I am no longer in a research role, so I do a lot less publication and reviews than I used to. But in many cases it is perfectly legitimate for a reviewer to ask that citations to that reviewer's work be added, because the editor has chosen that reviewer because they are a leading expert in the field. If it's a narrow specialty, it's quite likely that the expert has already published relevant material that the paper submitter missed.<p>The key is whether the added citation is relevant or not, and that's hard to judge based on statistics.<p>A more common issue I've seen is that there are cliques of academics who've staked out some corner of a field and cite each others' work a lot, even though it isn't that interesting. This doesn't have much to do with the review process.