Peer review depends on one foundational assumption: that as a peer reviewer, you are evaluating a manuscript on its scientific merits alone. A conflict of interest (COI), i.e., any circumstance that could bias your judgment or be perceived as doing so, puts that assumption at risk. Undisclosed COI is one of the most serious ethical violations a reviewer can commit, and the consequences for the author, the journal, and your own reputation can be significant. 

This explores COI in peer review, why the grey areas matter as much as the obvious cases, and how to handle situations where you are uncertain about declaring a conflict or recusing yourself from the review. 

What counts as a conflict of interest? 

Most reviewers know to flag a direct financial stake in the research they are reviewing. But COI is considerably broader than that. The standard definition used by the Committee on Publication Ethics (COPE), International Committee of Medical Journal Editors (ICMJE), and most major publishers describes a COI as any relationship or circumstance that could inappropriately influence, or be perceived to influence, your judgment as a reviewer. The word “perceived” is importanthere. It means that you are not only accountable for actual bias, but for the appearance of it. 

Broad types of conflicts 

Create a table with two columns = Category and Examples. List the examples as bullet points 

• Financial interests: Holding stock in a company whose product is the subject of the research; receiving consultancy fees, speaker honoraria, or grants from a funder with a stake in the outcome; having a financial relationship with a competing company that would benefit from a negative review. 

• Professional relationships: Co-authoring a paper with any of the manuscript’s authors within the past three to five years (the exact window varies by journal); sharing an institutional affiliation; having supervised or been supervised by one of the authors; collaborating on an active grant or research project. 

• Personal relationships: Close friendships or family connections with authors; personal animosity or a professional dispute that could lead you to unfairly disadvantage a manuscript.

There is also a fourth category that could be difficult for reviewers to identify by themselves: intellectual or competitive conflict. If you are working on closely related research or competing to be first to publish similar findings, your review may be colored by your own ambitions, even unconsciously. A reviewer with a direct stake in seeing a competing team’s paper delayed or rejected has a genuine COI, even if there is no financial relationship whatsoever. 

Why undisclosed COI is treated as serious misconduct 

Failing to disclose a conflict of interest is not a procedural oversight; it is considered active scientific misconduct as per COPE guidelines. This is because the consequences of undisclosed conflicts can go beyond that manuscript. 

When an undisclosed COI comes to light after publication, the journal is required to investigate. Depending on the type of conflict and its perceived impact, the outcome could be a published correction noting the oversight, a whole new peer review process with a different set of reviewers, or even retraction. COPE’s retraction guidelines explicitly list “compromised peer review” as grounds for retraction. In one documented case, a manuscript was accepted following a positive review, but it later emerged that the reviewer was a close collaborator of the research group, but a COI was not declared. The paper was retracted, and the manuscript was sent out for a fresh round of review. 

When undisclosed conflicts of interest in peer review come to light, they also tarnish the reputation of the stakeholders involved. The authors, whose work may be entirely valid, are tainted by association with a compromised process. The journal’s credibility is questioned. And the reviewer, even if the bias never actually affected the review, faces a charge of misconduct that can follow them through their career. In cases of misconduct such as undisclosed conflicts of interest, journals undertake a detailed investigation, the results of which are shared with the institutions of the authors and peer reviewers.  

How to deal with grey areas 

Most experienced reviewers know to decline a review invitation for a paper by their supervisor or collaborator. However, there are cases where conflicts of interest are not always so obvious. 

Former collaborations. Most journals set a three-to-five-year window for prior co-authorship or collaboration as a threshold for mandatory recusal. But what about a collaboration that ended six years ago, that too in a niche field where your paths cross regularly? The formal rule may not require disclosure, but if there is any genuine closeness in the relationship, it is worth flagging with the editor and letting them make the call. 

Competitive relationships. Reviewing work from a direct competitor, a team whose publications you are actively trying to outpace, is a COI that many reviewers underestimate. Your own research ambitions are a secondary interest that can lead to a bias in your assessment, even if that is unintentional or unconscious. If you are in direct competition with the authors, disclose it. 

Ideological or intellectual disagreement. Strong prior disagreement with an author’s theoretical framework or ideological position can color your reading of their work. This is complex to navigate because some intellectual disagreement is healthy and even desirable during peer review. But if you have a documented, public dispute with an author, particularly one conducted in print, it is worth flagging. The editor can decide whether it is material. 

Institutional proximity. Sharing a large university with an author is not automatically a conflict. Sharing a department or a building where you interact regularly is a different matter. The operative question is whether the relationship could reasonably affect your objectivity. 

Here’s a reliable test across all these grey areas. Ask yourself whether knowing about your conflict of interest would lead a reader of your review to question your objectivity. If the answer is “yes” or even “maybe,” you need to disclose the conflict of interest. Editors would rather have more information than less, and a brief note flagging a potential conflict shifts the decision-making responsibility where it belongs: with them. 

What to do when you identify a conflict of interest in peer review 

The moment you receive a review invitation, check for conflicts first. Doing this later will make it harder for you to assess your own impartiality. 

If you identify a clear conflict, the right action is to decline the invitation and tell the editor why. This is not a failure of any kind. It is exactly what the system asks of you as a responsible and ethical reviewer. 

If the conflict is minor or ambiguous, you can accept the invitation but proactively let the journal editor know before the review and in your peer review report. Transparency will allow the editor to factor your disclosure into how they weigh your review. Some journals have specific fields for this in their submission systems; if yours does not, a brief covering note works just as well. 

Always document your COI disclosures as useful protection if questions arise later. 

Identifying undisclosed COI in a manuscript 

Your responsibilities as a reviewer are not limited to managing your own conflicts. You are also well-placed to notice when an author’s conflicts of interest appear to be undisclosed or insufficiently declared. If the funding acknowledgements, institutional affiliations, or the nature of the research raise questions that the COI statement fails to address, you should flag this for the editor. 

Examples of this include a clinical trial with pharmaceutical industry funding that is not mentioned in the COI statement, or an evaluation of a software product developed by one of the co-authors. These are not necessarily grounds for rejection, but they are things the editor needs to know about.  

Join the ReviewerOne community to access more resources and tips on best and ethical peer review practices. Sign up here to share experiences, ask questions, and grow as a reviewer. 

Resources 

COPE Flowchart: Undisclosed Conflict of Interest in a Published Article: Practical decision-tree guidance for what happens when a COI comes to light after publication. Useful for understanding the full chain of consequences. 

COPE Core Practices on Conflicts of Interest: The foundational standards that member journals commit to upholding, covering definitions, processes, and expectations for all parties in the publication process. 

PLOS: Competing Interests for Reviewers: A clear, worked explanation of how PLOS defines competing interests for reviewers, with practical examples that apply beyond the PLOS ecosystem. 

ICMJE Recommendations: Conflicts of Interest: The authoritative standard for COI disclosure across medical and many scientific journals. Reviewers benefit from understanding what authors are being asked to declare, so they can assess completeness. 

Elsevier: Undisclosed Conflicts of Interest: Elsevier’s editorial guidance on handling undisclosed COI, including what editors are expected to do when conflicts are discovered during or after review. Valuable for understanding the process from the editor’s perspective. 

Resnik & Elmore (2018): Conflict of Interest in Journal Peer Review: A peer-reviewed analysis of how COI manifests specifically in the reviewer and editor roles, with recommendations for journal policy. The paper that most thoroughly addresses the reviewer-side gap in COI research.