How to Write the Introduction of a Research Paper: Steps, Outline, Examples

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Contents

• Glossary of Key Terms
• Key Takeaways
• What Does an Introduction Actually Need to Do?
• Argumentative Papers Versus Empirical Papers: What Changes?
• What Are the Five Components of an Introduction?
• How Long Should an Introduction Be, and How Many Citations Does It Need?
• What Language Helps Make the Gap to Purpose Pivot?
• Examples of Effective Introduction Sections
• How Do These Five Components Differ Across Disciplines?
• Common Pitfalls When Writing an Introduction
• Template for an Introduction Section
• Frequently Asked Questions

Glossary of Key Terms

This glossary defines the core terms used throughout this guide. Reviewing it first will make the rest of the guide easier to follow.

Term	Definition
Hook	The opening sentence or sentences of an introduction, written to establish relevance and draw the reader in.
CARS model	A three move framework (Create a Research Space) for understanding how academic introductions are typically built: establish a territory, establish a niche, occupy the niche.
Territory	The first CARS move: showing that a topic is important and providing background context.
Niche	The second CARS move: identifying a gap, limitation, or controversy that justifies new research.
Gap statement	A sentence or short passage that explains what remains unknown, untested, or unresolved in the existing literature.
Thesis statement	A clear, direct statement of the position an argumentative paper will defend; used in place of a research question in non empirical papers.
Research question	A direct or indirect question that states what an empirical study set out to answer.
Hypothesis	A specific, testable prediction about the relationship between variables, usually stated in the past tense once a study is complete.
IMRaD	A common paper structure: Introduction, Methods, Results, and Discussion.
Roadmap paragraph	An optional closing part of the introduction that previews the structure of the rest of the paper, section by section.
Mini literature review	A short, selective summary of the most relevant prior research, included in the introduction rather than as a separate chapter.

Key Takeaways

• An introduction moves from broad context to a single, specific research question; this shape is often called the funnel structure.
• The three core moves are: establish the topic’s importance, identify a gap in current knowledge, and state your specific purpose.
• Empirical papers end the introduction with a research question and, often, a hypothesis; argumentative papers end with a thesis statement.
• Background sections should stay selective; a full literature review belongs elsewhere, not in the introduction.
• A clear gap statement, often signaled by a phrase such as “despite this” or “however,” is the hardest sentence to write and the most important one to get right.
• Introductions typically run about ten percent of a paper’s total word count, commonly five hundred to one thousand words for a journal article.
• Citation habits, gap framing, and roadmap paragraphs differ noticeably across social science, biomedical science, and physical science writing.
• Many writers find it easier to draft or finalize the introduction after completing the methods, results, and discussion sections.

What Does an Introduction Actually Need to Do?

An introduction needs to move the reader from a broad, shared body of knowledge down to the one specific, unanswered question a paper will address, then explain how the paper will address it.

A strong introduction accomplishes five jobs:

• Hook the reader and establish the topic’s relevance.
• Supply background information or a mini literature review.
• Identify the gap, problem, or controversy the study responds to.
• State a specific objective: a thesis statement, research question, or hypothesis.
• Optionally, map out the structure of the rest of the paper.

What Is the CARS Model and Why Does It Matter?

The CARS model describes the three moves nearly every academic introduction makes: establish a territory, establish a niche, and occupy that niche with your own contribution.

Move	Plain language version	What it accomplishes
Establish a territory	Show the topic matters and give background	Orients the reader and demonstrates relevance
Establish a niche	Identify the gap, limitation, or controversy	Justifies why new research is needed
Occupy the niche	State your purpose, question, and hypothesis	Announces your specific contribution

This shape, broad to narrow to specific, is the single most consistent piece of advice for writing introductions across disciplines. Picture an inverted triangle: wide at the top, narrowing steadily until it ends in one precise research question.

Should I Write the Introduction First or Last?

Most writers find it easier to finish the introduction last, even though readers see it first, because the gap and contribution are easier to state accurately once the rest of the paper is written.

