell you which one you need to use.

What should I do once I have my research paper outlined?

If you have created your research paper outline, then you’re ready to start writing. Remember, the first copy will be a draft, so don’t leave it until the last minute to begin writing. Check out some tips for overcoming writer’s block if you’re having trouble getting started.

Research Paper Structure

There are 8 parts of a research paper that you should go through in this order:

The very first page in your research paper should be used to identify its title, along with your name, the date of your assignment, and your learning institution. Additional elements may be required according to the specifications of your instructors, so it’s a good idea to check with them to make sure you feature all the required information in the right order. You will usually be provided with a template or checklist of some kind that you can refer to when writing your cover page .

This is the very beginning of your research paper, where you are expected to provide your thesis statement ; this is simply a summary of what you’re setting out to accomplish with your research project, including the problems you’re looking to scrutinize and any solutions or recommendations that you anticipate beforehand.

Literature Review

This part of a research paper is supposed to provide the theoretical framework that you elaborated during your research. You will be expected to present the sources you have studied while preparing for the work ahead, and these sources should be credible from an academic standpoint (including educational books, peer-reviewed journals, and other relevant publications). You must make sure to include the name of the relevant authors you’ve studied and add a properly formatted citation that explicitly points to their works you have analyzed, including the publication year (see the section below on APA style citations ).

Research Methodology

Different parts of a research paper have different aims, and here you need to point out the exact methods you have used in the course of your research work. Typical methods can range from direct observation to laboratory experiments, or statistical evaluations. Whatever your chosen methods are, you will need to explicitly point them out in this section.

Data Analysis

While all the parts of a research paper are important, this section is probably the most crucial from a practical standpoint. Out of all the parts of a research paper, here you will be expected to analyze the data you have obtained in the course of your research. This is where you get your chance to really shine, by introducing new data that may contribute to building up on the collective understanding of the topics you have researched. At this point, you’re not expected to analyze your data yet (that will be done in the subsequent parts of a research paper), but simply to present it objectively.

From all the parts of a research paper, this is the one where you’re expected to actually analyze the data you have gathered while researching. This analysis should align with your previously stated methodology, and it should both point out any implications suggested by your data that might be relevant to different fields of study, as well as any shortcomings in your approach that would allow you to improve you results if you were to repeat the same type of research.

As you conclude your research paper, you should succinctly reiterate your thesis statement along with your methodology and analyzed data – by drawing all these elements together you will reach the purpose of your research, so all that is left is to point out your conclusions in a clear manner.

Reference Page

The very last section of your research paper is a reference page where you should collect the academic sources along with all the publications you consulted, while fleshing out your research project. You should make sure to list all these references according to the citation format specified by your instructor; there are various formats now in use, such as MLA, Harvard and APA, which although similar rely on different citation styles that must be consistently and carefully observed.

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Research Paper Examples

When you’re still learning about the various parts that make up a research paper, it can be useful to go through some examples of actual research papers from your exact field of study. This is probably the best way to fully grasp what is the purpose of all the different parts.

We can’t provide you universal examples of all the parts of a research paper, since some of these parts can be very different depending on your field of study.

To get a clear sense of what you should cover in each part of your paper, we recommend you to find some successful research papers in a similar field of study. Often, you may be able to refer to studies you have gathered during the initial literature review.

There are also some templates online that may be useful to look at when you’re just getting started, and trying to grasp the exact requirements for each part in your research paper:

Research Paper APA Formatting

When you write a research paper for college, you will have to make sure to add relevant citation to back up your major claims. Only by building up on the work of established authors will you be able to reach valuable conclusions that can be taken seriously on a academic context. This process may seem burdensome at first, but it’s one of the essential parts of a research paper.

The essence of a citation is simply to point out where you learned about the concepts and ideas that make up all the parts of a research paper. This is absolutely essential, both to substantiate your points and to allow other researchers to look into those sources in cause they want to learn more about some aspects of your assignment, or dig deeper into specific parts of a research paper.

There are several citation styles in modern use, and APA citation is probably the most common and widespread; you must follow this convention precisely when adding citations to the relevant part of a research paper. Here is how you should format a citation according to the APA style.

In a Nutshell

• There are eight different parts of a research paper that you will have to go through in this specific order.
• Make sure to focus on the different parts of a research paper one at a time, and you’ll find it can actually make the writing process much easier.
• Producing a research paper can be a very daunting task unless you have a solid plan of action; that is exactly why most modern learning institutions now demand students to observe all these parts of a research paper.
• These guidelines are not meant to make student’s lives harder, but actually to help them stay focused and produce articulate and thoughtful research that could make an impact in their fields of study.

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Open Access

Peer-reviewed

Research Article

Pharmacological and behavioral investigation of putative self-medicative plants in Budongo chimpanzee diets

Contributed equally to this work with: Elodie Freymann, Fabien Schultz

Roles Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Visualization, Writing – original draft, Writing – review & editing

* E-mail: [email protected] (EF); [email protected] (FS)

Affiliation Primate Models for Behavioural Evolution Lab, Institute of Human Sciences, Department of Anthropology and Museum Ethnography, University of Oxford, Oxford, United Kingdom

Roles Supervision, Writing – review & editing

Affiliations Primate Models for Behavioural Evolution Lab, Institute of Human Sciences, Department of Anthropology and Museum Ethnography, University of Oxford, Oxford, United Kingdom, Gorongosa National Park, Sofala, Mozambique, Interdisciplinary Centre for Archaeology and the Evolution of Human Behaviour, University of Algarve, Faro, Portugal

Roles Funding acquisition, Supervision, Writing – review & editing

Affiliations Ethnopharmacology & Zoopharmacognosy Research Group, Department of Agriculture and Food Sciences, Neubrandenburg University of Applied Sciences, Neubrandenburg, Germany, ZELT–Center for Nutrition and Food Technology gGmbH

Roles Formal analysis, Writing – original draft, Writing – review & editing

Affiliation Ethnopharmacology & Zoopharmacognosy Research Group, Department of Agriculture and Food Sciences, Neubrandenburg University of Applied Sciences, Neubrandenburg, Germany

Roles Resources, Supervision, Writing – review & editing

Affiliations Wild Minds Lab, School of Psychology and Neuroscience, University of St Andrews, St Andrews, United Kingdom, Budongo Conservation Field Station, Masindi, Uganda

Affiliation Wildlife Research Center, Inuyama Campus, Kyoto University, Inuyama, Japan

Roles Investigation

Affiliation Budongo Conservation Field Station, Masindi, Uganda

Roles Formal analysis

Affiliations Budongo Conservation Field Station, Masindi, Uganda, Czech University of Life Sciences Prague, Prague, Czech Republic

Roles Resources, Writing – review & editing

Affiliations Budongo Conservation Field Station, Masindi, Uganda, Department of Comparative Cognition, Institute of Biology, University of Neuchâtel, Neuchâtel, Switzerland

Roles Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Writing – original draft, Writing – review & editing

Affiliations Ethnopharmacology & Zoopharmacognosy Research Group, Department of Agriculture and Food Sciences, Neubrandenburg University of Applied Sciences, Neubrandenburg, Germany, Pharmacognosy and Phytotherapy, School of Pharmacy, University College of London, London, United Kingdom

• Elodie Freymann, 
• Susana Carvalho, 
• Leif A. Garbe, 
• Dinda Dwi Ghazhelia, 
• Catherine Hobaiter, 
• Michael A. Huffman, 
• Geresomu Muhumuza, 
• Lena Schulz, 
• Daniel Sempebwa, 

• Published: June 20, 2024
• https://doi.org/10.1371/journal.pone.0305219
• Reader Comments

Wild chimpanzees consume a variety of plants to meet their dietary needs and maintain wellbeing. While some plants have obvious value, others are nutritionally poor and/or contain bioactive toxins which make ingestion costly. In some cases, these nutrient-poor resources are speculated to be medicinal, thought to help individuals combat illness. In this study, we observed two habituated chimpanzee communities living in the Budongo Forest, Uganda, and collected 17 botanical samples associated with putative self-medication behaviors (e.g., bark feeding, dead wood eating, and pith-stripping) or events (e.g., when consumer had elevated parasite load, abnormal urinalysis, or injury). In total, we selected plant parts from 13 species (nine trees and four herbaceous plants). Three extracts of different polarities were produced from each sample using n -hexane, ethyl acetate, and methanol/water (9/1, v/v ) and introduced to antibacterial and anti-inflammatory in vitro models. Extracts were evaluated for growth inhibition against a panel of multidrug-resistant clinical isolates of bacteria, including ESKAPE strains and cyclooxygenase-2 (COX-2) inhibition activity. Pharmacological results suggest that Budongo chimpanzees consume several species with potent medicinal properties. In the antibacterial library screen, 45 out of 53 extracts (88%) exhibited ≥40% inhibition at a concentration of 256 μg/mL. Of these active extracts, 41 (91%) showed activity at ≤256μg/mL in subsequent dose-response antibacterial experiments. The strongest antibacterial activity was achieved by the n- hexane extract of Alstonia boonei dead wood against Staphylococcus aureus (IC50: 16 μg/mL; MIC: 32 μg/mL) and Enterococcus faecium (IC50: 16 μg/mL; MIC: >256 μg/mL) and by the methanol-water extract of Khaya anthotheca bark and resin against E . faecium (IC50: 16 μg/mL; MIC: 32 μg/mL) and pathogenic Escherichia coli (IC50: 16 μg/mL; MIC: 256 μg/mL). We observed ingestion of both these species by highly parasitized individuals. K . anthotheca bark and resin were also targeted by individuals with indicators of infection and injuries. All plant species negatively affected growth of E . coli . In the anti-inflammatory COX-2 inhibition library screen, 17 out of 51 tested extracts (33%) showed ≥50% COX-2 inhibition at a concentration of 5 μg/mL. Several extracts also exhibited anti-inflammatory effects in COX-2 dose-response experiments. The K . anthotheca bark and resin methanol-water extract showed the most potent effects (IC50: 0.55 μg/mL), followed by the fern Christella parasitica methanol-water extract (IC50: 0.81 μg/mL). This fern species was consumed by an injured individual, a feeding behavior documented only once before in this population. These results, integrated with associated observations from eight months of behavioral data, provide further evidence for the presence of self-medicative resources in wild chimpanzee diets. This study addresses the challenge of distinguishing preventative medicinal food consumption from therapeutic self-medication by integrating pharmacological, observational, and health monitoring data—an essential interdisciplinary approach for advancing the field of zoopharmacognosy.

Citation: Freymann E, Carvalho S, Garbe LA, Dwi Ghazhelia D, Hobaiter C, Huffman MA, et al. (2024) Pharmacological and behavioral investigation of putative self-medicative plants in Budongo chimpanzee diets. PLoS ONE 19(6): e0305219. https://doi.org/10.1371/journal.pone.0305219

Editor: Armel Jackson Seukep, University of Buea, CAMEROON

Received: January 9, 2024; Accepted: May 25, 2024; Published: June 20, 2024

Copyright: © 2024 Freymann et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript and its Supporting Information files.

Funding: Funding for this project was granted by the the Clarendon Fund at the University of Oxford (to EF), the British Institute of Eastern Africa (to EF), Keble College at the University of Oxford (to EF), Boise Trust Fund (to EF), German Federal Ministry of Education and Research (13FH026IX5, PI: L-AG and Co-I: FS) (to LAG, FS) and Neubrandenburg University of Applied Sciences (grant # 13310510) (to LAG, FS).

Competing interests: The authors have declared that no competing interests exist.

Introduction

‘Medicinal foods’ refer to resources in the diet that have potential curative value due to the presence of plant secondary metabolites (PSMs) [ 1 , 2 ]. PSMs are compounds that usually occur only in special, differentiated cells [ 3 ] and which help plants defend against predators, pathogens, and competitors [ 4 – 7 ]. PSMs can have a range of functions, including the inhibition of microbial, fungal, and competitor growth [ 8 ]. While some PSMs can be toxic at high doses, these compounds can also promote the health of human and non-human consumers [ 8 – 10 ]. Research suggests 15–25% of primate and other mammalian diets consist of medicinal foods [ 9 , 11 ]. These resources likely play a critical role in animal health-maintenance by passively preventing or reducing the impact of parasitic infections or other pathogens [ 9 – 14 ].

While most animals likely consume foods with medicinal properties as part of their normal diets, fewer species have been shown to engage in therapeutic self-medication. Huffman [ 15 ] defines this type of self-medicative behavior as the active extraction and ingestion, by an ill individual, of medicinal resources with little nutritional value. Instead of an individual passively benefiting from a plant’s medicinal properties through normal feeding, this form of self-medication requires basic awareness of the resource’s healing properties. One of the best-studied animals to engage in this form of self-medication is our closest living relative: the chimpanzee.

Wild chimpanzees ( Pan troglodytes ), across at least sixteen field sites [ 15 ] have demonstrated therapeutic self-medication using two well-established self-medicative behaviors: leaf swallowing [ 16 , 17 ] and bitter-pith chewing [ 18 ]. Leaf swallowing, first reported by Wrangham [ 19 , 20 ] and described by Wrangham & Nishida [ 21 ], involves the careful selection and ingestion of whole, hispid leaves. This behavior was later demonstrated to expel internal parasites (i.e. Oesophagostomum sp. and Bertiella studeri ) from the gut [ 16 , 17 , 22 , 23 ]. The functional mechanism responsible for this anthelminthic effect is considered to be primarily “mechanical” [ 9 ] as, rather than a chemical compound, the leaf’s indigestibility, brought about by the trichomes on its surface—stimulates gut motility in the swallower [ 17 , 23 , 24 ].

The second established behavior is bitter-pith chewing, which involves the stripping of outer bark and leaves from the soft new stem growth of the shrub, Vernonia amygdalina , exposing the inner pith. Individuals chew the pith and ingest only the bitter juices while spitting out the fibers [ 18 , 25 ]. Bitter-pith chewing is considered ‘phytochemical’ self-medication [ 9 ], as its anthelminthic effect appears to be the result of bioactive PSMs [ 26 – 29 ]. This behavior’s medicinal effect was associated with a significant drop in the infection intensity of Oesophagostomum stephanostomum nematodes [ 25 ], suggesting that the bitter compounds directly affect the adult worms. This hypothesis was supported by in vivo studies conducted by Jisaka et al. [ 30 ], demonstrating that extracts from the pith permanently paralyzed adult Schistosome parasites. V . amygdalina is also used to aid gastrointestinal discomfort and other signs of parasitosis in humans and livestock, symptoms also displayed by chimpanzees ingesting the plant’s bitter pith [ 9 , 18 , 25 , 31 ]. The bitter piths of other plant species are reported to be chewed by chimpanzees across field sites but detailed studies on their medicinal properties have yet to be conducted [ 9 ].

Beyond these two established behaviors, not much is known about the phytochemical self-medicative repertoires of wild chimpanzees, although some behaviors associated with the ingestion of specific plant parts or processing techniques have been recommended for further investigation [ 9 , 15 , 32 ]. One of these behaviors is bark feeding, which involves the ingestion of living stem bark and/or cambium [ 33 ], and which has been observed in at least eleven established field sites [ 33 – 43 ]. Bark feeding has been suggested as a medicinal behavior in chimpanzees and other primates, used to aid in the chemical control of intestinal nematode infection and to relieve gastrointestinal upset [ 9 ]. Bark is characteristically highly fibrous, heavily lignified, sometimes toxic, relatively indigestible, and nutrient-poor [ 44 ]. However, the contribution of bark in chimpanzee diets and toward general health is still poorly understood [though see: 45 ]. In this study, the bark of eight species ingested by Budongo chimpanzees ( Scutia myrtina , Cynometra alexandri , Alstonia boonei , Ficus exasperata , Ficus variifolia , Syzygium guineense , Desplatsia dewevrei , Khaya anthotheca) was screened for antibiotic and anti-inflammatory properties, to better understand the function of bark feeding behaviors and the role this behavior may play in the health maintenance of chimpanzees. For the species K . anthotheca , we tested a mixture of bark and congealed resin, which Budongo chimpanzees were observed to particularly target throughout the study period.

