A peer reviewer being added to the author list of the manuscript they reviewed looks, on its face, like exactly the kind of backchannel arrangement peer review is designed to prevent: the person judging the work ends up credited for it. COPE (the Committee on Publication Ethics) has addressed this scenario directly through several published case discussions, and its position is narrower than either “never allowed” or “fine if the author agrees.” A reviewer can legitimately become a co-author, but only if the contribution genuinely meets standard authorship criteria, the editor — not the author or the reviewer — verifies that independently, and the manuscript then goes back out for a fresh, independent re-review.
Why this is treated as a red flag by default
Peer review exists to give a manuscript an independent, arm’s-length evaluation. Most journals run single- or double-anonymized review (see CASRAI’s single-anonymized and double-anonymized peer review definitions), where direct contact between reviewer and author during the review process is itself against the rules regardless of what that contact leads to. COPE’s case “Peer reviewer contacted by author” addresses exactly this: COPE’s guidance is that the reviewer should have gone to the journal to discuss the situation rather than engaging directly, and that recusal followed by assignment of an independent reviewer was the appropriate path once contact occurred.
That’s the baseline problem a reviewer-to-author conversion has to overcome: if it happened through direct, undisclosed contact between reviewer and author, the arrangement is compromised regardless of how much work the reviewer subsequently did. COPE’s “Suspected contact between reviewer and an author led to coauthorship of the reviewer” case is built around exactly that suspicion — an editor noticing that a reviewer had, at some point, become a listed author, and needing to work out whether that happened properly.
The test: did the reviewer actually meet authorship criteria?
COPE does not treat “the reviewer gave detailed comments” as sufficient on its own. The applicable bar is the same one used for any other prospective author — most journals point to the ICMJE authorship criteria, which require all four of the following:
- Substantial contribution to the conception or design of the work, or the acquisition, analysis, or interpretation of data;
- Drafting the work or critically revising it for important intellectual content;
- Final approval of the version to be published; and
- Agreement to be accountable for all aspects of the work, including ensuring questions about its accuracy or integrity are appropriately investigated and resolved.
A reviewer who submits a normal review report — even a thorough, expert one — does not meet this bar. What can meet it is a reviewer who, at the editor’s invitation and with the editor’s knowledge, goes substantially further: contributing new analysis, rewriting sections, or otherwise doing work that would qualify anyone else as an author. COPE’s guidance on the suspected-contact case above frames it this way: authorship criteria might genuinely be satisfied if the reviewer made a substantial contribution to the revised paper — but that is a factual question the editor has to establish, not something the author or reviewer gets to assert to each other and self-certify. See CASRAI’s Authorship and Co-Authorship definitions for the general criteria this test draws on, and the research-software-engineer co-authorship guide for a worked example of applying the same four-part test to a different edge case.
Editor verification: why the editor has to be the one who checks
The reason COPE’s cases route this through the editor rather than trusting the author’s account is straightforward: an author reporting that “the reviewer did enough work to be an author” is precisely the claim that would need to be true for an improper deal to look legitimate on paper. The editor’s role is to independently confirm what actually happened — what the reviewer contributed, when, and whether it meets the journal’s authorship standard — rather than accept the addition at face value because the corresponding author has vouched for it.
In practice this means the editor should be able to point to concrete, documented contribution: tracked-changes evidence of what the reviewer actually wrote or analyzed, correspondence showing the editor (not just the author) invited and was aware of the reviewer’s expanded role, and a clear account of why the contribution goes beyond standard review comments. COPE’s companion guidance on recognizing authorship problems flags several adjacent warning signs worth checking for in this scenario specifically: an unnotified authorship change appearing only in later manuscript versions, and a corresponding author who is otherwise unresponsive to editorial and reviewer queries. Neither is proof of misconduct on its own, but either should prompt the editor to look more closely before accepting the reviewer as a legitimate co-author.
The re-review requirement
Even once the editor is satisfied the reviewer’s contribution is genuine, COPE’s guidance does not treat that as the end of the process. The reviewer who is now a co-author can no longer serve the independent-evaluation function peer review exists for — they have a direct stake in the paper’s acceptance. COPE’s cases are consistent on the remedy: the manuscript, now carrying its new author list, needs to be re-reviewed, and that re-review needs a different, independent reviewer standing in for the one who has just changed sides.