It is common practice to draft or substantially revise the introduction after the methods, results, and discussion sections are complete. Trying to write a polished introduction before you fully know what you found often forces rewrites later, so many researchers treat the first draft of the introduction as a placeholder.

Argumentative Papers Versus Empirical Papers: What Changes?

The main difference is the closing move: argumentative papers end the introduction with a thesis statement, while empirical papers end with a research question, a hypothesis, or both.

Paper type	What it does	How the introduction ends
Argumentative or humanities style	Builds a case from existing sources	A clear, direct thesis statement
Empirical or original research	Reports a study the author conducted	A research question and, often, a hypothesis

Most social science, biomedical, and physical science research papers are empirical, so the rest of this guide focuses primarily on that structure, while noting where an argumentative framing would differ.

What Are the Five Components of an Introduction?

An empirical introduction reliably contains five components, usually in this order: a hook, background, a gap statement, a purpose statement, and an optional roadmap.

Component 1: The Hook

The hook is the opening sentence or two, written to orient an outside reader and signal relevance without resorting to a sweeping generality.

A hook should already gesture toward the specific subfield rather than opening with something like “Throughout history, humans have…”. For example, a paper about soil should not open with “Agriculture is important”; it should open with something closer to “Sustainable crop production depends on the physical, chemical, and biological properties of soil.”

Component 2: Background, or the Mini Literature Review

Background sections summarize the most relevant prior research in a few sentences to a few paragraphs, narrowing steadily from the field, to the subfield, to the specific line of work.

This is not a full literature review chapter. Citations should be selective, and review articles are often a better citation choice than every individual primary source when a field is large.

Component 3: The Gap or Problem Statement

The gap statement is the pivot point of the entire introduction; it explains what remains unknown, untested, or unresolved despite the prior work just summarized.

This move usually requires mentioning past attempts to address the problem and clarifying exactly how the present research differs from them.

Component 4: The Purpose Statement, Research Question, or Hypothesis

Element	What it is	When it’s used	Example phrasing
Purpose statement/Research Objective	A general statement of what the study aims to do	Common in any empirical paper, often as a lead-in to the more specific question or hypothesis	“This study aims to evaluate the effect of X on Y.”
Research question	A direct or indirect question the study is designed to answer	Used when the goal is exploratory or when a precise prediction isn’t yet warranted	“Does X affect Y?” or “We investigated whether X affects Y.”
Hypothesis	A specific, testable prediction about the relationship between variables	Used when prior evidence supports a directional prediction, common in confirmatory or experimental research	“We hypothesized that increased X would lead to decreased Y.”

A purpose statement is the broadest of the three, a research question narrows it into something answerable, and a hypothesis goes one step further by predicting the answer in advance. Many introductions use a purpose statement to frame the study, then follow it with either a research question or a hypothesis, not always both.

This is the single most specific sentence or set of sentences in the introduction, and it can take several grammatical forms.

• A direct question, for example: does X affect Y?
• A numbered set of hypotheses, for example: H1, H2, H3.
• An infinitive phrase, for example: to examine the effect of X on Y.
• A declarative statement, for example: we investigated whether X affects Y.

Component 5: The Roadmap Paragraph

The roadmap is an optional closing paragraph that outlines the structure of the rest of the paper, and it is most useful when the paper does not follow a predictable structure.

Roadmaps are common in economics, engineering and technology papers and far less common in medicine, life sciences, and psychology, where the standard introduction, methods, results, and discussion structure is simply assumed.

How Long Should an Introduction Be, and How Many Citations Does It Need?

A typical introduction runs about ten percent of a paper’s total word count, often five hundred to one thousand words, though specific journal guidelines should always take priority.

Guideline	Typical range
Share of total paper length	Roughly ten percent of the body word count
Example for a 4000-word paper	400-600 words across two to three paragraphs
Paragraph length	Roughly 150-300 words each
General target for journal articles	500-1000 words

Citation density matters as much as length. Over citing is a common failure mode: a sentence such as “many studies have found a significant association between X and Y” followed by a long string of citation numbers fails to differentiate what each study actually found. The fix is to split a broad claim into narrower sub-claims, each supported by its own smaller citation cluster.

