Imagine this scenario. You are industriously working away on your most recent paper (ignoring other pressing data analyses, administrative duties, and grant proposals). You have just begun to get into the zone of intense focus, writing nirvana, when DING!!! a new email appears in your inbox. It is a request to review a paper from Journal X. You immediately ask yourself: to review or not to review?
Deciding whether to accept or decline a request to review a manuscript is a question that we must all frequently ask ourselves. There are two possible actions to this question: agree to review or decline to review. Furthermore, there are two possible logical outcomes to your reasoning process: sound or faulty. This dilemma can be summarized with a two by two matrix (Figure 1) and I discuss each of these scenarios in turn.
The first outcome is often the easiest decision to make; when one agrees to review a paper for valid reasons. Valid reasons for reviewing a paper include having a genuine interest in the paper topic (at least as presented via the abstract), having sufficient depth and breadth of knowledge, and having just enough time to provide a quality review (see here for just some excellent posts on how to review a scientific paper: link, link, link). You will know you have made this decision if you are genuinely excited to review the paper.
The second outcome (agree/faulty) usually occurs (in retrospect) when you begrudgingly accept to review a paper. If you think about why you have accepted begrudgingly, it may help you to not make this mistake again in the future. Reasons for making this choice include: feeling like you are obligated to review a paper even though you do not find the paper topic interesting (good to ask yourself why you feel obligated), truly not having sufficient time to adequately review the paper, consciously (or subconsciously) realizing that you are not qualified to review the manuscript, and any number of morally dubious reasons (e.g., author Y is a good pal*).
The third outcome (decline/sound) is also a desirable end result. If you rightly think that you cannot provide a rigorous, thoughtful, unbiased, and timely review, then you should decline to review the paper. If you do choose to decline, you should at least try and recommend several other individuals that could serve as reviewers. I have heard statements like “if you publish 4 papers a year, then you should be reviewing 12 papers a year to do your part to the scientific community,” but I think this is faulty logic. You should not review a paper because you feel like you have to; you should only review for the right reasons (see above).
That being said, there are some people (who shall remain nameless), who never agree to review papers (decline/faulty). However, I think that these people are by far the exception rather than the rule. Other faulty reasons for declining to review a paper include genuine laziness (very rare in academia), or a faulty perception that you are not qualified to review the paper (very difficult to self assess). This latter fallacy typically occurs early in ones career, and can be avoided by asking for second opinions from your peers, asking for a mentor to help with the review, and by trial and error**.
If every reviewer spent a little bit more time thinking about their motivations for agreeing or refusing to review a paper, I think that the peer review system could function at an even higher level. Doing so would help the right people agree to review manuscripts and not the wrong ones (i.e. ones with faulty reasoning). Sometimes it is difficult to make a decision without all of the information (i.e., just the title and abstract), and there may be ways for journals to provide potential reviewers with additional information (e.g., figures, parts of the cover letter, the entire manuscript). Nevertheless, with a little more reflection, you may just find that you are on your way to becoming a more satisfied reviewer.
* I think it is possible to provide objective, fair reviews for colleagues that you know well (within reason). However, it is important to notify the editor of any potential conflict of interests and if this is the sole criteria for accepting to review a paper (i.e., you do not find the subject matter interesting or important), then this becomes morally dubious.
** It is okay to admit to an editor that parts of a paper were beyond your comprehension. This is preferable to pretending that you understand the 5th order differential equations.