What is the purpose of the “Comments only to the editors” section in peer reviews? It seems lack of transparency to me. What couldn’t you possibly tell the authors?
A few examples:
- "I happen to know one of the authors personally and I know that they didn't consent to their name on the manuscript."
- "Are you sure this topic is within the scope of your special issue?"
- "I can't understand the authors. The other reviewers seem able to, however. Send the paper to them, not me. I don't want to see this manuscript again."
- "Did you notice that between revisions there is a new author?"
- "I'm not an expert on part X of the manuscript, but I'm an expert on part Y, and ..."
- "I previously reviewed this manuscript for ____ journal. I attach the review I wrote for that journal, since the authors have not addressed my comments."
- "I have reviewed so many papers for this group of authors. They all apply the same method to slightly-different problems. Scientifically, there's nothing wrong with this particular manuscript, but it's salami slicing."
You can convey information that would reveal your identity.
E.g., you had already refereed a previous version of the paper for another journal and the authors ignored your feedback. Or: you know that the authors intentionally refuse to cite some existing work.
You can be more direct and blunt.
E.g., if the main result of a paper is roughly at the same level of difficulty as a homework exercise one could assign to a grad student, I might state this bluntly in more or less these words in the private field, but I would phrase it in a more diplomatic way in the report. The reason for doing this is not so that we can have a laugh behind the author's back, but to avoid discouraging and hurting the authors.
This applies especially if the authors are e.g. Ph.D. students, whom you don't want to discourage from pursuing research even if the paper should clearly be rejected.
There is nothing nefarious about this: clearly different norms of respect/politeness apply when talking to vs. when talking about a person. If they did not apply, it would be impossible to have fully honest conversations about other people's research.
You can use it to give a concise informal summary of the report.
"Strong paper. Clear accept." or "Borderline. Either way is fine with me." or "Not a great paper but probably publishable".
You can use it if you are uncertain about what to recommend.
One can choose to write an ambiguous report which the editor can use to justify either decision, and then convey your personal subjective opinion (which you might be uncertain about) privately.
You can use it to convey information that is irrelevant to the author.
E.g., that you might not be able or willing to referee further revisions.
You can use it to address potential conflicts of interest.
E.g., if you ask the authors to cite a number of your papers, you might use this field to reassure the editor that this is a legitimate request and you're not just abusing your position as a referee to get more citations.
There's differing opinions about this due to transparency, but I have been told by at least two editors (one from a big name journal) that they prefer your suggested disposition of the paper (accept without changes, minor revision, reject, etc.) and your opinions and justification on that matter should go into the editor comments.
This is because the editor makes the final decision, which may differ from the reviewers.
For example, consider the situation where all the reviewers say in the author comments to accept because they focus on the technical aspects. However, the editor is unconvinced that the novelty or interest is sufficient for the journal and rejects the paper. This causes confusion and complaints about why the paper was rejected despite two good reviews.
The other direction occurs too: a reviewer says in the author comments, reject due to novelty, but the editor believes they are unduly harsh and, overrides and accepts the paper. This obviously leads to less author complaints, but is awkward, can lead to some ill will with the reviewer and may give the impression of cheapening the journal.
Some people think that this is unfair due to transparency, but you can also find guides saying you should do this.
Apart from what others have already said, you can also use the "comment to the editor" function if you are not 100% sure about your asssessment--you can use the comment to tell the editor so and why. When I did my very first peer review, I told the editor that this was my very first review, and although I did feel confident in my review, I was torn between suggesting "reject" and "major revision".
Your report to the authors and your comments to the editor serve two different purposes and should address two different audiences. Your report to the authors should help them improve the paper. When writing these reports, regardless of my opinions of the paper, I maintain a positive tone. I point out problems, areas of concern, etc. and propose ways they could address them.
There are components that feed into my evaluation that are not helpful to include in reports. e.g., my accept/revise/reject recommendation (particularly if it does not agree with the editor's final decision). This is where comments to the editor can be useful.
My comments to the editor are intended to help them make then communicate their decision. In my comments, I clearly state my accept/revise/reject recommendation and summarize the major reasons for that decision. If I worry that my concerns cannot be addressed by revision or I'm on the fence about a decision, I say as much and explain why. This is never more than a paragraph or two tops, but provides the editor a high-level summary of the thinking behind my recommendation which might otherwise be less clear in my longer report to the authors and whether I am likely to eventually recommend acceptance if revisions are requested.
By revealed preference these comments must at times be useful because I have seen editors closely paraphrase them in their decision letters (particularly when providing guidelines for a successful revision). This may also address OP's concern with transparency. Editors are free to repeat information they receive in the comments if they feel it will be useful to the authors.