Unlike the methods section, which often needs few or no citations, the introduction requires citations throughout to support background claims, prior findings, and the identified gap.

What Language Helps Make the Gap to Purpose Pivot?

A small set of transition phrases reliably signals the shift from summarizing prior work to identifying what is missing, which is consistently the hardest sentence to write in the whole introduction.

Phrase pattern	What it signals
Although X has been studied in detail, insufficient attention has been paid to Y.	You are filling a previously overlooked angle.
The implications of a prior study deserve to be explored further.	You are extending or deepening existing work.
It is generally assumed that X; however, this paper suggests that Y.	You are challenging the consensus.
Despite this, the topic or method has yet to be robustly researched.	A straightforward gap statement.

Examples of Effective Introduction Sections

The three annotated examples below show the same five component structure applied to social science, biomedical science, and physical science writing, with bracketed labels marking each component and italicized notes marking the transitions between them.

Annotated Example: Social Science

This survey-based example follows the funnel structure from a societal trend down to a single testable hypothesis about adolescent social media use.

[Hook] The rise of social media has been accompanied by a sharp increase in the prevalence of body image issues among women and girls.

Transition: from a broad societal claim to a scoped review of prior literature.

[Background] This correlation has received significant academic attention. Various empirical studies have examined Facebook usage among adolescent girls, consistently finding that the platform’s visual and interactive features exert the greatest influence on body image concerns. Subsequent work has extended these findings to broader samples and additional outcome measures, reinforcing the idea that image centric platforms carry distinct psychological risks compared with text based ones.

Transition: narrowing from social media in general to the one untested platform.

[Gap statement] Despite this, highly visual social media such as Instagram, which differs structurally from Facebook in its emphasis on curated, filtered imagery, have yet to be robustly researched. It remains unclear whether the effects documented for Facebook generalize to platforms built around continuous image consumption.

Transition: pivot sentence announcing the present study.

[Purpose and research question] This paper sets out to address this research gap. We investigated the effects of daily Instagram use on the prevalence of body image issues among adolescent girls. It was hypothesized that daily Instagram use would be associated with an increase in body image concerns and a decrease in self-esteem ratings.

Transition: optional roadmap, often dropped in social science journal articles.

[Roadmap, optional] The remainder of this paper first describes the survey methodology, then presents results from a sample of three hundred and twelve adolescent girls, before discussing implications for platform design and adolescent media literacy interventions.

This example relies on named author and date citations, consistent with social science convention, and frames its gap around a structural difference between platforms, a common move when prior work does not clearly transfer to a new context.

Annotated Example: Biomedical Science

This clinical example moves quickly from a public health burden statement to a tightly framed comparative hypothesis, in keeping with biomedical convention.

[Hook] Healthcare-associated infections caused by multidrug resistant organisms remain a leading cause of preventable illness and death in hospitalized patients worldwide.

Transition: from the broad clinical problem to the specific intervention category.

[Background] Environmental contamination of patient rooms is now recognized as a major reservoir for transmission of these organisms. Standard terminal cleaning protocols reduce, but do not eliminate, residual contamination, and several disinfection technologies, including ultraviolet light, hydrogen peroxide vapor, and enhanced manual cleaning bundles, have each demonstrated reductions in surface contamination in single site trials.

Transition: from individual technologies working in isolation to the unanswered comparative question.

[Gap statement] However, these interventions have rarely been compared directly within the same hospital system, and their relative effectiveness at the level of hospital-wide infection rates, rather than surface contamination alone, remains poorly characterized.

Transition: pivot to the present study’s design.

[Purpose and objective] We aimed to assess the comparative effectiveness of four disinfection strategies on hospital wide incidence of multidrug resistant organisms and Clostridioides difficile across a multisite hospital network. We hypothesized that combined ultraviolet light and enhanced manual cleaning would be associated with the greatest reduction in incidence relative to standard terminal cleaning alone.

Transition: a roadmap paragraph is rare in clinical papers, since the standard structure is assumed.