Another putative self-medicative behavior is dead wood eating [ 9 , 35 ], which involves the consumption of decomposing cambium from dead trees. To date, the majority of studies examining this behavior in apes have focused on exploring potential mineral and nutritional benefits, rather than investigating pharmacological properties [ 46 – 49 ]. Many of these studies suggest that dead wood is exploited by chimpanzees as a source of sodium in environments where this mineral is otherwise scarce [ 48 , 49 ]. Our study evaluates the pharmacology of two species of dead wood ( A . boonei and Cleistopholis patens) consumed by the Sonso community of chimpanzees to determine whether this behavior may have multiple functions or health benefits.

The ingestion of pith material from other species has also been suggested as putatively self-medicative [ 34 , 50 , 51 ]. However, unlike V . amygdalina bitter-pith, some of these plant piths appear bland or tasteless. While Wrangham et al. have previously suggested that pith is likely a high-fiber fallback food [ 52 ], De la Fuente et al. review several pith species targeted by chimpanzees with proposed medicinal properties [ 32 ]. In our study, two species of non-bitter piths ( Marantachloa leucantha and Acanthus polystachyus) , were collected for pharmacological assessment. M . leucantha was observed on several occasions being stripped, masticated, and spat out after the juice was extracted from the pith, whereas A . polystachyus was observed being stripped, masticated, and swallowed. Both of these species are also ingested by chimpanzees in Kibale National Park, Uganda [ 52 ].

Establishing phytochemical self-medicative behaviors in wild animals is difficult and time consuming, as the burden of proof is high, self-medicative events can be rare relative to other behaviors, and methods often require multidisciplinary expertise and collaboration [ 9 ]. Past studies have utilized ethnopharmacological methods to determine specific medicinal properties of foods consumed by primates [ 11 ], greatly advancing our understanding of the relationship between primate diets and health. However, a key challenge for establishing novel self-medicative behaviors is differentiating between medicinal food consumption and therapeutic self-medication. While pharmacological data interpreted on its own is crucial for establishing the presence of medicinal resources in chimpanzee diets, the integration of observational and health monitoring data is needed to parse therapeutic self-medicative behaviors from normal feeding behaviors with inadvertent health benefits. Furthermore, the importance of collecting in situ samples from the locations where putative self-medicative behaviors are observed is paramount, as ecological, climatic, and anthropogenic variables can cause variation in the bioactivity of plants across habitats [ 53 ].

In total, we investigated the bioactivity of 51 plant extracts produced from 17 part-specific samples (across 13 species), collected in the Budongo Forest. Each extract was tested for inhibition of bacterial growth as well as anti-inflammatory COX-2 inhibition activity. Due to limitations in scope, funding, and the unavailability of anthelminthic assays for wild animal parasites, none were not conducted in this study, restricting specific identification of parasiticidal behaviors. Assay results are reported and contextualized in this study with direct behavioral evidence and health monitoring data.

Materials and method

Study site and subjects.

Behavioral data, health monitoring metrics, and botanical samples were collected from the Budongo Central Forest Reserve in Uganda (1°35′– 1°55′ N, 31°18′–31°42′ E). An overview of methodological workflow can be found in S2 Fig . The Budongo Conservation Field Station (BCFS) site, founded in 1990, is composed of continuous, semi-deciduous forest and contains two habituated Eastern chimpanzee ( Pan troglodytes schweinfurthii ) communities [ 54 ]. The Sonso community has been studied continuously since 1992, and the ages, social relationships, demographics, and diet of its members are well documented [ 55 , 56 ]. The Sonso population was ~68 individuals at the time of data collection, and the home range covered an area of ~5.33 km 2 [ 57 ]. Waibira, a larger group of at least 105 individuals, was more recently habituated, with consistent data collection beginning in 2011. The Waibira maximum home range area was ~10.28 km 2 [ 57 ].

Behavioral data collection

All samples were collected in the Budongo Forest within the Sonso home range, based on behavioral observations from the study period and supporting evidence from the site’s long-term data of their use. Behavioral and health data were collected from two neighboring chimpanzee communities, each for one four-month field season (Sonso: June-October 2021, Waibira: June-October 2022). Data collected between June-September 2021 informed subsequent plant sample collection for pharmacological analysis, which occurred in early September 2021. Behavioral data collected after sample collection provided additional behavioral context for ingestion of these species. Behavioral data were collected between 07:00 and 16:30 in Sonso and between 06:30 and 17:00 in Waibira using day-long focal animal follows sensu Altman et al. [ 58 ]. This data was recorded using Animal Observer (AO) on iPad and ad libitum feeding events were recorded for any unusual feeding behaviors, including but not limited to bark ingestion, dead wood eating, pith stripping, and geophagy. All feeding events were filmed on a Sony Handycam CX250. We prioritized focal follows on individuals with wounds, high or diverse parasite loads identified through on-going monitoring, or known ailments. However, consecutive day follows of priority individuals were not always possible—or were avoided when they might contribute to increased stress in particularly vulnerable individuals. Throughout the study, using this protocol, 27 Sonso individuals (♂:11; ♀:16) and 24 Waibira individuals (♂:14; ♀:10) were observed. Authors collecting behavioral data were blind to pharmacological results during both study periods.

Health monitoring

Individual health data were recorded in both communities, including opportunistic macroscopic and microscopic fecal analysis and urinalysis testing. While anthelminthic assays were not run in this study, parasite load was opportunistically assessed to provide additional health context for each observation. As the presence of certain helminths may impair a host’s immunological response to bacterial, viral, and protozoal pathogens [ 59 ], parasite load can provide a proxy measurement for overall health. Similarly, a reduced immune system and increased stress caused by co-infections could render a host more susceptible to virulent endoparasites [ 60 , 61 ]. When helminths and/or proglottids were found in samples, they were collected and preserved in ethanol for later identification. To quantify parasite loads, fecal samples were analyzed using the McMaster Method [ 9 , 25 , 62 ]. Urinalysis samples were taken opportunistically using multi-reagent Urine Dipstick Test 9-RC for Urotron RL9 to assess the health and physiological status of group members following methods established by Kaur & Huffman [ 63 ]. Urinalysis metrics considered in this study included: leukocytes (LEU) associated with pyuria caused by UTI, balanitis, urethritis, tuberculosis, bladder tumors, viral infections, nephrolithiasis, foreign bodies, exercise, glomerulonephritis, and corticosteroid and cyclophosphamide use; blood (BLO) associated with peroxidase activity of erythrocytes, and UTIs; and ketones (KET) associated with pregnancy, carbohydrate-free diets, starvation, and diabetes [ 64 ]. Test results were interpreted in situ using a colorimetric scale. We considered a result ‘abnormal’ if the colorimetric scale indicated a positive result when the expected result was negative or if the result was outside the specified test parameters according to the manufacturer.

Plant sample selection for bioactivity testing

Plants were selected for pharmacological testing after three months of data collection in the Sonso community. We selected 10 samples (from 9 species) based on direct observations during this period. These observations included individuals targeting plant parts associated with putative self-medicative behaviors (i.e., bark feeding, dead wood eating, pith-stripping) or sick/wounded individuals seeking out unusually consumed resources. We then selected an additional five species, the ingestion of which had not been directly observed, for testing based on their historical inclusion in Sonso chimpanzees’ bark feeding repertoire. GM, who has worked at the field station for over thirty-years, has previously observed bark feeding on each of these selected species. These historic observations enabled collection of bark samples from specific trees known to have been previously stripped. In two cases, leaf samples were collected from tree species that were also selected for bark samples ( S . guineense and F . exasperata) . While neither Sonso nor Waibira chimpanzees have been observed ingesting the leaves of S . guineense , a sample was collected to enable comparison of bioactivity across plant parts. F . exasperata leaves are consumed in both communities; however, we found no behavioral evidence for use in unusual contexts. In some cases, direct observation of an event involving one of the collected species occurred after botanical collection was complete. These post hoc behavioral observations are reported in this paper, although they did not impact sample selection.

Collection of sample material

Plants were collected from the Sonso community home range following best practice procedures [ 65 ], using sustainable harvesting methods [ 66 ]. See S1 File for more information. Voucher accession numbers are reported in Table 3 . Digital images of voucher specimens can be found in S3 Fig . The currently recognized scientific names of each species were confirmed on https://mpns.science.kew.org/ . Plant family assignments were done in accordance with The Angiosperm Phylogeny Group IV guidance [ 67 ].

Ethnobotanical literature review

We conducted a post-hoc ethnomedicinal review of all species collected for this study using Google Scholar, PROTA, and Kokwaro’s ethnomedicinal pharmacopeia [ 68 ]. To search databases, we used scientific names and synonyms for each plant as keywords [ 65 ].

Plant processing and extractions

At Neubrandenburg University of Applied Sciences, samples were ground using a food processor. Extractions were produced using two solvents and a solvent mixture ( n -hexane, ethyl acetate, and methanol/water ( v/v 9/1)), allowing for the selective isolation of components with varying solubilities and polarities. Methanol-water, the solvent with the highest polarity, generally extracts primary plant metabolites (e.g., polar compounds such as proteins, amino acids, and carbohydrates). Nonpolar solvents like n- hexane extract nonpolar compounds like lipids, making n-hexane a preferred solvent for oil or wax extraction. Extractions with each solvent were achieved through double maceration of new material (non-successively). Extraction suspensions were placed on a shaker at 80 rpm at room temperature for minimum 72h, followed by vacuum filtration. Processes were repeated with the leached material. Filtrates were then combined and dried using a vacuum evaporator, labeled, and stored at -20°C until needed for assays.

Sample solution preparation

To create sample solutions, each crude extract was dissolved in DMSO (Carl Roth) at a concentration of 10 mg/mL. To ensure a homogenous solution, samples were mixed with a vortex mixer and, if necessary, treated with sonication at room temperature or up to 55°C for samples with low solubility. Each extract solution was then tested for inhibition of bacterial growth as well as anti-inflammatory COX-2 inhibition activity. Solutions were stored at -20°C when not in use.

Antibacterial susceptibility tests

A. bacterial strains..

For antibacterial assays, eleven multidrug-resistant clinical isolate strains from nine species were used. This process increased the study’s applicability for early-stage drug discovery, specifically relevant to the threat of antimicrobial resistance (AMR). Seven of these strains (from six species) are classified as ESKAPE pathogens, including Enterococcus faecium (DSM 13590), Staphylococcus aureus (DSM 1104; DSM 18827), Klebsiella pneumoniae (DSM 16609), Acinetobacter baumannii (DSM 102929), Pseudomonas aeruginosa (DSM 1117), and Enterobacter cloacae (DSM 30054), meaning they are highly virulent and resistant to antibiotics [ 69 ]. A strain of the foodborne pathogen Escherichia coli (DSM 498) with AMR as well as a non-resistant E . coli strain (DSM 1576) were also included in the study. Although not an ESKAPE pathogen, E . coli is widely known for causing bacterial diarrhea and AMR strains are a major cause of urinary tract infections [ 70 , 71 ]. Strains of Stenotrophomonas maltophilia (DSM 50170) and Salmonella enterica subsp. enterica (DSM 11320) were also tested. More information on specific clinical isolates/strains, their individual resistance profiles, and antibiotics used can be found in the S5 & S6 Tables in S2 File . Clinical and Laboratory Standards Institute (CLSI) guidelines for broth microdilution testing (M100-S23) were followed [ 72 ].

b. Growth inhibition screening and dose-response study.

The broth dilution in vitro methods for bacterial susceptibility assessment have previously been described by Schultz et al. [ 69 ]. The standardized bacterial working cultures were pipetted into sterile 96-well microtiter plates (Greiner Bio-One International, CELLSTAR 655185). Extracts and antibiotic (64–1 μg/mL), vehicle and sterility controls, were then added into respective wells. Initial optical density measurement (600 nm) was performed, accounting for absorbance of extracts. Plates were incubated at 37°C for 18 h, except for A . baumannii which was incubated for 22h in accordance with strain characteristics ( S5 Table in S2 File ) . After incubation, a final optical density reading (600 nm) was conducted. Percent inhibition values were calculated and the IC 50 and MIC values were determined [ 69 , 73 ]. The IC 50 value is defined as the lowest concentration at which an extract showed ≥ 50% inhibition, and the MIC is the lowest concentration at which an extract displayed ≥ 90% inhibition. A total of 51 samples underwent single-dose pre-screening for growth inhibition (in triplicate) at the concentration of 256 μg/mL on eleven pathogens. Samples showing ≥40% growth inhibition were further tested in a dose-response study with two-fold serial dilution at descending concentrations from 256 to 4 μg/mL. The dose-response experiments were done as biological replicates on separate days in triplicate (technical replicates) to validate reproducibility. Positive controls (antibiotics) and negative controls (vehicle control and sterile media control) were always included. Further details on bacteria standardization can be found in S1 File . Information on plate setup for bacterial library screens and dose-response assays can be found in S4 Fig .

COX-2 inhibition assay

Anti-inflammatory assays were assessed using an in vitro COX inhibitor screening assay kit (Cayman Item No: 701080), with modifications previously described in Schultz et al. [ 74 ]. All extracts were first screened in duplicate for inhibition against human recombinant COX-2 at an initial concentration of 50 μg/mL. For extracts exhibiting at least 50% inhibition, the concentration was then lowered to 10 μg/mL, 5 μg/mL, and 2.5 μg/mL. The most active extracts were taken to dose-response experiments for determination of IC 50 values ( Table 5 ). The assay was done in two steps: 1) the COX reaction step in which the prostaglandin H 2 (PG) was produced (which was further reduced to the more stable prostaglandin F 2α by addition of stannous chloride), and 2) an acetyl choline esterase competitive ELISA step to quantify the produced prostaglandin and calculate a potential enzyme inhibition caused by the extracts. The pure compound and selective COX-2 inhibitor DuP-769 was included as a positive control. DMSO was included as the vehicle control for determining 100% enzyme activity. Information on ELISA plate setup for anti-inflammation assays can be found in S5 Fig .

Ethics statements

Behavioral data used in this study were collected with the approval of the Uganda Wildlife Authority (permit #: COD/96/05) and the Uganda National Council for Science and Technology (permit #: NS257ES). Exportation of samples for pharmacological testing were conducted under UNCST permit #: NS104ES. Behavioral data collection adhered to International Primatological Society’s Code of Best Practice for Field Primatology [ 75 ]. No exported samples were listed under CITES. Plant samples were exported in collaboration with Makerere University (permit #: UQIS00005033/93/PC), issued by the Ugandan government, and transported to Neubrandenburg University of Applied Sciences in accordance with the Nagoya Protocol. A CUREC was approved by the University of Oxford (Ref No.: SAME_C1A_22_080). The authors report no conflict of interest.

Behavioral observations

Several unusual feeding events and putative self-medicative behaviors were recorded over 116 total field days. Table 1 reports all species collected for pharmacological testing and provides behavioral justifications for collection. Images from some of these events can be found in S1 Fig .

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https://doi.org/10.1371/journal.pone.0305219.t001

Individuals with injuries were directly observed ingesting K . anthotheca bark and resin, W . elongata young leaves, C . alexandri bark, and C . parasitica ferns. Individuals exhibiting respiratory symptoms were observed ingesting C . alexandri bark and K . anthotheca bark and resin. Individuals with abnormal urinalysis results (e.g., positive for leukocytes, elevated ketones, and presence of blood) were observed feeding on C . patens dead wood, K . anthotheca bark and resin, and M . leucantha pith. Individuals with recent cases of diarrhea were observed consuming A . boonei and C . patens dead wood, K . anthotheca bark and resin, and W . elongata leaves. Parasitological analyses further suggest individuals with varying degrees of endoparasite infections consumed S . myrtina and C . alexanderi bark, A . boonei and C . patens dead wood, K . anthotheca bark and resin, W . elongata leaves, as well as A . polystachyus and M . leucantha pith. On a day when two individuals were observed leaf swallowing, a scientifically established self-medicative behavior, one was observed consuming K . anthotheca bark and resin, while the other was observed stripping A . polystachyus pith prior to the event. Ingestion of F . variifolia , D . dewevrei , and S . guineense bark were never directly observed during the study period. Examples of bark feeding, dead wood eating, and pith-stripping marks are shown in Fig 1 .