This holds even when the reviewer-to-author transition is discovered only after the fact. In COPE’s case “Reviewer requests to be added as an author after publication”, the guidance is that editors should be willing to let the authors resubmit the paper with the corrected, full author list — and have that resubmission go through re-review with new handling editors and new reviewers, rather than simply amending the byline on the already-published version. The mechanism differs (pre-publication re-review versus a post-publication resubmission-and-correction cycle) but the underlying principle is the same: a compromised review can’t be patched by trusting the people involved to self-report accurately; it has to be re-run independently.
What editors and reviewers should each do
For reviewers
If a reviewer believes their input has gone well beyond a normal review — to the point that authorship might genuinely be warranted — COPE’s Ethical Guidelines for Peer Reviewers are clear that this is a conversation to have with the editor, not a private arrangement with the author. Reviewers are expected to keep manuscripts confidential, not use unpublished material for personal advantage, and not negotiate their own role changes directly with authors. The safe path is: raise it with the editor, let the editor decide whether the journal’s authorship threshold is met, and step back from any further reviewing role on that manuscript once it’s under discussion.
For editors
A practical checklist for handling a reviewer-to-author request: confirm with the reviewer directly (not only the author) what they contributed and when; check for documentary evidence of that contribution rather than relying on description alone; apply the same authorship criteria (commonly ICMJE’s four-part test) the journal applies to any other prospective author; document the decision in the manuscript’s editorial record; and route the manuscript to a new, independent reviewer once the author list changes. Many journals also require a formal change-of-authorship form signed by all authors, including the newly added one, as part of this process — consistent with COPE’s broader guidance on ghost, guest, and gift authorship, which treats any authorship change without full transparency and editorial sign-off as a risk factor regardless of the specific route it took.
How this differs from routine reviewer recognition
It’s worth distinguishing this scenario from the much more common, uncontroversial ways journals credit reviewers: named acknowledgment, reviewer certificates, or entries on a reviewer’s ORCID record via the journal’s peer-review recognition workflow. None of that requires meeting authorship criteria, and none of it changes who is accountable for the paper’s content. Reviewer-to-author conversion is categorically different because it changes accountability, not just recognition — which is exactly why COPE applies a higher bar and an editor-led verification step to it, rather than treating it as an extension of ordinary reviewer thanks.
Frequently asked questions
Can a peer reviewer become a co-author on the paper they reviewed?
Yes, but only in narrow circumstances: the reviewer’s contribution must independently satisfy the journal’s authorship criteria (typically ICMJE’s four-part test), the editor must verify this directly rather than accept the author’s account, and the manuscript must then be re-reviewed by a new, independent reviewer.
What does COPE say about a reviewer becoming an author?
COPE addresses this through several published case discussions rather than a single flowchart. Its consistent position across those cases is that the transition is not automatically improper, but it requires editorial verification of a genuine substantial contribution and independent re-review before the manuscript proceeds.
Is it a problem for an author to contact a reviewer directly during review?
Yes. Under standard single- or double-anonymized peer review, direct contact between reviewer and author during the review process is against the rules regardless of its outcome. COPE’s guidance in this situation is that the reviewer should recuse and refer the matter to the editor rather than respond to the author directly.
Does the manuscript need to be re-reviewed if a reviewer becomes a co-author?
Yes. Once a reviewer has a stake in the paper as an author, they can no longer serve as an independent evaluator of it. COPE’s guidance calls for the manuscript to go out to a new, independent reviewer as part of accepting the authorship change.
What if the request to add a reviewer as an author comes after the paper is already published?
COPE’s guidance for this scenario is that editors should allow the authors to resubmit the paper with the corrected author list and have that resubmission go through re-review with new handling editors and reviewers, rather than simply editing the byline on the published version.
What should a reviewer do if an author asks them to join as a co-author?
Raise it with the editor rather than agreeing directly with the author. COPE’s Ethical Guidelines for Peer Reviewers treat unauthorized direct arrangements between reviewer and author as outside acceptable practice, independent of whether the underlying contribution turns out to be genuine.