[Roadmap, usually omitted] Most clinical journals skip this step entirely, moving directly from the stated objective into the methods section.

Biomedical introductions tend to be terser about the gap and move quickly to a formally stated aim and hypothesis. Medicine is also the field most likely to withhold a preview of results and most likely to use numbered citation styles rather than narrative author and date citations.

Annotated Example: Physical Science

This materials science example demonstrates how to show, rather than simply state, why a technical gap matters, and it includes the roadmap paragraph common in engineering writing.

[Hook] The rapid growth of lithium-ion batteries and their expanding use in applications such as electric vehicles and grid scale energy storage demands more reliable methods for predicting battery performance and degradation over time.

Transition: from the application context to the specific scientific bottleneck.

[Background] Recent advances in synchrotron X ray tomography have enabled three dimensional imaging of electrode microstructure at sub micron resolution, revealing that capacity fade is closely linked to the evolution of porosity and crack networks within the cathode. Computational models incorporating these microstructural features have improved predictions of cycle life under laboratory conditions.

Transition: from models that work in the lab to models that fail under realistic conditions; this is the show, do not tell move.

[Gap statement] These models, however, have been validated almost exclusively under constant current cycling protocols. Electric vehicle batteries are routinely subjected to highly variable current profiles during regenerative braking and rapid acceleration, and it is not known whether microstructure-based degradation models remain accurate under such dynamic loading, a gap with direct consequences for real world battery lifetime prediction and warranty design.

Transition: pivot to the present study, framed with an infinitive construction.

[Purpose and objective] In this work, we aim to evaluate the predictive accuracy of microstructure resolved degradation models under realistic, variable current driving profiles, using operando X ray tomography to track electrode evolution over five hundred simulated drive cycles. We hypothesize that crack propagation rates under variable loading will exceed those predicted by constant current models by a significant margin.

Transition: a roadmap paragraph, common in engineering and physical science papers.

[Roadmap] Section II describes the experimental cell design and imaging protocol. Section III presents the tomography and electrochemical results. Section IV compares observed degradation rates with model predictions, and Section V discusses implications for battery management system design.

This example demonstrates the show, do not tell principle directly: rather than asserting that predicting degradation is important, it explains what breaks, namely warranty design and lifetime prediction, if the problem goes unsolved.

How Do These Five Components Differ Across Disciplines?

The table below compares hook style, citation convention, gap framing, objective phrasing, and roadmap use across the three example disciplines.

Component	Social science	Biomedical science	Physical science
Hook	A social trend or societal statistic	A clinical burden statistic	An applied or technological driver
Citation style	Narrative, author and date	Numbered, often clustered	Numbered, tied to specific techniques
Gap framing	Untested in a new population or context	Interventions not compared directly	Models fail under realistic conditions
Roadmap use	Optional, often omitted except in economics and econometrics	Rare, almost always omitted	Common, especially in engineering

Common Pitfalls When Writing an Introduction

The most common pitfalls are starting too broad, writing a full literature review instead of a brief one, stacking citations without differentiating them, and telling readers a topic matters instead of showing them why.

• Starting too broad and staying there, such as opening with “since the dawn of time, humans have sought…”; the fix is to open with the narrowest true general statement, not the broadest one.
• Writing a full literature review instead of a mini one; cite selectively, summarize trends rather than every paper, and use review articles to compress large bodies of work.
• Stacking citations without differentiating them; break broad claims into sub-claims, each with its own narrower citation cluster.
• Telling instead of showing importance; demonstrate the downstream consequence rather than simply asserting that a topic matters.
• Burying too much methodological or results detail in the introduction; save granular numbers for the results section.
• Forgetting the gap to purpose pivot sentence; without an explicit transition, readers cannot tell where background ends and the contribution begins.
• Treating the introduction as something that must be finalized first; it is often easier to state the gap and contribution accurately after the rest of the paper is drafted.

Template for an Introduction Section

Yes. The template below summarizes the five components in order, with approximate sentence counts for each part.