[ a ]: Evidence of F. exasperata bark feeding [ b ] Evidence of C. patens dead wood eating [ c ] Evidence M. leucantha pith-stripping and wadging.

https://doi.org/10.1371/journal.pone.0305219.g001

Ethnobotanical review

Based on our analysis of ethnomedicinal literature spanning various African regions from 1976 to 2022, 11 out of the 13 species tested also had documented ethnomedicinal uses ( Table 2 ).

https://doi.org/10.1371/journal.pone.0305219.t002

Production of extracts and sample information

Taxonomic information and extraction details for the 13 plant species studied, including the plant family, local name (when available), plant part used, solvent for extraction, yield of extraction, extract identification numbers (extract IDs), herbarium accession numbers, and collection location are summarized in Table 3 . Overall, the highest extraction yields were obtained with methanol-water (9/1) as a solvent. The yields from methanol-water extractions for C . parasitica , F . exasperata leaves, and S . guineense stem bark were higher than the other extractions from these samples. The plant samples which had higher yield values with n -hexane, such as the leaves of W . elongata and bark extract of A . boonei , likely have a higher content of lipids (i.e., fatty molecules).

https://doi.org/10.1371/journal.pone.0305219.t003

Library screening against multidrug-resistant human and food bacterial pathogens

Initial screening of extracts involved checking for growth inhibition against each bacterium at a concentration of 256 μg/mL. In total, 45 of the 51 plant extracts (88%) showed activity ≥40% inhibition against at least one of the 11 strains and were thus considered active and brought to dose-response experiments to determine their IC 50 value and MIC. Results from the library screening are reported in S1 Table in S2 File . As all tested plant species in the library screen had at least one extract that was active ( in vitro ) against at least one bacterial strain, no entire species was eliminated for further experimentation. However, as no extracts (at any concentration) inhibited the growth of K . pneumoniae , no further tests were conducted on this bacterium. The extract active against the most bacterial strains (n = 11) was the methanol-water extract of S . guineense stem bark (mwE098a, active against eight strains), followed by the methanol-water S . guineense leaves (mwE098b), the ethyl acetate P . patens dead wood, and the n -hexane A . boonei dead wood (hE092b) extracts, which were each active against seven, seven, and six strains, respectively. The only extract that demonstrated significant inhibition against P . aeruginosa at the highest test concentration was the methanol-water extract from S . guineense bark (mwE098a). This was also the only extract to display significant inhibition at 256 μg/mL against E . cloacae . Of all bacteria in this study, the two strains of E . coli (DSM 498 and DSM 15076) were the most susceptible, with at least one extract from all plant species inhibiting their growth. The E . coli strain with nine known antibiotic resistances (DSM 15076) surprisingly showed growth inhibition in 80% of tested extracts.

Dose-response antibacterial experiments

In dose-response assays, 41 out of the 45 tested extracts (91%) showed activity at ≤256μg/mL, though not all extracts reached MIC values (see Table 4 ). The results, along with standard deviations, are reported in S2 Table in S2 File , while S3 Table in S2 File provides a summary of the number of strains each extract was active against. The strongest in vitro growth inhibition was reported for the methanol-water extract of K . anthotheca bark and resin (mwE088) against Gram-positive E . faecium and the n- hexane extract of A . boonei dead wood (hE092b) against Gram-positive S . aureus (DSM 1104). Both extracts had low IC 50 values of 16 μg/mL (showing strong inhibition), with MIC values of 32 μg/mL against respective strains. E . faecium showed the most general susceptibility to K . anthotheca , with all extracts of this species achieving MIC values (mwE088: 32 μg/mL, eE088: 64 μg/mL, hE088: 128 μg/mL). The ethyl acetate extract of A . boonei dead wood (eE092b) also strongly inhibited the growth of E . faecium (IC 50 : 16 μg/mL; MIC: 64 μg/mL), as did the n- hexane extract of A . boonei dead wood, producing an IC 50 value of 16 μg/mL but failing to reach a MIC value. S . aureus (DSM 1104) was also highly susceptible to the ethyl acetate extracts of A . boonei dead wood (IC 50 : 32 μg/mL; MIC: 128 μg/mL).

https://doi.org/10.1371/journal.pone.0305219.t004

Only one extract, the methanol-water extract of S . guineense bark (mwE098a), was active against the gram-negative P . aeruginosa . This extract exhibited moderate growth inhibition (IC 50 : 64 μg/mL) with no MIC value reached. Despite E . coli (DSM 498) being highly susceptible on the library screen, only two extracts, the methanol-water extract of A . boonei dead wood (mwE092b; IC 50 : 256 μg/mL) and the methanol-water extract of S . guineense leaves (mwE098b; IC 50 : 128 μg/mL), reached IC 50 values at the concentration range tested, with no MICs reached. Interestingly, the strain of E . coli with nine known resistances (DSM 1576) was more susceptible, with 89% (N = 40) of extracts achieving IC 50 values ≤ 256 μg/mL. The most active extract against this strain was the methanol-water extract of K . anthotheca (mwE088; IC 50 : 16 μg/mL; MIC: 256 μg/mL). S . guineense exhibited the highest overall inhibition of S . maltophilia , with all extracts except hE098a displaying IC 50 values of ≤ 256 μg/mL against the bacterium. At the concentration range tested, no extracts yielded MIC values for S . aureus (DSM 18827), A . baumannii , E . cloacae , P . aeruginosa or E . coli (DSM 498).

Anti-inflammatory COX-2 inhibition library screen

Results from the in vitro COX-2 inhibition library screen at descending concentrations are reported in S4 Table in S2 File . At the initial concentration of 50 μg/mL, 43 out of 51 extracts (84%) exhibited an enzyme inhibition of at least 50%, displaying anti-inflammatory activity. This included at least one extract of every plant species. In the next stage of screening, at 10 μg/mL, 18 samples were eliminated. During the final step, at 5 μg/mL, five more were eliminated. The remaining 17 extracts from 10 plant species which displayed inhibition ≥50% at 5 μg/mL, were then introduced to dose-response experiments. The ethyl acetate S . myrtina bark extract (eE089b) was taken to the COX-2 dose-response despite not showing inhibition past 50 μg/mL, as it almost reached the selection limit during analysis and had a relatively high standard deviation. No extracts from W . elongata , C . patens or D . dewevrei showed COX-2 inhibition at 5 μg/mL and thus were excluded from further testing.

COX-2 inhibition dose-response experiments

The most active COX-2 inhibitors were extracts from K . anthotheca (mwE088; hE088; eE088), C . parasitica (mwE087; hE087), F . exasperata (hE093a; eE093a), S . myrtina (hE089a; eE089b), F . variifolia (eE097; hE097), A . polystachyus (hE099; eE099), M . leucantha (hE094), S . guineense (hE098a), A . boonei (hE092b), and C . alexandri (hE096). Results are reported in Table 5 . The strongest COX-2 inhibitor was the K . anthotheca methanol-water bark and resin extract (mwE088) (IC 50 of 0.55 μg/mL), followed by the C . parasitica methanol-water fern extract (mwE087) (IC 50 of 0.81 μg/mL). In contrast, all extracts of the species W . elongata , C . patens , and D . dewevrei failed to show ≥50% inhibition, mostly at the second screening concentration (10 μg/mL). W . elongata extracts notably showed low activity in both antibacterial and COX-2 inhibition assays.

https://doi.org/10.1371/journal.pone.0305219.t005

Plant species with strong pharmacological activity

This study provides the first pharmacological and behavioral evidence of its kind, based on in situ sampling, for the medicinal benefits of bark feeding, dead wood eating, and non-bitter pith stripping behaviors in Budongo chimpanzees. In the following sub-sections, we describe and discuss specific results from five of the tested plant species in further detail. For scope, we selected the two species with the strongest antibacterial properties ( K . anthotheca and A . boonei ) to profile, both of which were the only species to reach 40% inhibition at 16 μg/mL. We also selected C . parasitica to discuss as this species, along with K . anthotheca , exhibited the strongest anti-inflammatory properties. We then discuss results from our S . guineense samples, as this species was effective against the most bacterial strains in our antibacterial assays. Lastly, we selected S . myrtina , as we have behavioral evidence and health data that anecdotally support the use of this species for therapeutic self-medication by Budongo chimpanzees.

Alstonia boonei . Numerous in vitro and in vivo studies, reviewed by Adotey [ 76 ], have reported pharmacological activity in A . boonei bark. However, none of these studies investigated dead wood samples of A . boonei . Consistent with these findings, we found high levels of antibacterial and anti-inflammatory activity in the extracts of this species. Interestingly, extracts from A . boonei dead wood generally exhibited higher activity than living bark. This difference could be due either to a change in active ingredient composition, or possible fungal growth following the tree’s death. While the A . boonei dead wood n -hexane extract (hE092b) exhibited strong growth inhibition against S . aureus (DSM 1104; DSM 18827) and E . faecium at low concentrations in the dose-response assays, the n -hexane bark extract (hE092a) showed no activity <256 μg/mL. Similarly, the ethyl acetate extract of dead wood (eE092b) also strongly inhibited S . aureus (DSM 1104) (IC 50 : 16 μg/mL; MIC: 128 μg/mL) and E . faecium (IC 50 : 16 μg/mL; MIC: 64 μg/mL), while the ethyl acetate bark extract of this species did not even exhibit enough inhibition in the antibacterial library screen to be taken to dose-response assays. However, the methanol-water extract of A . boonei bark (mwE092a) did show activity against E . coli (DSM 498) (IC 50 : 128 μg/mL), as did the methanol-water dead wood extract (mwE092a) (IC 50 : 128 μg/mL), with no MIC values reached in either case. Overall, extracts from A . boonei displayed more potent activity in Gram-positive bacteria, although this effect is more apparent in dead wood than stem bark. In the COX-2 inhibition assays, the n -hexane extract of A . boonei dead wood also showed strong anti-inflammatory inhibition, while the n -hexane extract of the bark only exhibited weak inhibition (at the highest test concentration of 50 μg/mL).

A . boonei is a known medicinal plant across East Africa, commonly used for a variety of reproductive, bacterial, and gastro-intestinal issues, as well as for snake bites, asthma, and dizziness [ 68 , 76 , 77 ]. The bark and latex are intensely bitter, a reliable signal of the presence of bioactive secondary compounds and toxicity [ 94 – 96 ]. Budongo chimpanzees in both communities have been reported to consume both bark and dead wood of A . boonei , often travelling long distances to access these trees and only consuming small amounts of bark per feeding bout [ 45 ]. In an observation reported in this study (see Table 1 : A . boonei , Case 1 ), three males ingested A . boonei dead wood while outside the community’s core area for 1-minute. Two days before the event, one of the individuals had been observed with diarrhea, while also shedding visible tapeworm proglottids ( Bertiella sp.). This sample also contained unidentified protozoa, and Taenia sp. eggs. Pebsworth et al. [ 34 ] also reported an event in which four adult males, all with diverse parasite loads, traveled to a large A . boonei tree and ingested bark.

In the long-term site data, A . boonei bark ingestion was only documented 17 times between 2008–2021 [ 45 ], although this behavior was not systematically reported. In addition, the direct observation of only one A . boonei dead wood eating event, and no A . boonei bark ingesting events over the two four-month periods of observation in this study, suggest that consumption of this species is relatively rare across both communities. While specific pathogenic catalysts for selection of this species remain unknown, based on pharmacological, ethnobotanical, and behavioral data, we propose that A . boonei may be a therapeutic self-medicative resource for Budongo chimpanzees. The relatively strong inhibitory activity of this species against S . aureus , a bacteria associated with causing contamination on the skin leading to chronic wounds [ 97 ], as well as its anti-inflammatory properties, suggests that A . boonei ingestion may have beneficial effects in wound care contexts.

Khaya anthotheca . Previous studies have demonstrated that K . anthotheca bark contains biologically active compounds like gedunins, mexicanolide, phragmalin, and andirobins [ 98 ]. One limonoid identified in the species, anthothecol, has anti-cancer properties [ 99 ]. A study by Obbo et al. [ 100 ] on K . anthotheca bark collected in the Budongo Forest, found strong antiprotozoal activity against Plasmodium falciparum (IC 50 0.96 μg/mL) and Trypanosoma brucei rhodesiense (IC 50 5.72 μg/mL). A related species, K . senegalensis , has been shown to cause cell lysis in some gram-negative bacteria, including Salmonella Typhimurium , Escherichia coli , Shigella sp. and Salmonella sp., by targeting cytoplasmic membranes [ 101 ].

In our antibacterial library screen, of all extracts tested, only the methanol-water extract inhibited growth of A . baumannii (although no IC 50 values were reached in dose-response). The methanol-water extract also inhibited the growth of E . coli (DSM 498) in the library screen, as did the ethyl acetate (eE088) extract, though again no IC 50 values were reached. In our antibacterial dose-response assays, all extracts of K . anthotheca stem bark and resin exhibited strong inhibition against the Gram-positive E . faecium . The most active extract against this strain, which was also the strongest antibacterial result reported in this study, was methanol-water (mwE088) (IC 50 : 16 μg/mL; MIC: 32 μg/mL). All extracts of this species were also found to inhibit E . coli (DSM 1576) in the dose-response experiments, with the methanol-water extract once again also showing the strongest inhibition (IC 50 : 16 μg/mL; MIC: 256 μg/mL). This extract also inhibited the growth of S . maltophilia (IC 50 : 64 μg/mL) in the library screen. Only weak inhibition was found against the food pathogen S . enterica ( n -hexane extract, IC 50 : 256 μg/mL).

K . anthotheca exhibited potent anti-inflammatory activity. Of all extracts tested, the methanol-water K . anthotheca extract (mwE088) displayed the strongest COX-2 inhibition activity (IC 50 : 0.55 μg/mL). Past phytochemical studies on methanol and ethanol-water stem bark extracts from the related species, K . senegalensis , revealed many phenolic compounds, including flavonoids and tannins e.g., [ 101 , 102 ]. Flavonoids act on the inflammatory response, and may block molecules like COXs, cytokines, nuclear factor-кB and matrix metalloproteinases [ 103 ]. Some tannins have also been proven to have strong free radical-scavenging and antioxidant activities [ 104 ]. These compounds are antagonists of particular hormone receptors or inhibitors of particular enzymes such as COX enzymes [ 103 ]. If Khaya species are phytochemically similar, this could help explain K . anthotheca ’s strong COX-2 inhibitory activity.

Across Africa, K . anthotheca is traditionally used for ailments including allergies, fever, headaches, jaundice, bacterial infections, and as a disinfectant for bleeding wounds [ 105 – 107 ]. Our behavioral observations suggest that this species is also a common resource for Sonso chimpanzees, with a total of 65 feeding events recorded throughout the first field season. Of these events, several involved individuals with imbalanced health states (see Table 1 : K . anthotheca ) . On at least three independent occasions, K . anthotheca bark and resin were consumed by wounded individuals. Two adult females on different days tested positive for leukocytes on urinalysis tests within hours of ingesting K . anthotheca , suggesting the presence of infection. One of these individuals was also experiencing severe diarrhea the day prior, the other was found to have trace levels of blood in her urine. A juvenile female with a persistent cough was also observed consuming K . anthotheca bark. On several occasions individuals with high parasite loads or diverse species infection were observed targeting this resource while shedding tapeworm proglottids ( Bertiella sp.). An elderly female was also observed eating bark and resin a few hours prior to leaf-swallowing, a well-established self-medicative behavior known to rid the gut of endoparasites [ 9 , 23 ]. The frequency of K . anthotheca ingestion in the Sonso diet during this period, suggests that individuals have consistent exposure to the antibacterial and anti-inflammatory compounds present in this species. Whether this is a case of passive prevention through intake of a medicinal food, or therapeutic self-medication for a common and wide-spread condition will need further investigation. If used therapeutically, our results suggest this species could be used for treating wounds, bacterial or infections, and/or reducing internal parasite loads.

Christella parasitica.

Extracts of C . parasitica produced notably high anti-inflammatory activity in COX-2 testing, with the methanol-water extract (mwE087) achieving an IC 50 value of 0.81 μg/mL. This same extract, however, exhibited the lowest general activity in the antibacterial library screen. The only antibacterial activity from this species was on E . coli (DSM 498) by the ethyl acetate and n- hexane extracts (eE087; hE087), and on E . coli (DSM 1576) by the n-hexane extract (hE087). The n -hexane extract reached an IC 50 of 128 μg/mL in dose-response assays with no MIC value. Prior to this study, there had been limited pharmacological testing on C . parasitica (though see [ 108 ]), so comparison across studies is not possible.

When we considered the associated behavioral observation involving C . parasitica , we found a notable relevance to our pharmacological results (see Table 1 : C . parasitica , Case 1 ). This observation involved a wounded Sonso adult male (PS) travelling outside of his core area with a large group. It was unclear if this was an inter-community patrol. PS had been observed earlier in the day with a severe hand injury which impacted his mobility, though no open wound was observed. PS separated himself from the group and moved a few meters to a patch of ferns where he began consuming the leaflets. The bout lasted approximately 3-minutes. No other group members were observed feeding on this species, and this was only the second case of fern ingestion reported in Budongo in over 30-years of observations (unpublished site data). Health states of individuals from the past event were unfortunately not recorded. Whether or not C . parasitica ’s highly anti-inflammatory properties were the principal motivator for the selection of this species remains unknown, however, regardless of intention, this plant may have benefitted PS by reducing pain and swelling in his injured hand.

Syzygium guineense.

S . guineense bark and leaves have both previously been found to exhibit a range of pharmacological activity, reviewed by Uddin et al. [ 109 ]. The antioxidant, analgesic, and anti-inflammatory activities of this plant have been attributed to flavonoids, tannins, saponins, carbohydrates, alkaloids, and cardiac glycosides in the extracts [ 109 – 112 ]. In our assays, S . guineense bark exhibited high antibacterial growth inhibition effects in vitro . The methanol-water bark extract (mwE098a) showed some level of inhibition against all bacteria tested in the dose-response assays, except for E . faecium and S . enterica . This was also the only extract, out of all tested, to inhibit growth of P . aeruginosa (IC 50 : 64 μg/mL; MIC: >256 μg/mL) a pathogen known to cause infections in the blood, lungs, and other body parts after surgeries [ 113 ], and was one of two extracts to reach a MIC value against S . maltophilia (IC 50 : 32μg/mL; MIC: 256 μg/mL). The other extract to reach a MIC value was the ethyl acetate S . guineense bark extract (eE098a; IC 50 : 64 μg/mL; MIC: 256 μg/mL). All bark and leaf extracts showed strong inhibition against E . coli (DSM 1576) in the dose-response assays, with the strongest results coming from the methanol-water extracts (mwE098a and mwE098b). All bark and leaf extracts of this species, except for the n -hexane bark extract (hE098a), inhibited E . cloacae , and were the only extracts in the study to do so. E . cloacae , while part of normal intestinal flora, can cause UTI’s and respiratory infections in humans [ 114 ]. S . guineense extracts were also the only extracts to inhibit A . baumannii at a concentration <256 μg/mL, with the methanol-water bark extract showing the strongest inhibition. A . baumannii can cause infections in wounds, blood, urinary tracts, and lungs [ 115 ]. The efficacy of methanolic extracts from this species suggests that the active compounds are polar molecules. In the anti-inflammatory COX-2 inhibition dose-response assays, only the n -hexane bark extract displayed strong inhibitory effects (IC 50 : 2.42 μg/mL), while the other extracts failed to exhibit significant activity during the pre-screening or ≥ 50% inhibition at 10 μg/mL. The COX-2 inhibition assays showed no inflammatory inhibition amongst leaf extracts at tested concentrations.

S . guineense can be found throughout Sub-Saharan Africa and is a common traditional medicine, for malaria [ 116 ]. The bark is also used for stomach aches, diarrhea, internal parasites, and infertility [ 68 , 109 ]. Ingestion of S . guineense bark is rare in Budongo, with no direct observations in either community throughout the study period, and only six total cases between 2008–2021 documented in the site’s long-term data. No observations of leaf ingestion of this species have ever been reported. The infrequent ingestion of S . guineense bark implies a more targeted use, making it unlikely to be a medicinal food. Instead, our pharmacological findings make this resource a strong candidate as a putative, therapeutic self-medicative resource. Unfortunately, as there is currently no health data associated with individuals who have recently consumed S . guineense bark, we do not yet know which properties chimpanzees may be targeting. However, based on pharmacological results, we recommend further investigation into this species as a curative agent for respiratory-related infections.

Scutia myrtina.

Kritheka et al. [ 117 ] in their study on the bioactivity of S . myrtina , found in vivo evidence that this species possesses dose-dependent anti-inflammatory, antimicrobial, and antifungal properties. Across our antibacterial assays, the bark sample of this species collected from the stem inhibited E . faecium (eE089a) and E . coli DSM 1576 (eE089a; mwE089a) in dose-response tests at concentrations ≤256 μg/mL. The refuse sample, collected from the ground below the plant’s stem, inhibited A . baumannii (hE089b), E . faecium (eE089b), and E . coli DSM 1576 (mwE089b; eE089b; hE089b) in dose-response tests below the specified concentration. Interestingly, the refuse sample inhibited more bacteria species overall than the fresh bark. The most potent antibacterial growth inhibition effects came from the ethyl acetate bark sample against E . faecium (eE089a; IC 50 : 64 μg/mL), though no MIC value was reached. In the COX-2 inhibition assays, the n- hexane bark extract had the fifth strongest inhibitory effect in vitro (hE089a; IC 50 : 1.19 μg/mL) out of all samples, while the ethyl acetate refuse bark sample was less potent, though still moderately active (E089b; IC 50 : 7.49 μg/mL).

As far as the authors know, this is the first published report presenting both behavioral and pharmacological evidence for S . myrtina bark as a putative medicinal resource amongst free-ranging chimpanzees (though see [ 118 ] for evidence based on food-combinations). Our behavioral observations indicate that an individual with a diverse and intense parasite infection deliberately sought out the bark of this species. The Budongo chimpanzees may, therefore, utilize S . myrtina as an anthelminthic. Across traditional accounts from multiple regions, S . myrtina is commonly used by people as an anthelminthic to treat intestinal worms [ 68 ], while aerial parts are also used to treat various bacterial infections. As we were not able to conduct urinalysis on the consumer during or after this event, we cannot determine whether the individual also harbored a bacterial infection at the time of ingestion. However, this possibility cannot be ruled out. Based on these findings, we propose S . myrtina be added to the list of putative chimpanzee self-medication behaviors as a treatment for internal parasites, and we encourage further exploration into the other specific chimpanzee health conditions that this species may help ameliorate.

Assessment of putative self-medicative behaviors

We synthesized pharmacological and behavioral evidence to assess therapeutic use of species associated with bark feeding, dead wood eating, and pith stripping behaviors. A summary of the antibacterial and anti-inflammatory results for each species is reported in S3 Table in S2 File . Overall, stem bark and dead wood samples were notable for their activity. Bark samples from every species showed >40% antibacterial inhibition against at least one bacterial strain. This activity was also true of the dead wood samples. When plant parts of the same species were tested ( S . guineense and F . exasperata ), barks generally exhibited more potent antibacterial and COX-2 inhibition activity than the leaves, likely to do with the higher concentration of plant secondary metabolites in bark. Our findings offer strong support that bark and dead wood eating of certain species could constitute novel self-medicative behaviors in wild chimpanzees. We also encourage more investigation into the bioactivity of non-bitter pith stripping, as the pith of A . polystachius showed strong antibacterial activity against E . faecium (hE099; IC 50 : 32 μg/mL; MIC: 128 μg/mL), and the piths of both A . polystachius and M . leucantha demonstrated significant anti-inflammatory properties at low concentrations. Future primatological research should prioritize the establishment of multi-disciplinary long-term projects that look systematically at health states of individuals who engage in bark, dead wood, and pith ingestion behaviors. We also encourage further pharmacological testing on other species used for these behaviors in Budongo and across primate field sites.

Drug discovery

Multidisciplinary studies on this topic have potential to lead to the discovery of new medicines which may benefit our own species [ 119 – 122 ]. Historically, PSMs have played a major role in the development of modern human medicine, and even today, a large portion of medicines are derived either directly or indirectly from plants and other natural materials [ 123 – 127 ]. Antimicrobial resistance is rising to dangerously high levels according to the World Health Organization [ 128 ] requiring the rapid creation of new antibacterial treatments. Infections caused by multi-drug resistant bacteria kill hundreds of thousands of people annually. Our findings of strong antibacterial growth inhibition across numerous plant species growing in Budongo have promising implications for our ability to discover novel compounds in existing forest habitats. Extracts should also be tested against additional bacteria and for anti-virulence effects, e.g., inhibition and disruption of biofilm formation, quorum sensing and toxin production, pursuing development of new therapeutic strategies that apply less evolutionary pressure, likely resulting in emergence of less antibiotic resistances in the future. Phytochemical characterization using advanced techniques, such as LC-ToF-MS and NMR, as well as potentially AI-assisted untargeted metabolomics approaches, are now needed to identify substances present in the most active extracts. This may eventually lead to the isolation and structure elucidation of yet unknown active ingredients and make way for determining their pharmacological selectivity and toxicity, while also taking potential synergistic effects into account.

Simultaneously, we are currently faced with a pressing need for more effective treatments to combat symptoms of acute inflammation and mediate long-term consequences of chronic inflammatory diseases [ 129 ]. The prostaglandin-producing cyclooxygenase-2 (COX-2) mediates and regulates pain, fever, wound inflammation, and many other medical disorders, as it plays a crucial role in the host organism’s defense against pathogens and injury. COX-2 inhibition has the same mechanism of action as non-steroidal anti-inflammatory drugs (NSAIDs). While inflammation is a normal part of the body’s defense against injury or infection, it can be damaging when occurring in healthy tissues or over a protracted period. Chronic inflammation can lead to cardiovascular diseases (CVD) and cancer, the two leading global causes of death [ 130 ]. Past studies have shown that the IC 50 values of Aspirin and ibuprofen (pure compounds and common NSAIDs) are 210 μg/mL and 46 μg/mL respectively for COX-2, and 5 μg/mL and 1 μg/mL respectively for COX-1 [ 131 , 132 ]. The in vitro COX-2/COX-1 selectivity ratio for Aspirin and ibuprofen is 42 and 46 respectively. Surprisingly, the 17 most active extracts in our COX-2 assays display lower IC 50 values than these popular NSAIDs, meaning our extracts have more potent inhibitory effects on the inhibition of COX-2 than the most common anti-fever and anti-pain drugs on the market. While COX-1 assays were beyond the scope of this study, future research should investigate COX-1 inhibition activity of these 17 extracts to calculate COX-2/COX-1 selectivity ratios. Doing so will allow for preliminary assessment of potential side effects, selectivity, and efficacy before future in vivo experiments can commence.

Future directions

Future research on this topic would benefit from the inclusion of control samples (plants or plant parts not consumed by chimpanzees); however, in this study, assay costs were a prohibiting factor. Additional information regarding the nutritional and mineral content of the species mentioned in this study is needed to better understand the motivations for ingestion. However, bioactivity and nutritional/mineral content are by no means mutually exclusive. It is, therefore, highly likely that these resources provide multiple benefits to consumers.

Future studies should also consider ecological variables. For example, different individual plants of the same species should be tested across habitat types to determine whether bioactivity varies based on location, age, life history, or time of harvest. Situating samples in their ecological context will provide a better understanding of whether chimpanzees select resources based on species alone, or other more nuanced criteria. Lastly, climatic studies in combination with pharmacological testing should examine how climate change may impact bioactivity of these plants, as shifting weather patterns have already been shown to alter nutritional content [ 133 ]. This information will be critical for establishing protected habitats that can sustain healthy, wild, primate populations.

Conclusions

As we learn more about the pharmacological properties of plants ingested by chimpanzees in the wild, we can expand our understanding of their health maintenance strategies. Our results provide pharmacological evidence, from in vitro assays of plant parts consumed by wild chimpanzees collected in situ , for the presence of potent bioactive secondary plant metabolites in Budongo chimpanzee diets for a variety of potential illnesses previously not considered. Whether these resources are consumed intentionally as a form of therapeutic self-medication or passively as medicinal foods, must be assessed on a case-by-case basis, taking behavioral observations into account.

For the field of zoopharmacognosy to progress, we encourage continued multidisciplinary collaboration between primatologists, ethnopharmacologists, parasitologists, ecologists, and botanists [ 9 ]. Beyond improving our broad understanding of chimpanzee health maintenance, multidisciplinary studies will benefit our own species, potentially leading to the discovery of novel human medicines to combat the looming problem of growing drug-resistance. For this to happen, however, it is imperative that we urgently prioritize the preservation of our wild forest pharmacies as well as our primate cousins who inhabit them.

Materials availability

Voucher specimens for each species were deposited at the Makerere University Herbarium in Kampala, Uganda for taxonomic identification and storage. A duplicate set was deposited at the University of Oxford Herbarium for permanent storage.

Supporting information

S1 fig. budongo chimpanzees consuming resources tested in this study..

a.) IN eating K . anthotheca bark and resin b.) MZ eating S . myrtina bark c.) KC stripping A . polystachyus pith d.) MB eating C . patens dead wood e.) OZ eating S . guineense bark (post-study period) g.) MZ eating F . exasperata bark.

https://doi.org/10.1371/journal.pone.0305219.s001

S2 Fig. Generalized multi-method workflow used in this study.

https://doi.org/10.1371/journal.pone.0305219.s002

S3 Fig. Voucher samples collected in duplicate.

a . ) C . alexandri (00243133G) b . ) A . polystachius (00243136J) c . ) W . elongata (00243129L) d . ) C . parasitica (00243122E) e . ) K . anthotheca (00243123F) f . ) F . variifolia (51195) g . ) M . leucantha (51203) h . ) A . boonei (51204) i . ) D . dewevrei (00243132F) j . ) S . guineense (00243135I) k . ) S . myrtina (00243128K) l . ) F . exasperata (00243130D).

https://doi.org/10.1371/journal.pone.0305219.s003

S4 Fig. Plate layouts for growth inhibition assays.

[Top] Library Screen: done in 96-wells-mikrotiterplate; AB: Antibiotic as positive control; DMSO: vehicle control / negative control; GC: growth control: containing working culture, to check whether the bacterium grew/active; [Bottom] Dose-Response: done in descending concentration of samples, DMSO, and antibiotic. MB: Media blank, consisted of CAMHB as negative/ sterile media control; DMSO as negative/ vehicle control; GC: growth control, consisted of working culture.

https://doi.org/10.1371/journal.pone.0305219.s004

S5 Fig. ELISA assay setup for anti-inflammatory assay.

https://doi.org/10.1371/journal.pone.0305219.s005

S1 File. Supplementary materials: Methods .

https://doi.org/10.1371/journal.pone.0305219.s006

S2 File. Supplementary tables.

https://doi.org/10.1371/journal.pone.0305219.s007

Acknowledgments

We are grateful to all the field staff working in Budongo who provided invaluable instruction and guidance, generously sharing both scientific insight and traditional knowledge. This study could not have been done without their contributions. Specifically, we would like to thank members of the Perspectives Collective: Chandia Bosco, Monday Mbotella Gideon, Adue Sam, Asua Jackson, Steven Mugisha, Atayo Gideon, and Kizza Vincent, and Walter Akankwasa, as well as site director David Eryenyu. We would also like to thank Godwin Anywar for his assistance with plant identification at the Makerere Herbarium, Stephen Harris at the University of Oxford’s Herbarium for his facilitation of voucher storage, and the Natural History Museum in London for their aid in parasite identification. We are grateful to Vernon Reynolds who founded the field site and to the Royal Zoological Society of Scotland for providing core support. We also gratefully acknowledge the Uganda Wildlife Authority and the Uganda National Council for Science and Technology for granting permission to conduct research in Uganda. Lastly, thank you to the staff and students at Neubrandenburg University of Applied Sciences who made this collaboration possible, and to research assistant, Finn Freymann, for his help with botanical extractions.

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Minimal resolutions of Iwasawa modules

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• Published: 21 June 2024
• Volume 10 , article number  64 , ( 2024 )

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• Takenori Kataoka   ORCID: orcid.org/0000-0001-5682-367X 1 &
• Masato Kurihara 2  

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In this paper, we study the module-theoretic structure of classical Iwasawa modules. More precisely, for a finite abelian p -extension K / k of totally real fields and the cyclotomic \(\mathbb {Z}_p\) -extension \(K_{\infty }/K\) , we consider \(X_{K_{\infty },S}={{\,\textrm{Gal}\,}}(M_{K_{\infty },S}/K_{\infty })\) where S is a finite set of places of k containing all ramifying places in \(K_{\infty }\) and archimedean places, and \(M_{K_{\infty },S}\) is the maximal abelian pro- p -extension of \(K_{\infty }\) unramified outside S . We give lower and upper bounds of the minimal numbers of generators and of relations of \(X_{K_{\infty },S}\) as a \(\mathbb {Z}_p[[{{\,\textrm{Gal}\,}}(K_{\infty }/k)]]\) -module, using the p -rank of \({{\,\textrm{Gal}\,}}(K/k)\) . This result explains the complexity of \(X_{K_{\infty },S}\) as a \(\mathbb {Z}_p[[{{\,\textrm{Gal}\,}}(K_{\infty }/k)]]\) -module when the p -rank of \({{\,\textrm{Gal}\,}}(K/k)\) is large. Moreover, we prove an analogous theorem in the setting that K / k is non-abelian. We also study the Iwasawa adjoint of \(X_{K_{\infty },S}\) , and the minus part of the unramified Iwasawa module for a CM-extension. In order to prove these theorems, we systematically study the minimal resolutions of \(X_{K_{\infty },S}\) .