Part	Length	Purpose
Hook	One to two sentences	A specific, true, attention getting claim already situated in your subfield
Background	Two to five sentences	Narrow from field to subfield to specific line of work, citing selectively
Gap	One to three sentences	Name precisely what is missing, untested, or unresolved
Purpose	One to three sentences	State the research question, hypothesis, or thesis directly

A roadmap, when used, typically adds one sentence per major section of the paper, and should only be included when the paper’s structure is not a standard, predictable one.

Frequently Asked Questions

Is it acceptable to use an AI writing tool to draft an introduction?

Most journals and institutions allow AI tools like Paperpal for brainstorming, outlining, or polishing language, but expect the underlying ideas, gap analysis, and claims to be the author’s own and require disclosure if AI assistance was used. Policies vary widely by journal and institution, so check the specific submission guidelines before relying on one, and always verify that any citations or factual claims an AI tool produces are accurate, since fabricated references are a known risk.

Can I reuse the introduction from an earlier paper of mine in a new one?

Reusing substantial blocks of text from your own previously published introduction is generally considered self-plagiarism by most journals, even though you wrote the original words. It is fine to reuse the same background facts or general framing, but the wording, citations, and gap statement should be rewritten for the new paper, and any closely related prior work should be cited rather than silently recycled.

What is the difference between an introduction and a background section?

When a journal asks for both, the introduction is usually short and gives a high level overview of the topic and study objectives, while the background section is longer and includes the theoretical framework and relevant preliminary data. When only one section exists, it typically combines both functions, opening broad and narrowing into the specific objective as described throughout this guide.

Introduction vs Background Section

Element	Introduction	Background
Length	Short, usually a few paragraphs	Longer, can run several pages or a full section
Focus	High-level overview of the topic, gap, and study objective	Theoretical framework, detailed prior research, and context needed to understand the study
Purpose	Orients the reader and states what the paper will do	Builds the foundation of knowledge the reader needs before the methods
When used separately	Appears first, stays brief and forward-looking	Often follows the introduction as its own section, especially in theses or longer papers
When combined	Most journal articles merge both into a single section that opens broad and narrows into the objective	Not applicable, background content is folded into the introduction itself

Whether these are split into two sections or combined into one depends mainly on the journal or institution’s required format. When only one section exists, it does the work of both: establishing relevance, summarizing relevant prior work, and ending in the specific research gap and purpose.

My advisor says my introduction is too long. What should I cut first?

Start by trimming the background section rather than the hook or the purpose statement, since reviewers and advisors most often flag an overlong literature summary. Look for

1. citation clusters that repeat the same point,
2. sentences that restate something already said in the abstract, and
3. any specific results or numbers that belong in the results section instead.

Do reviewers actually read the entire introduction carefully?

Reviewers generally read the introduction closely because it is where they judge whether the study’s premise, novelty, and significance are sound, and a weak or unclear gap statement is one of the most common reasons a paper is criticized at the review stage. A confusing or padded introduction can color a reviewer’s expectations for the rest of the paper, even if the methods and results are strong.

How is a thesis introduction different from a journal article introduction?

A thesis or dissertation introduction is typically much longer, often around ten percent of the total thesis word count, and may run several thousand words because it often previews multiple chapters or studies rather than a single result. A thesis introduction also commonly includes its own roadmap of chapters, which is far more standard practice in a thesis than in a typical journal article.

Is it a problem if my introduction does not match the order of my methods or results sections?

A slight mismatch in order is common and usually not a problem, since the introduction is organized around the logic of the argument rather than the chronology of the study. If the mismatch is large enough that a reader cannot connect the stated objectives to what the results section eventually reports, that is a sign the purpose statement needs to be revised for clarity rather than the introduction restructured to mirror the results.

Should I cite my own prior work in the introduction?

Citing your own relevant prior work is normal and often expected when it directly relates to the gap you are addressing, but excessive self-citation, especially citing your own work in places where it is not the most relevant source, is viewed unfavorably by many reviewers and editors. As a general guideline, your own work should appear only where it is genuinely the most directly relevant source, not as a default habit.