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1 Introduction

Throughout this paper we fix a prime number p . We write \(F_{\infty }\) for the cyclotomic \(\mathbb {Z}_p\) -extension of F for any number field F .

Let K / k be a finite abelian p -extension of totally real fields (see Theorem 3.3 for the non-abelian case). We consider the abelian extension \(K_{\infty }/k\) , whose Galois group we denote by \(\mathcal {G}={{\,\textrm{Gal}\,}}(K_{\infty }/k)\) . Suppose that S is a finite set of places of k , containing all archimedean places and all places that ramify in \(K_{\infty }\) . In particular, S contains all p -adic places. Let \(M_{K_{\infty },S}\) denote the maximal abelian pro- p -extension of \(K_{\infty }\) unramified outside S . Our main purpose in this paper is to study the classical Iwasawa module \(X_{K_{\infty },S}={{\,\textrm{Gal}\,}}(M_{K_{\infty },S}/K_{\infty })\) over the Iwasawa algebra \(\mathcal {R}= \mathbb {Z}_p[[\mathcal {G}]]\) .

Define \(I_{\mathcal {G}}\) to be the augmentation ideal of \(\mathcal {R}= \mathbb {Z}_p[[\mathcal {G}]]\) , namely \(I_{\mathcal {G}}={{\,\textrm{Ker}\,}}(\mathbb {Z}_p[[\mathcal {G}]] \rightarrow \mathbb {Z}_p)\) . We write \(Q(\mathcal {R})\) for the total quotient ring of \(\mathcal {R}\) . We consider an \(\mathcal {R}\) -submodule \(\mathcal {R}^{\sim }\) of \(Q(\mathcal {R})\) , which consists of elements \(x \in Q(\mathcal {R})\) , satisfying \(x I_{\mathcal {G}} \subset \mathcal {R}\) . This is the module of pseudo-measures of \(\mathcal {G}\) in the sense of Serre. The p -adic L -function of Deligne and Ribet is an element \(g_{K_{\infty }/k,S}\) in \(\mathcal {R}^{\sim }\) , satisfying the following property. Suppose that \(\kappa :\mathcal {G}\rightarrow \mathbb {Z}_p^\times \) is the cyclotomic character. For a character \(\psi \) of \(\mathcal {G}\) of finite order with values in an algebraic closure \(\overline{\mathbb {Q}}_{p}\) of \(\mathbb {Q}_p\) and for a positive integer n , one can extend a character \(\kappa ^n \psi : \mathcal {G}\rightarrow \overline{\mathbb {Q}}_{p}^{\times }\) to a ring homomorphism \(\mathcal {R}\rightarrow \overline{\mathbb {Q}}_{p}\) , and also to \(\mathcal {R}^{\sim } \rightarrow \overline{\mathbb {Q}}_{p}\) . Then \(g_{K_{\infty }/k,S}\) satisfies

for any character \(\psi \) of \(\mathcal {G}\) of finite order and for any positive integer \(n \in \mathbb {Z}_{>0}\) , where \(L_{S}(s, \psi \omega ^{-n})\) is the S -truncated L -function, and \(\omega \) is the Teichmüller character.

Put \(G = {{\,\textrm{Gal}\,}}(K_{\infty }/k_{\infty })\) . In [ 5 , Theorem 3.3] and [ 6 , Theorem 4.1], as a refinement of the usual main conjecture, Greither and the second author computed the Fitting ideal of \(X_{K_{\infty },S}\) as an \(\mathcal {R}\) -module to obtain

where \({\mathfrak a}_{G}\) is a certain ideal of \(\mathcal {R}\) which is determined only by the group structure of G . The explicit description of \({\mathfrak a}_{G}\) is obtained in [ 7 , Sect. 1.2] by Greither, Tokio and the second author. We do not explain this ideal \({\mathfrak a}_{G}\) in this paper, but only mention two facts. If s is the p -rank of G (i.e., \(s = \dim _{\mathbb {F}_p}(\mathbb {F}_p \otimes _{\mathbb {Z}} G)\) ) and \({\mathfrak m}_{\mathcal {R}}\) is the maximal ideal of \(\mathcal {R}\) , then we have \({\mathfrak a}_{G} \subset {\mathfrak m}_{\mathcal {R}}^{s(s-1)/2}\) . Also, if G is isomorphic to \((\mathbb {Z}/p^m)^{\oplus s}\) , then \({\mathfrak a}_{G}=(p^m\mathcal {R}+I_{\mathcal {G}})^{s(s-1)/2}\) . We also note here that the classical main conjecture in Iwasawa theory studies the character component \(X_{K_{\infty },S}^{\psi }\) for K which corresponds to the kernel of \(\psi \) . In this case, G is cyclic, and only the case \(s=1\) is studied.

The above computation of \({{\,\textrm{Fitt}\,}}_{\mathcal {R}}(X_{K_{\infty },S})\) suggests that \(X_{K_{\infty },S}\) is complicated as an \(\mathcal {R}\) -module when the p -rank s of G is large. To understand why and in which way the module has to be complicated, we study in this paper the minimal numbers of generators and relations of \(X_{K_{\infty },S}\) . Let \({{\,\textrm{gen}\,}}_{\mathcal {R}}(X_{K_{\infty }, S})\) (resp.  \(r_{\mathcal {R}}(X_{K_{\infty }, S})\) ) be the minimal number of generators (resp. of relations) of \(X_{K_{\infty }, S}\) as an \(\mathcal {R}\) -module.

In order to state the main result of this paper, we need the maximal abelian pro- p -extension \(M_{k, S}\) of k unramified outside S . By our choice of S , we have

Now we state the main result of this paper. For any abelian group A , we define its p -rank by \({{\,\textrm{rank}\,}}_p A = \dim _{\mathbb {F}_p}(\mathbb {F}_p \otimes _{\mathbb {Z}} A)\) , which is finite in all cases we consider in this paper.

Theorem 1.1

Let us write

for the p -ranks of the Galois groups. Then we have

We note that Leopoldt’s conjecture for k is equivalent to that the extension \(M_{k, S}/k_{\infty }\) is finite though we do not assume it in Theorem 1.1 . We get the equalities of the minimal numbers of generators in the following special cases.

If Leopoldt’s conjecture holds and \(p \ge 3\) , then we may take K so that \(K_{\infty } = M_{k, S}\) . In this case, we have \(t = 0\) , so the theorem says

where \(s = {{\,\textrm{rank}\,}}_p{{\,\textrm{Gal}\,}}(M_{k, S}/k_{\infty })\) .

In case \(K_{\infty } = k_{\infty }\) , we have \(s = 0\) , so the theorem says

Indeed, this follows directly from Lemma 6.1 .

Except for these cases, we have no theoretical method to determine the exact value of \({{\,\textrm{gen}\,}}_{\mathcal {R}}(X_{K_{\infty }, S})\) so far.

In Sect. 7 we give several numerical examples for \(p=2\) , 3. We take \(k=\mathbb {Q}\) and \(K/\mathbb {Q}\) to be a real abelian extension such that \({{\,\textrm{Gal}\,}}(K/\mathbb {Q}) \simeq (\mathbb {Z}/p\mathbb {Z})^{\oplus s}\) . Here, we pick up some typical examples from Sect. 7 .

Take \(p=3\) . For two primes \(\ell _i\) with \(i=1\) , 2 such that \(\ell _i \equiv 1\) (mod 3), let K be the unique \((\mathbb {Z}/3\mathbb {Z})^{\oplus 2}\) -extension over \(\mathbb {Q}\) with conductor \(\ell _1\ell _2\) . Take \(\ell _1=7\) , and \(\ell _2\) satisfying \(\ell _2 \equiv 1\) (mod 9), and consider \(S=\{3, \ell _1, \ell _2, \infty \}\) . In this case \(s=2\) and \(t=1\) . So Theorem 1.1 says that

For \(\ell _2\) less than 200, we have

Thus the above inequality is sharp in this case.

Take \(p=2\) . Suppose that \(\ell _1\) , \(\ell _2\) , \(\ell _3\) are three distinct primes such that \(\ell _{i} \equiv 1\) (mod 4). We take \(K=\mathbb {Q}(\sqrt{\ell _1},\sqrt{\ell _2}, \sqrt{\ell _3})\) and \(S=\{2, \ell _1, \ell _2, \ell _3, \infty \}\) . Then \({{\,\textrm{Gal}\,}}(K/\mathbb {Q})\simeq (\mathbb {Z}/2\mathbb {Z})^{\oplus 3}\) , so \(s=3\) . Taking account of the archimedean place, we know \(t=1+3=4\) . Since \(s(s+1)/2=6\) , Theorem 1.1 says in this case

Take \(\ell _1=5\) . For any \(5< \ell _2 < \ell _3 \le 100\) , we have \( {{\,\textrm{gen}\,}}_{\mathcal {R}}(X_{K_{\infty }, S}) =7 \) except for

for which we have \({{\,\textrm{gen}\,}}_{\mathcal {R}}(X_{K_{\infty }, S}) =8\) . Also we have \({{\,\textrm{gen}\,}}_{\mathcal {R}}(X_{K_{\infty }, S}) =9\) for \((\ell _1,\ell _2,\ell _3)=(17,73,89)\) , and \({{\,\textrm{gen}\,}}_{\mathcal {R}}(X_{K_{\infty }, S}) =10\) for \((\ell _1,\ell _2,\ell _3)=(73,89,97)\) . We never observed \({{\,\textrm{gen}\,}}_{\mathcal {R}}(X_{K_{\infty }, S}) =6\) at least in this range.

In this paper, we prove not only the above Theorem 1.1 but also its non-abelian generalization in Theorem 3.3 . We also study and determine the minimal numbers of generators and relations of the dual (Iwasawa adjoint) of \(X_{K_{\infty }, S}\) (see Theorem 3.4 ). This is relatively easier than Theorem 3.3 . Also, we give in Sect. 3.3 some applications to the minus part of certain Iwasawa modules of CM-fields (see Corollary 3.5 ), using Kummer duality.

A key to the proof of our theorems is the existence of certain exact sequences, called Tate sequences. We remark here that Greither also used a different kind of Tate sequence in [ 3 ] to get information on the minimal numbers of generators of class groups of number fields. Our method of using the Tate sequences is totally different from Greither’s.

This paper is organized as follows. After algebraic preliminaries in Sect. 2 , we will state the main results in Sect. 3 . The proofs are given in Sects. 4 – 6 . Finally in Sect. 7 , we will study numerical examples.

2 Algebraic preliminaries

2.1 minimal resolutions.

Let R be a Noetherian local ring, which we do not assume to be commutative. Let \(\mathfrak {m}\) be the Jacobson radical of R , that is, \(\mathfrak {m}\) is the maximal left (right) ideal of R . For simplicity, let us assume that \(\textbf{k} := R/\mathfrak {m}\) is a commutative field. We will often consider the case \(R = \mathbb {Z}_p[[\mathcal {G}]]\) for a pro- p group \(\mathcal {G}\) , in which case R is indeed local and we have \(\textbf{k} = \mathbb {F}_p\) (see [ 13 , Proposition 5.2.16 (iii)]).

Definition 2.1

For a finitely generated (left) R -module M , we write \({{\,\textrm{gen}\,}}_R(M)\) for the minimal number of generators of M as an R -module. Also, we write \(r_R(M)\) for the minimal number of relations of M as an R -module (see Definition 2.4 below).

The following observations will be often used.

By Nakayama’s lemma (for non-commutative rings, see e.g., [ 8 , Corollary 13.12]), we have

for any two-sided ideal \(I \subset \mathfrak {m}\) of R . In particular, we have

Therefore, for a finitely generated \(\mathbb {Z}_p\) -module M , we have

If we have an exact sequence

of finitely generated R -modules, we have

The proof is standard and easy.

In item (2) above, if we assume that R is a discrete valuation ring (DVR), the formula may be refined to

This follows from the structure theorem for finitely generated modules over principal ideal domains.

Example 2.3

Let us observe an example for which the formula in item (3) above does not hold when R is not a DVR. Let \(R = \mathbb {Z}_p[[T]]\) . Consider \(M = \mathbb {Z}_p[[T]]\) and its submodule

with \(n \ge 1\) . Then we have \({{\,\textrm{gen}\,}}_{R}(M') = n + 1\) and \({{\,\textrm{gen}\,}}_{R}(M) = 1\) , so \({{\,\textrm{gen}\,}}_{R}(M') \le {{\,\textrm{gen}\,}}_{R}(M)\) does not hold.

Next we introduce the minimal resolutions of modules.

Definition 2.4

Let M be a finitely generated R -module. We can construct an exact sequence of R -modules

such that the image of each homomorphism \(R^{r_{n+1}} \rightarrow R^{r_n}\) ( \(n \ge 0\) ) is contained in \(\mathfrak {m}^{r_n}\) . Such a sequence is called a minimal resolution of M . In this case, since \(R^{r_{n+1}} \rightarrow R^{r_n}\) induces the zero map on \((R/\mathfrak {m})^{r_{n+1}} \rightarrow (R/\mathfrak {m})^{r_n}\) , by the definition of the \({{\,\textrm{Tor}\,}}\) functor, the integer \(r_n\) coincides with

for \(n \ge 0\) . In particular, the integer \(r_n\) is independent of the choice of the minimal resolution. By definition we have

If G is a finite p -group, we have

for all \(n \ge 0\) .

This follows from \(H_n(G, \mathbb {F}_p) \simeq {{\,\textrm{Tor}\,}}_n^{\mathbb {Z}_p[G]}(\mathbb {Z}_p, \mathbb {F}_p) \simeq {{\,\textrm{Tor}\,}}_n^{\mathbb {Z}_p[G]}(\mathbb {F}_p, \mathbb {Z}_p)\) and the formula in Definition 2.4 . \(\square \)

2.2 Group homology

In this subsection, we summarize facts about group homology.

Let G be a finite group. The following lemma is well-known.

the abelianization of G , and

for \(M \in \mathbb {Z}_{\ge 1}\) .

As for the second homology groups, if G is abelian, it is known that \(H_2(G, \mathbb {Z})\) is isomorphic to \(\bigwedge ^2 G\) (see [ 1 , Chap. V, Theorem 6.4 (iii)]). If G is not abelian, \(H_2(G, \mathbb {Z})\) is much harder to study, and is also known as the Schur multiplier of G (cf. [ 11 ]).

For now, we observe a relation between \(H_n(G, \mathbb {Z})\) and \(H_n(G, \mathbb {Z}/M\mathbb {Z})\) for a p -power M .

Let \(n \ge 2\) . For any \(m \ge 1\) , we have

In particular, as the right hand side is independent from m , we have

This follows from the universal coefficient theorem (see [ 1 , Chap. I, Proposition 0.8], for example), which says in our case that

is split exact. \(\square \)

In case G is abelian, it is not hard to compute the p -rank of the n -th homology group:

Suppose G is abelian and put \(s = {{\,\textrm{rank}\,}}_p G\) . Then we have

for \(n \ge 0\) (when \(n = 0\) , the right hand side is understood to be 1).

By replacing G by its p -Sylow subgroup, we may assume that G is a p -group. As in [ 5 , Sect. 1.2] or [ 12 , Sect. 4.3], we can construct an explicit minimal free resolution of \(\mathbb {Z}_p\) as an R -module

with \(s_{n}=s(s+1) \cdots (s+n-1)/n!\) . Thus, the lemma follows from Lemma 2.5 . \(\square \)

We also need the following duality theorem between the cohomology groups and the homology groups (see [ 1 , Chap VI Proposition 7.1], for example).

Let G be a finite group and M a (discrete) G -module. We define its Pontryagin dual \(M^{\vee }\) by \(M^{\vee }={{\,\textrm{Hom}\,}}(M, \mathbb {Q}/\mathbb {Z})\) . Then for any \(n \in \mathbb {Z}_{\ge 0}\) , we have an isomorphism between \(H^{n}(G, M)\) and \({{\,\textrm{Hom}\,}}(H_{n}(G,M^{\vee }), \mathbb {Q}/\mathbb {Z})\) .

3 The main results

3.1 setting.

As in Sect. 1 , let p be any prime number, k a totally real field, and \(k_{\infty }\) its cyclotomic \(\mathbb {Z}_p\) -extension. For a finite set S of places of k such that S contains all the archimedean places and all p -adic places, we write \(M_{k, S}\) for the maximal abelian pro- p -extension of k unramified outside S .

Let \(K_{\infty }/k\) be a pro- p Galois extension of totally real fields such that \(K_{\infty }\) contains \(k_{\infty }\) and the extension \(K_{\infty }/k_{\infty }\) is finite. We do not assume that \(K_{\infty }/k\) is abelian, but we have to assume the following.

Assumption 3.1

There exists an intermediate finite Galois extension K / k of \(K_{\infty }/k\) such that

In other words, the map induced by the restriction maps

is an isomorphism.

If \(K_{\infty }/k\) is abelian, then Assumption 3.1 holds.

We consider the restriction homomorphism \(f: {{\,\textrm{Gal}\,}}(K_{\infty }/k) \twoheadrightarrow {{\,\textrm{Gal}\,}}(k_{\infty }/k)\) . Since f is a homomorphism of \(\mathbb {Z}_p\) -modules and the target is free, f has a section. We take a section and define K to be the fixed field of the image of the section. A point is that K / k is then automatically Galois as \(K_{\infty }/k\) is abelian. \(\square \)

Set \(\mathcal {G}= {{\,\textrm{Gal}\,}}(K_{\infty }/k)\) and \(G = {{\,\textrm{Gal}\,}}(K_{\infty }/k_{\infty })\) . We take an S such that \(K_{\infty }/k\) is unramified outside S . Let \(M_{K_{\infty }, S}/K_{\infty }\) be the Galois group of the maximal abelian pro- p -extension of \(K_{\infty }\) that is unramified outside places lying above S , and \(X_{K_{\infty }, S} = {{\,\textrm{Gal}\,}}(M_{K_{\infty }, S}/K_{\infty })\) as in the Introduction. Then it is known that \(X_{K_{\infty }, S}\) is a finitely generated torsion module over the associated Iwasawa algebra \(\mathcal {R}= \mathbb {Z}_p[[\mathcal {G}]]\) . Since \(K_{\infty }/k\) is a pro- p extension, the algebra \(\mathcal {R}\) is a local ring whose residue field is \(\mathbb {F}_p\) .

3.2 The statements

We use the notation in Sect. 3.1 . To state the result, let us put

for \(n \ge 0\) (recall \(G = {{\,\textrm{Gal}\,}}(K_{\infty }/k_{\infty })\) ). For instance, we have \(s_0 = 1\) and \(s_1 = {{\,\textrm{rank}\,}}_p G^{{{\,\textrm{ab}\,}}}\) by Lemma 2.6 . Recall that Lemma 2.8 tells us an explicit formula of \(s_n\) in case \(K_{\infty }/k_{\infty }\) is abelian ; in particular, we have \(s_2 = s(s+1)/2\) and \(s_3 = s(s+1)(s+2)/6\) with \(s = {{\,\textrm{rank}\,}}_p G (= s_1)\) .

The following is the main result, which contains a non-abelian generalization of Theorem 1.1 .

Theorem 3.3

When Assumption 3.1 is satisfied, the following inequalities and equalities hold.

Put \(t = {{\,\textrm{rank}\,}}_p {{\,\textrm{Gal}\,}}(M_{k, S}/M_{k, S} \cap K_{\infty })\) . Then we have

for \(n \ge 2\) and

It is easy to see that Theorems 3.3 implies 1.1 , thanks to Lemma 3.2 .

We also prove corresponding theorems concerning the dual (Iwasawa adjoint) of \(X_{K_{\infty }, S}\) . For a finitely generated torsion \(\mathcal {R}\) -module M , we define the dual (Iwasawa adjoint) of M by

Using the field K in Assumption 3.1 , we put \(\Lambda = \mathbb {Z}_p[[{{\,\textrm{Gal}\,}}(K_{\infty }/K)]]\) . As a consequence of Assumption 3.1 , \(\mathcal {R}\) is isomorphic to \(\Lambda [G]\) . Then we have

because \({{\,\textrm{Hom}\,}}_{\mathcal {R}}(N, \mathcal {R}) \simeq {{\,\textrm{Hom}\,}}_{\Lambda }(N, \Lambda )\) for any \(\mathcal {R}\) -module N . Therefore, our \(M^{*}\) coincides with the Iwasawa adjoint of M in [ 13 , Definition 5.5.5], [ 9 , Sect. 5.1], [ 10 , Sect. 1.3].

We are interested in the \(\mathcal {R}\) -module \(X_{K_{\infty }, S}^{*}\) . It is known that the structure of \(X_{K_{\infty }, S}^{*}\) is often simpler than \(X_{K_{\infty }, S}\) itself (e.g., when we are concerned with their Fitting ideals). The following theorem implies that we encounter such a phenomenon when we are concerned with the minimal resolutions.

Theorem 3.4

When Assumption 3.1 is satisfied, the following equalities hold.

If \(K_{\infty } \supsetneqq k_{\infty }\) , then we have

for \(n \ge 3\) ,

If \(K_{\infty } = k_{\infty }\) , then we have \(r_n(X_{K_{\infty }, S}^{*}) = 0\) for \(n \ge 2\) and \(r_1(X_{K_{\infty }, S}^{*}) - r_0(X_{K_{\infty }, S}^{*}) = 0\) .

In Sect. 5 , we will prove \(s_2 \le {{\,\textrm{gen}\,}}_{\mathcal {R}}(X_{K_{\infty , S}})\) in Theorems 3.3 (1), 3.3 (2), and 3.4 (2). These parts follow only from the existence of the Tate sequence introduced in Sect. 4 . The rest of the statements ( \(t \le {{\,\textrm{gen}\,}}_{\mathcal {R}}(X_{K_{\infty }, S}) \le s_2 + t\) in Theorems 3.3 (1) and 3.4 (1)) will be proved in Sect. 6 .

3.3 Applications for the minus parts of Iwasawa modules for CM-extensions

In this subsection we apply the main theorems in the previous subsection to CM-extensions. We keep the notation in Sect. 3.1 , so \(K_{\infty }/k\) is an extension of totally real fields satisfying Assumption 3.1 . Only in this subsection we assume that p is odd, which is mainly for making the functor of taking the character component exact for characters of \({{\,\textrm{Gal}\,}}(K_{\infty }(\mu _{p})/K_{\infty })\) .

We consider the field \(\mathcal {K}_{\infty }=K_{\infty }(\mu _{p})\) obtained by adjoining all p -th roots of unity to \(K_{\infty }\) . So \(\mathcal {K}_{\infty }/k\) is a CM-extension. We also use an intermediate field \(\mathcal {K}_{n}\) of the \(\mathbb {Z}_p\) -extension \(\mathcal {K}_{\infty }/K(\mu _{p})\) such that \([{\mathcal K}_{n}:K(\mu _{p})]=p^n\) for each \(n \ge 0\) . Let \(\mathcal {L}_{n}\) be the maximal abelian pro- p -extension of \(\mathcal {K}_{n}\) unramified everywhere . So \({{\,\textrm{Gal}\,}}(\mathcal {L}_{n}/{\mathcal K}_{n})\) is isomorphic to the p -component \(A_{\mathcal {K}_{n}}\) of the ideal class group of \(\mathcal {K}_{n}\) by class field theory. We denote by \(A_{\mathcal {K}_{\infty }}\) the inductive limit of \(A_{\mathcal {K}_{n}}\) , which is a discrete \(\mathbb {Z}_p[[{{\,\textrm{Gal}\,}}({\mathcal K}_{\infty }/k)]]\) -module. We write \(\mathcal {X}_{{\mathcal K}_{\infty }}\) for the projective limit of \(A_{\mathcal {K}_{n}}\) . Then \(\mathcal {X}_{\mathcal {K}_{\infty }}\) is a compact \(\mathbb {Z}_p[[{{\,\textrm{Gal}\,}}(\mathcal {K}_{\infty }/k)]]\) -module. Defining \({\mathcal L}_{\infty }\) to be the maximal abelian pro- p -extension of \(\mathcal {K}_{\infty }\) unramified everywhere, we know that \(\mathcal {X}_{\mathcal {K}_{\infty }}={{\,\textrm{Gal}\,}}({\mathcal L}_{\infty }/\mathcal {K}_{\infty })\) . Let \(n_{0}\) be the smallest integer such that all p -adic places are totally ramified in \(\mathcal {K}_{\infty }/\mathcal {K}_{n_0}\) . We define a submodule \(\mathcal {Y}_{\mathcal {K}_{\infty }}\) of \(\mathcal {X}_{{\mathcal K}_{\infty }}\) by \(\mathcal {Y}_{{\mathcal K}_{\infty }}={{\,\textrm{Gal}\,}}(\mathcal {L}_{\infty }/\mathcal {L}_{n_0}{\mathcal K}_{\infty })\) .

Put \(\Delta ={{\,\textrm{Gal}\,}}(\mathcal {K}_{\infty }/K_{\infty })\) , which is of order prime to p by our assumption \(p \ne 2\) in this subsection. Therefore, since \({{\,\textrm{Gal}\,}}(\mathcal {K}_{\infty }/k)\simeq \mathcal {G}\times \Delta \) , any \(\mathbb {Z}_p[[{{\,\textrm{Gal}\,}}(\mathcal {K}_{\infty }/k)]]\) -module M is decomposed into \(M=\bigoplus _{\chi } M^{\chi }\) where \(\chi \) runs over all characters of \(\Delta \) with values in \(\mathbb {Z}_p^{\times }\) , and \(M^{\chi }\) is the \(\chi \) -component of M defined by

Note that each \(M^{\chi }\) is an \(\mathcal {R}\) -module. Let \(\omega : \Delta \rightarrow \mathbb {Z}_p^{\times }\) be the Teichmüller character, giving the action on \(\mu _{p}\) . Using our main results in Sect. 3.2 , we study \(A_{\mathcal {K}_{\infty }}^{\omega }\) and \({\mathcal Y}_{\mathcal {K}_{\infty }}^{\omega }\) . Note that \(\omega \) is an odd character, so the complex conjugation acts on these modules as \(-1\) .

Let \(S_{p}\) be the set of all p -adic places and all archimedean places. Recall that we write \((-)^{\vee }\) for the Pontryagin dual. By Kummer pairing (see [ 13 , Theorem 11.4.3] or [ 15 , Proposition 13.32]), we have an isomorphism

where (1) is the Tate twist. Also, by [ 13 , Theorem 11.1.8] we have

Therefore, we have

For any finitely generated torsion \(\mathcal {R}\) -module M which has no nontrivial finite submodule, we know \((M^{*})^{*} \simeq M\) (see, for example, [ 13 , Proposition 5.5.8 (iv)]). Since \({\mathcal X}_{\mathcal {K}_{\infty },S_{p}}\) has no nontrivial finite submodule, so does \(\mathcal {Y}_{\mathcal {K}_{\infty }}^{\omega }\) . Therefore, it follows from the previous isomorphism that

Thus, from Theorems 3.3 and 3.4 we get

Corollary 3.5

In Theorem 3.3 , we further assume that \(p >2\) and \(S = S_p\) (so \(K_{\infty }/k_{\infty }\) is unramified outside p ).

Then we have

Put \(t'={{\,\textrm{rank}\,}}_p {{\,\textrm{Gal}\,}}(M_{k, S}/k_{\infty })\) . Then we have \({{\,\textrm{gen}\,}}_{\mathcal {R}}(\mathcal {Y}_{\mathcal {K}_{\infty }}^{\omega }) = t'\) and

4 The Tate sequence

A key ingredient to prove Theorems 3.3 and 3.4 is an exact sequence that \(X_{K_{\infty }, S}\) satisfies, which is often called the Tate sequence. Indeed, as noted in the final paragraph of Sect. 3.2 , some parts of our main theorems can be deduced by using the Tate sequence and not much else. On the other hand, the other parts require additional arithmetic study that we will do in Sect. 6 . The Tate sequence also played a key role in computing the Fitting ideal of \(X_{K_{\infty }, S}\) in the work [ 5 , 6 , 7 ] that we mentioned in Sect. 1 .

In order to prove the main theorems, we need the Tate sequence of the following type.

Theorem 4.1

There exists an exact sequence of \(\mathcal {R}\) -modules

where P and Q are finitely generated torsion \(\mathcal {R}\) -modules whose projective dimensions are \(\le 1\) . Moreover, this sequence is functorial when \(K_{\infty }\) varies. More precisely, for a finite normal subgroup H of \({{\,\textrm{Gal}\,}}(K_{\infty }/k)\) , we have an exact sequence

over the Iwasawa algebra \(\mathbb {Z}_p[[{{\,\textrm{Gal}\,}}(K_{\infty }^H/k)]]\) , where \(P_H\) , \(Q_H\) denote the H -coinvariant modules, and \(\phi _H\) the homomorphism induced by \(\phi \) .

For an intermediate field F of \(K_{\infty }/k\) with \([F:k]<\infty \) , we use a perfect complex \(\textsf{R}\Gamma _{c}(\mathcal {O}_{F,S}, \mathbb {Z}_p(1))\) in Burns–Flach [ 2 , Proposition 1.20]. Note that this complex works well even for \(p=2\) (here, we use our assumption that S contains all archimedean places). Taking the project limit, we get a perfect complex \(C^{\bullet }=\textsf{R}\Gamma _{c}(\mathcal {O}_{K_{\infty },S}, \mathbb {Z}_p(1))\) which is quasi-isomorphic to a complex of the form \([C^0 \overset{d^0}{\rightarrow }\ C^1 \overset{d^1}{\rightarrow }\ C^2]\) concentrated on degrees 0, 1, 2 with \(C^i\) finitely generated projective over \(\mathcal {R}\) and whose cohomology groups are

(see [ 2 , p. 86, line 6]) and \(H^i(C^{\bullet })=0\) for \(i \ne 1\) , 2, where we used the weak Leopoldt conjecture which is proven in this case (see [ 13 , Theorem 10.3.25]). By the definition of cohomology groups, we have an exact sequence

where we regard \(C^0\) as a submodule of \(C^1\) via \(d^0\) and \(\Phi \) is induced by \(d^1\) . Take a non-zero-divisor f in the center of \(\mathcal {R}\) that annihilates \(\mathbb {Z}_p\) . Then the image of \(\Phi \) contains \(f C^2\) , so by the projectivity of \(C^2\) we can construct a commutative diagram of \(\mathcal {R}\) -modules

Then defining P and Q as the cokernel of these vertical maps respectively, we obtain the Tate sequence as claimed. The functoriality follows from that of \(\textsf{R}\Gamma _{c}({\mathcal O}_{K_{\infty },S}, \mathbb {Z}_p(1))\) . \(\square \)

5 Abstract Tate sequences

Set \(\Lambda = \mathbb {Z}_p[[T]]\) . Let G be a (not necessarily abelian) finite p -group.

Motivated by Theorem 4.1 , we study a \(\Lambda [G]\) -module X that admits an abstract Tate sequence, that is:

Setting 5.1

There exists an exact sequence of \(\Lambda [G]\) -modules

where both P and Q are finitely generated torsion \(\Lambda [G]\) -modules whose projective dimensions are \(\le 1\) .

In this section, we show that the existence of a Tate sequence gives a severe constraint on the integers \({{\,\textrm{gen}\,}}_{\Lambda [G]}(X) = r_0(X)\) , \(r_{\Lambda [G]}(X) = r_1(X)\) , and \(r_n(X)\) ( \(n \ge 2\) ).

5.1 The statements

To state the result, let us define

for \(n \ge 0\) .

The following are the main theorems in this section. As noted in the final paragraph of Sect. 3.2 , those are enough to show parts of Theorems 3.3 and 3.4 .

Theorem 5.2

Let X be a \(\Lambda [G]\) -module that satisfies a Tate sequence as in Setting 5.1 .

We have \({{\,\textrm{gen}\,}}_{\Lambda [G]}(X) \ge s_2\) .

Claims (1) and (2) will be proved respectively in Sects. 5.3 and 5.4 .

For a \(\Lambda [G]\) -module M , we define its dual (Iwasawa adjoint) by

The corresponding theorem for the dual is:

Theorem 5.3

We have \({{\,\textrm{gen}\,}}_{\Lambda [G]}(X^{*}) \ge s_1\) .

If G is non-trivial, then we have

If G is trivial, then we have \(r_n(X^{*}) = 0\) for \(n \ge 2\) and \(r_1(X^{*}) - r_0(X^{*}) = 0\) .

This theorem will be proved in § 5.6 . The idea is basically the same as that of Theorem 5.2 . However, we need an additional algebraic proposition shown in § 5.5 .

5.2 Specialization

We consider modules over \(\Lambda = \mathbb {Z}_p[[T]]\) . As explained in Example 2.3 , \({{\,\textrm{gen}\,}}_{\Lambda }(-)\) does not behave very well for short exact sequences. A key idea to prove the main theorems is to apply a specialization method to reduce to modules over DVRs.

Here, a monic distinguished polynomial is by definition a polynomial of the form

where \(a_1, \dots , a_e \in p\mathbb {Z}_p\) . By the Weierstrass preparation theorem, any prime element of \(\Lambda \) can be written as the product of a unit element and an element of \(\mathcal {F}\) in a unique way.

For each \(f \in \mathcal {F}\) , put

which is a domain. We define a subset \(\mathcal {F}_0 \subset \mathcal {F}\) by

The following lemma tells us a concrete description of \(\mathcal {F}_0\) . Although the lemma is unnecessary for the proof of the main results, we include it in this paper to clarify the situation.

We have \(\mathcal {F}_0 = \{p\} \cup \mathcal {F}_1 \cup \mathcal {F}_2\) , where we put

It is clear that \(\{p \} \subset \mathcal {F}_0\) and \(\mathcal {F}_1 \subset \mathcal {F}_0\) . Also, \(\mathcal {F}_2 \subset \mathcal {F}_0\) holds by the Eisenstein irreducibility criterion. Therefore, it remains to only show \(\mathcal {F}_0 \setminus \{p\} \subset \mathcal {F}_1 \cup \mathcal {F}_2\) .

Let \(f \in \mathcal {F}_0 \setminus \{p\}\) . Since \(\mathcal {O}_f\) is a DVR, it is the integral closure of \(\mathbb {Z}_p\) in the p -adic field \(K_f = {{\,\textrm{Frac}\,}}(\mathcal {O}_f)\) . Moreover, since the residue field of \(\mathcal {O}_f\) is the same as that of \(\Lambda \) , namely \(\mathbb {F}_p\) , we see that the extension \(K_f/\mathbb {Q}_p\) is totally ramified. In case the extension \(K_f/\mathbb {Q}_p\) is trivial, we have \(\deg (f) = 1\) , so we obtain \(f \in \mathcal {F}_1\) . In case \(K_f/\mathbb {Q}_p\) is non-trivial, the image of T in \(\mathcal {O}_f\) must be a uniformizer of \(\mathcal {O}_f\) , so its minimal polynomial f is in \(\mathcal {F}_2\) (see [ 14 , Chap. I, Proposition 18]). This completes the proof. \(\square \)

5.3 Proof of Theorem 5.2 (1)

Let us now study a \(\Lambda [G]\) -module X admitting a Tate sequence as in Setting 5.1 . We define a \(\Lambda \) -module \(X_{(G)}\) by

where \(P_G\) and \(Q_G\) denote the G -coinvariant modules and \(\phi _G\) denotes the induced homomorphism. Note that \(X_{(G)}\) does not coincide with the coinvariant module \(X_G\) in general; in fact, the difference is what we shall investigate from now on.

The following proposition is a key to prove the main theorem.

Proposition 5.5

Let \(f \in \mathcal {F}\) be an element that is prime to both \({{\,\textrm{char}\,}}_{\Lambda }(P)\) and \({{\,\textrm{char}\,}}_{\Lambda }(Q)\) , where \({{\,\textrm{char}\,}}_{\Lambda }(-)\) denotes the characteristic polynomial. We set \(m = {{\,\textrm{ord}\,}}_p(f(0)) \ge 1\) . Then we have an exact sequence of finitely generated torsion \(\mathcal {O}_f\) -modules

Firstly note that \(f(0) \ne 0\) since \({{\,\textrm{char}\,}}_{\Lambda }(Q)\) is divisible by \({{\,\textrm{char}\,}}_{\Lambda }(\mathbb {Z}_p) = (T)\) . By taking modulo f of the sequence ( 5.1 ), we obtain an exact sequence of finitely generated torsion \(\mathcal {O}_{f}[G]\) -modules

Let L denote the image of the map \(\overline{\phi }: P/f \rightarrow Q/f\) . Since both P / f and Q / f are G -cohomologically trivial, taking the G -homology, we obtain exact sequences

and also an isomorphism \(H_2(G, \mathbb {Z}_p/p^m \mathbb {Z}_p) \simeq H_1(G, L)\) .

We can combine these observations with the exact sequence obtained by taking modulo f of sequence ( 5.2 ) to construct a diagram

This is a commutative diagram of finitely generated torsion \(\mathcal {O}_f\) -modules. By applying the snake lemma, we obtain the proposition. \(\square \)

Proof of Theorem 5.2(1)

In Proposition 5.5 , we take f so that \(f \in \mathcal {F}_0\) , i.e., \(\mathcal {O}_f\) is a DVR. Then the injective homomorphism from \(H_2(G, \mathbb {Z}_p/p^m \mathbb {Z}_p)\) to \((X/f)_G\) in Proposition 5.5 implies

where the first equality follows from Nakayama’s lemma. Since \(H_2(G, \mathbb {Z}_p/p^m \mathbb {Z}_p)\) is annihilated by T and \(\mathcal {O}_f/(T) \simeq \mathbb {Z}_p/p^m\mathbb {Z}_p\) , we have

where the second equality follows from Lemma 2.7 . Combining these formulas, we obtain \({{\,\textrm{gen}\,}}_{\Lambda [G]}(X) \ge s_2\) , as claimed. \(\square \)

This argument also shows

with \(\pi \) as in Proposition 5.5 . Then the full statement of Theorem 3.3 (1) follows if we can take f so that \({{\,\textrm{gen}\,}}_{\mathcal {O}_f}({{\,\textrm{Ker}\,}}(\pi ))\) coincides with the t in Theorem 3.3 (1). This is indeed possible, but in § 6 we will give a more direct proof for the rest of Theorem 3.3 (1) instead.

5.4 Proof of Theorem 5.2 (2)

We introduce several lemmas. We abbreviate \(r_n^{\Lambda [G]}(-)\) as \(r_n(-)\) and we never omit the coefficient ring otherwise.

Let P be a finitely generated torsion \(\Lambda [G]\) -module whose projective dimension is \(\le 1\) . Then we have \(r_n(P) = 0\) for \(n \ge 2\) and \(r_1(P) = r_0(P)\) .

By the assumption on P , there exists a presentation of P of the form \(0 \rightarrow \Lambda [G]^a \rightarrow \Lambda [G]^a \rightarrow P \rightarrow 0\) . The lemma follows immediately from this. \(\square \)

Let \(0 \rightarrow M' \rightarrow P \rightarrow M \rightarrow 0\) be a short exact sequence of finitely generated torsion \(\Lambda [G]\) -modules such that the projective dimension of P is \(\le 1\) . Then we have

This follows immediately from the long exact sequence of \({{\,\textrm{Tor}\,}}_*^{\Lambda [G]}(\mathbb {F}_p, -)\) applied to the given sequence, taking Lemma 5.7 into account. \(\square \)

Let M be a finitely generated \(\mathbb {Z}_p[G]\) -module that is free over \(\mathbb {Z}_p\) . We regard M as a \(\Lambda [G]\) -module so that T acts trivially on M . Then we have

for \(n \ge 0\) . Here, we set \(r_{-1}^{\mathbb {Z}_p[G]}(M) = 0\) .

Let us take a minimal resolution of M as a \(\mathbb {Z}_p[G]\) -module

where we put \(\rho _n = r_n^{\mathbb {Z}_p[G]}(M)\) . We have an exact sequence

which may be regarded as a minimal resolution of \(\mathbb {Z}_p\) as a \(\Lambda \) -module. Then we take the tensor product over \(\mathbb {Z}_p\) of the two complex above (omitting M and \(\mathbb {Z}_p\) respectively). As a result, we obtain an exact sequence

By construction, this is a minimal resolution of M as a \(\Lambda [G]\) -module. This completes the proof of the lemma. \(\square \)

Now we are ready to prove Theorem 5.2 (2).

Proof of Theorem 5.2(2)

First we recall \(s_n = r_n^{\mathbb {Z}_p[G]}(\mathbb {Z}_p)\) by Lemma 2.5 . By applying Lemma 5.9 to \(M = \mathbb {Z}_p\) , we obtain

for \(n \ge 0\) , where we set \(s_{-1} = 0\) . We apply Lemma 5.8 to the two short exact sequences obtained by splitting the Tate sequence. As a consequence, we obtain

This completes the proof. \(\square \)

5.5 An algebraic proposition

This subsection provides preliminaries to the proof of Theorem 5.3 . Let \(\mathcal {C}\) be the category of finitely generated torsion \(\Lambda [G]\) -modules whose projective dimension over \(\Lambda \) is \(\le 1\) , that is, those that do not have nontrivial finite submodules.

We also write \(\mathcal {P}\) for the subcategory of \(\mathcal {C}\) that consists of modules whose projective dimension over \(\Lambda [G]\) is \(\le 1\) .

For a module \(M \in \mathcal {C}\) , it is known that the dual

is also in \(\mathcal {C}\) and \((M^*)^* \simeq M\) ([ 13 , Propositions 5.5.3 (ii) and 5.5.8 (iv)]). Moreover, if \(P \in \mathcal {P}\) , we have \(P^{*} \in \mathcal {P}\) . These facts are also explained in [ 12 , Sect. 3.1].

In this subsection, we prove the following proposition.

Proposition 5.10

Let \(d \ge 0\) be an integer. Let us consider exact sequences

in \(\mathcal {C}\) such that \(P_i, P_i' \in \mathcal {P}\) for \(1 \le i \le d\) . Then the following hold.

If \(M \simeq M'\) , then we have

Similarly, if \(N \simeq N'\) , then we have

Remark 5.11

It is easy to deduce claim (1) from Lemma 5.8 . Indeed, we have

On the other hand, claim (2) cannot be deduced from Lemma 5.8 . Roughly speaking, claims (1) and (2) are respectively what we need to prove Theorems 5.2 and 5.3 .

To prove Proposition 5.10 , it is convenient to use the concept of axiomatic Fitting invariants introduced by the first author [ 12 ]. More concretely, inspired by [ 4 , Sect. 3.2], we use the notion of \(\mathcal {P}\) -trivial Fitting invariant defined as follows.

Definition 5.12

A \(\mathcal {P}\) -trivial Fitting invariant is a map \(\mathcal {F}: \mathcal {C}\rightarrow \Omega \) , where \(\Omega \) is a commutative monoid, satisfying the following properties:

If \(P \in \mathcal {P}\) , we have \(\mathcal {F}(P)\) is the identity element of \(\Omega \) .

For a short exact sequence \(0 \rightarrow M' \rightarrow M \rightarrow P \rightarrow 0\) in \(\mathcal {C}\) with \(P \in \mathcal {P}\) , we have \(\mathcal {F}(M') = \mathcal {F}(M)\) .

For a short exact sequence \(0 \rightarrow P \rightarrow M \rightarrow M' \rightarrow 0\) in \(\mathcal {C}\) with \(P \in \mathcal {P}\) , we have \(\mathcal {F}(M') = \mathcal {F}(M)\) .

It is an important fact [ 12 , Proposition 3.17] that conditions (ii) and (iii) are equivalent to each other (assuming (i)). Note that in this setting we do not have to assume \(\Omega \) is a commutative monoid, and instead a pointed set structure suffices.

A fundamental example of a Fitting invariant is, of course, given by the Fitting ideal; more precisely, the Fitting ideal modulo principal ideals satisfies the axioms of \(\mathcal {P}\) -trivial Fitting invariants.

The following proposition introduces another kind of Fitting invariants.

Proposition 5.13

For \(n \ge 2\) , define \(\mathcal {F}_n: \mathcal {C}\rightarrow \mathbb {N}\) by \(\mathcal {F}_n(M) = r_n(M)\) . We also define \(\mathcal {F}_{0, 1}: \mathcal {C}\rightarrow \mathbb {Z}\) by \(\mathcal {F}_{0, 1}(M) = r_1(M) - r_0(M)\) . Then all the maps \(\mathcal {F}_n\) and likewise the map \(\mathcal {F}_{0, 1}\) are \(\mathcal {P}\) -trivial Fitting invariants.

We check the conditions (i) and (ii). Firstly, (i) is a restatement of Lemma 5.7 . Secondly, (ii) follows from the associated long exact sequence, taking Lemma 5.7 into account again. Indeed, the long exact sequence collapses into isomorphisms for degree \(\ge 2\) and a 6-term exact sequence for degrees 0, 1. \(\square \)

Note that (iii) cannot be shown in a similar manner. This is because the lower degree part of the associated long exact sequence becomes an 8-term exact sequence. It is important that (iii) follows from (i) and (ii).

Proof of Proposition 5.10

By Proposition 5.13 , it is enough to show \(\mathcal {F}(N) = \mathcal {F}(N')\) (resp.  \(\mathcal {F}(M) = \mathcal {F}(M')\) ) if \(M \simeq M'\) (resp.  \(N \simeq N'\) ) for any \(\mathcal {P}\) -trivial Fitting invariant \(\mathcal {F}\) . For this, we apply the theory of shifts \(\mathcal {F}^{\langle d \rangle }(-)\) , \(\mathcal {F}^{\langle -d \rangle }(-)\) of Fitting invariants of the first author [ 12 , Theorem 3.19]. By the exact sequence involving M , N , and \(P_i\) , the definition of the shifts implies

and similarly for \(M'\) , \(N'\) . Then what we have to show is just a reformulation of the well-definedness of the shifts, which is already established by the first author in [ 12 , Theorem 3.19]. \(\square \)

5.6 Proof of Theorem 5.3

Proof of theorem 5.3.

(1) By taking the dual of the Tate sequence, we obtain an exact sequence

where we used \(\mathbb {Z}_p^{*} \simeq \mathbb {Z}_p\) .

As in Sect. 5.3 , we use the specialization method. Let us take any element \(f \in \mathcal {F}_0\) that is coprime to \({{\,\textrm{char}\,}}_{\Lambda }(P)\) and \({{\,\textrm{char}\,}}_{\Lambda }(Q)\) . Put \(m = {{\,\textrm{ord}\,}}_p(f(0)) \ge 1\) . Then ( 5.3 ) yields an exact sequence

Observe that both \(P^{*}/f\) and \(Q^{*}/f\) are G -cohomologically trivial. So we have

where \(\hat{H}^{-1}\) denotes the Tate cohomology group. By definition, \(\hat{H}^{-1}(G, X^*/f)\) is a submodule of \(H_0(G, X^*/f)\) , so the above isomorphism shows

as \(\mathcal {O}_f\) is a DVR. By Nakayama’s lemma, the left hand side is equal to \({{\,\textrm{gen}\,}}_{\Lambda [G]}(X^{*})\) . Also, as in the proof of Theorem 5.2 (1) in Sect. 5.3 ,

where the second equality follows from Lemma 2.9 and the third from Lemma 2.7 . Thus we obtain (1).

(2) In case G is trivial, since the projective dimension of X as a \(\Lambda \) -module is \(\le 1\) , we may apply Lemma 5.7 to obtain the assertion.

From now on, we assume G is non-trivial. Since \(s_n = r_n^{\mathbb {Z}_p[G]}(\mathbb {Z}_p)\) by Lemma 2.5 , a minimal resolution of \(\mathbb {Z}_p\) as a \(\mathbb {Z}_p[G]\) -module is of the form

We truncate it to an exact sequence

Since its dual \((-)^{*}\) is also exact and we have \(\mathbb {Z}_p^{*} \simeq \mathbb {Z}_p\) and \(\mathbb {Z}_p[G]^{*} \simeq \mathbb {Z}_p[G]\) , we obtain an exact sequence

By comparing ( 5.3 ) and ( 5.5 ), Proposition 5.10 (2) implies

for \(n \ge 2\) .

Let us compute \(r_n({{\,\textrm{Ker}\,}}(d_1)^{*})\) for any \(n \ge 0\) . We combine ( 5.5 ) with ( 5.4 ) to an exact sequence

By construction, this is a minimal resolution of \({{\,\textrm{Ker}\,}}(d_1)^{*}\) as a \(\mathbb {Z}_p[G]\) -module. For this we need the hypothesis that G is non-trivial; the map \(\varepsilon ^{*} \circ \varepsilon \) can be identified with the map \(\mathbb {Z}_p[G] \rightarrow \mathbb {Z}_p[G]\) that sends 1 to the norm element \(N_G = \sum _{g \in G} g\) , and \(N_G\) is in the Jacobson radical of \(\mathbb {Z}_p[G]\) if and only if G is non-trivial. Therefore, we obtain

By applying Lemma 5.9 , we obtain

6 Proof of the rest of Theorems 3.3 (1) and 3.4 (1)

Now we return to the arithmetic situation described in Sect. 3.1 .

6.1 Proof of Theorem 3.4 (1)

First we recall the following well-known fact.

We have an isomorphism

In particular, by Nakayama’s lemma we have

To prove Theorem 3.4 (1), we also need the following general lemma.

As in Sect. 5 , consider \(\Lambda = \mathbb {Z}_p[[T]]\) and a finite p -group G . For a finitely generated torsion \(\Lambda [G]\) -module M whose projective dimension is \(\le 1\) , we have

As in Lemma 5.7 , the minimal resolution of M is of the form \(0 \rightarrow \Lambda [G]^a \rightarrow \Lambda [G]^a \rightarrow M \rightarrow 0\) . By taking \({{\,\textrm{Ext}\,}}\) -functor, this induces an exact sequence \(0 \rightarrow \Lambda [G]^a \rightarrow \Lambda [G]^a \rightarrow M^* \rightarrow 0\) , which is again a minimal resolution. Thus we obtain the lemma. \(\square \)

Proof of Theorem 3.4(1)

Applying the dual of the Tate sequence introduced in ( 5.3 ) to our setting, we obtain an exact sequence

By the compatibility of the Tate sequences, we obtain an isomorphism

By Lemma 6.2 (with G trivial), this is then equal to \({{\,\textrm{gen}\,}}_{\Lambda }(X_{k_{\infty }, S})\) . By Lemma 6.1 , this completes the proof. \(\square \)

6.2 Proof of Theorem 3.3 (1)

Note that applying Theorem 5.2 (1) to the Tate sequence introduced in Theorem 4.1 , we already obtained the inequality \(s_2 \le {{\,\textrm{gen}\,}}_{\mathcal {R}}(X_{K_{\infty }, S})\) . Therefore, it remains only to prove \(t \le {{\,\textrm{gen}\,}}_{\mathcal {R}}(X_{K_{\infty },S}) \le s_2 + t\) .

We have an exact sequence

of \(\Lambda \) -modules.

By the Hochschild–Serre spectral sequence we have an exact sequence

where we used the weak Leopoldt conjecture \(H^2({\mathcal O}_{K_{\infty },S},\mathbb {Q}_p/\mathbb {Z}_p)=0\) . For \(i = 1, 2\) , the Pontryagin dual of \(H^i(G, \mathbb {Q}_p/\mathbb {Z}_p)\) is the projective limit of \(H_{i}(G, \mathbb {Z}/p^m)\) by Lemma 2.9 . But since G is a finite p -group, it is finite and isomorphic to \(H_{i}(G, \mathbb {Z})\) . Therefore, taking the Pontryagin dual of the above exact sequence and using Lemma 2.6 , we get the conclusion.

Note that \(G^{{{\,\textrm{ab}\,}}} = {{\,\textrm{Gal}\,}}(M_{k_{\infty }, S} \cap K_{\infty }/k_{\infty })\) . Then putting \(Y={{\,\textrm{Gal}\,}}(M_{k_{\infty },S}/M_{k_{\infty }, S} \cap K_{\infty })\) , by Lemma 6.3 we obtain two exact sequences

The first sequence implies

On the other hand, by taking the \(\Gamma ={{\,\textrm{Gal}\,}}(k_{\infty }/k)\) -coinvariant of the second sequence, we obtain an exact sequence

By Lemma 6.1 , this is reformulated as

This sequence, together with \(M_{k_{\infty },S} \cap K_{\infty }=M_{k,S} \cap K_{\infty }\) and the definition of t , implies that

By combining ( 6.1 ) and ( 6.2 ), we obtain

where we used Lemma 2.7 to get the final equality. Note that if G is abelian, we have \(H_{2}(G, \mathbb {Z}) \simeq \bigwedge ^2 G\) and \({{\,\textrm{gen}\,}}_{\mathbb {Z}_p}(\bigwedge ^2 G)=s(s-1)/2\) (with \(s = {{\,\textrm{rank}\,}}_p G\) ), which imply explicitly

This completes the proof of Theorem 3.3 (1).

7 Numerical examples

In this section, we numerically check the inequality concerning \({{\,\textrm{gen}\,}}_{\mathcal {R}}(X_{K_{\infty }, S})\) in Theorem 1.1 by using the computer package PARI/GP. We consider \(k = \mathbb {Q}\) and a finite abelian p -extension K of k that is totally real. Let S be a finite set of places of \(\mathbb {Q}\) containing p and the archimedean place \(\infty \) , such that \(K/\mathbb {Q}\) is unramified outside S .

The basic method is as follows. First, by Nakayama’s lemma and Lemma 6.1 , we have

We observe that \(M_{K, S}\) is the union of the maximal p -extensions of K in the ray class fields of modulus \(p^m \prod _{v \in S \setminus \{p\}} v\) for all \(m \ge 0\) . Note here that, since K is abelian over \(\mathbb {Q}\) , the Leopoldt conjecture is shown to be true for K by work of Brumer (see [ 13 , Theorem 10.3.16]). Therefore, \({{\,\textrm{Gal}\,}}(M_{K, S}/K_{\infty })\) is finite, and we can compute it by computing the ray class groups for finitely many m . In this way we can determine the quantity \({{\,\textrm{gen}\,}}_{\mathbb {Z}_p}({{\,\textrm{Gal}\,}}(M_{K, S}/K_{\infty })_{{{\,\textrm{Gal}\,}}(K/\mathbb {Q})})\) .

7.1 The case \(p = 3\)

Let us take \(p = 3\) , though the discussion is basically valid for any odd prime p . We write \(S \setminus \{p, \infty \} = \{\ell _1, \dots , \ell _s\}\) . By the theorem of Kronecker–Weber, the Galois group of the maximal abelian extension of \(\mathbb {Q}\) unramified outside S is isomorphic to \(\mathbb {Z}_p^{\times } \times \prod _{i = 1}^s \mathbb {Z}_{\ell _i}^{\times }\) , so we have

As is well-known, we may assume \(\ell _i \equiv 1 (\bmod \ p)\) for \(1 \le i \le s\) without loss of generality. For such an S , let us take K as the unique intermediate field of \(\mathbb {Q}(\mu _{\ell _1}, \dots , \mu _{\ell _s})/\mathbb {Q}\) such that

In this case, the above information on \(M_{\mathbb {Q}, S}\) implies that

so the integer t in Theorem 1.1 is determined as \(t = \# \{1 \le i \le s \mid \ell _i \equiv 1 (\bmod \ p^2)\}\) . To ease the notation, we write \({{\,\textrm{gen}\,}}(X) = {{\,\textrm{gen}\,}}_{\mathbb {Z}_p[[{{\,\textrm{Gal}\,}}(K_{\infty }/\mathbb {Q})]]}(X_{K_{\infty }, S})\) . Then Theorem 1.1 asserts

Let P be the set of all prime numbers \(\ell \) such that \(\ell \equiv 1 (\bmod \ p)\) . We divide P into two subsets \(P_1\) and \(P_2\) defined by

Then we have \(t = \# \{1 \le i \le s \mid \ell _i \in P_2\}\) .

First we consider the case \(t = 0\) , that is, \(\ell _{1}, \dots , \ell _{s}\) are s distinct primes in \(P_1\) . Then Theorem 1.1 asserts \({{\,\textrm{gen}\,}}(X) = s(s+1)/2\) as in Remark 1.2 (1). By using PARI/GP, we numerically checked \({{\,\textrm{gen}\,}}(X) = 1\) if \(s = 1\) and \(\ell _1 \le 100\) , and \({{\,\textrm{gen}\,}}(X) = 3\) if \(s = 2\) and \(\ell _1, \ell _2 \le 100\) , as Theorem 1.1 says.

Suppose that \(t = 1\) and \(s = 1\) , that is, \(S=\{3, \ell _1, \infty \}\) with \(\ell _1 \in P_2\) . Then Theorem 1.1 asserts

By numerical computation, we find only \({{\,\textrm{gen}\,}}(X) = 2\) in the range \(\ell _1 \le 200\) , and did not encounter \({{\,\textrm{gen}\,}}(X) = 1\) .

Next consider the case \(t = 1\) and \(s = 2\) , that is, \(S=\{3, \ell _1, \ell _2, \infty \}\) with \(\ell _1 \in P_1\) and \(\ell _2 \in P_2\) . Then we have

by Theorem 1.1 . In the range \(\ell _1 < 100\) and \(\ell _2 < 200\) , we find \({{\,\textrm{gen}\,}}(X)=3\) except for

for which \({{\,\textrm{gen}\,}}(X) = 4\) . Thus the above inequality on \({{\,\textrm{gen}\,}}(X)\) is sharp in this case.

Finally we consider the case \(t = 2\) and \(s = 2\) , that is, \(S=\{3, \ell _1, \ell _2, \infty \}\) with \(\ell _1\) , \(\ell _2 \in P_2\) . Then Theorem 1.1 says

By numerical computation, in the range \(\ell _1 < \ell _2 \le 200\) , we find \({{\,\textrm{gen}\,}}(X) = 4\) except for \((\ell _1, \ell _2) = (109, 199)\) , for which \({{\,\textrm{gen}\,}}(X) = 5\) . We did not encounter \({{\,\textrm{gen}\,}}(X) = 3\) in this situation.

Due to the limitation of the machine power we could not handle \(s \ge 3\) when \(p = 3\) .

7.2 The case \(p = 2\)

Let us discuss the case \(p = 2\) . Suppose that \(S=\{2, \ell _1, \dots , \ell _s, \infty \}\) where \(\ell _1, \dots , \ell _s\) are s distinct odd prime numbers. We have

Let us set K as \(K = \mathbb {Q}(\sqrt{\ell _1}, \dots , \sqrt{\ell _s})\) , so we have \({{\,\textrm{Gal}\,}}(K/\mathbb {Q}) \simeq (\mathbb {Z}/2\mathbb {Z})^s\) and

As in the previous subsection, we define P to be the set of all odd prime numbers, \(P_2\) to be the subset of P consisting of \(\ell \) such that \(\ell \equiv 1 (\bmod \ 4)\) , and \(P_1=P \setminus P_2\) . The integer t in Theorem 1.1 is then \(t = 1 + \# \{1 \le i \le s \mid \ell _i \in P_2\}\) .

We first consider the case \(s=1\) , so \(K=\mathbb {Q}(\sqrt{\ell _1})\) . If \(\ell _1\) is in \(P_1\) , then \(t=1\) and Theorem 1.1 says \(1 \le {{\,\textrm{gen}\,}}(X) \le 2\) . For \(\ell _1 \in P_1\) less than 100, we always have \({{\,\textrm{gen}\,}}(X) = 2\) . If \(\ell _1\) is in \(P_2\) , then \(t=2\) and Theorem 1.1 says \(2 \le {{\,\textrm{gen}\,}}(X) \le 3\) . For \(\ell _1 \in P_2\) less than 100, we have \({{\,\textrm{gen}\,}}(X) = 2\) except for \(\ell _1=73\) , 89, 97, for which we have \({{\,\textrm{gen}\,}}(X) = 3\) . So in this case the inequality is sharp.

In the following we focus on the case \(\ell _1, \dots , \ell _s\) are all in

Then \(t=s+1\) , and the inequality in Theorem 1.1 becomes

for \(s \ge 2\) .

For \(s=2\) , we have

by Theorem 1.1 . By numerical computation, we find \({{\,\textrm{gen}\,}}(X) = 4, 5, 6\) in the range \(\ell _1 < \ell _2 \le 100\) . Concretely, we have \({{\,\textrm{gen}\,}}(X) = 6\) if

\({{\,\textrm{gen}\,}}(X) = 5\) if

and \({{\,\textrm{gen}\,}}(X) = 4\) otherwise.

For \(s=3\) , Theorem 1.1 says

For \(\ell _1 = 5\) and \(5< \ell _2 < \ell _3 \le 100\) , we have \({{\,\textrm{gen}\,}}(X)=7\) except for

for which we have \({{\,\textrm{gen}\,}}(X)=8\) . Also, we find examples for \({{\,\textrm{gen}\,}}(X) = 9, 10\) by taking respectively \((\ell _1, \ell _2, \ell _3) = (17, 73, 89), (73, 89, 97)\) .

7.3 A variant

As a final remark, let us briefly discuss a variant that matters only when \(p = 2\) . So far we always assumed that S contains all archimedean places, so we studied the narrow class groups. Theoretically this assumption is necessary to use the Tate sequence in Theorem 4.1 . However, the numerical computation in this section is possible (and simpler) even if we remove the archimedean places from S .

Suppose that \(p=2\) , \(k=\mathbb {Q}\) , \(K=\mathbb {Q}(\sqrt{\ell _1}, \dots , \sqrt{\ell _s})\) as in the previous subsection, and \(S'=S \setminus \{\infty \}\) . We consider \({{\,\textrm{gen}\,}}_{\mathbb {Z}_p[[{{\,\textrm{Gal}\,}}(K_{\infty }/\mathbb {Q})]]}(X_{K_{\infty }, S'})\) , which we abbreviate as \({{\,\textrm{gen}\,}}(X')\) . We have a natural surjective homomorphism from \(X_{K_{\infty }, S}\) to \(X_{K_{\infty }, S'}\) whose kernel is a cyclic module (since \(k = \mathbb {Q}\) ). Therefore, we have

Still assuming \(\ell _i \equiv 1 (\bmod \ 4)\) for any \(1 \le i \le s\) , we find the following numerical examples.

When \(s = 1\) , we find examples for \({{\,\textrm{gen}\,}}(X') = 1, 2\) .

When \(s = 2\) , we find examples for \({{\,\textrm{gen}\,}}(X') =3, 4, 5\) .

When \(s = 3\) , we find examples for \({{\,\textrm{gen}\,}}(X') = 6, 7, 8, 9\) .

These results suggest that \(s(s+1)/2 \le {{\,\textrm{gen}\,}}(X') \le s(s+1)/2 + s\) , but this does not follow directly from Theorem 1.1 . The above computations suggest that Theorem 1.1 holds true without assumption that S contains all archimedean places.

Data availability

The code used to generate the results presented in Sect. 7 is available from the authors upon a reasonable request.

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Acknowledgements

The authors would like to thank Yuta Nakamura, who computed \({{\,\textrm{gen}\,}}_{\mathcal {R}}(X_{K_{\infty }, S})\) for several examples in his master’s thesis in a slightly different situation from ours. They also thank Cornelius Greither heartily for his interest in the subject of this paper and for giving them some valuable comments. They are very grateful to the anonymous referees for their careful reading. The first and the second author is supported by JSPS KAKENHI Grant Numbers 22K13898 and 22H01119, respectively.

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Takenori Kataoka

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Kataoka, T., Kurihara, M. Minimal resolutions of Iwasawa modules. Res. number theory 10 , 64 (2024). https://doi.org/10.1007/s40993-024-00549-y

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Received : 23 April 2024

Accepted : 10 June 2024

Published : 21 June 2024

DOI : https://doi.org/10.1007/s40993-024-00549-y

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