Why do peer reviewers decline to review? A survey.

Background: Peer reviewers are usually unpaid and their efforts not formally acknowledged. Some journals have difficulty finding appropriate reviewers able to complete timely reviews, resulting in publication delay.
Objectives and Methods: A survey of peer reviewers from five biomedical journals was conducted to determine why reviewers decline to review and their opinions on reviewer incentives. Items were scored on 5-point Likert scales, with low scores indicating low importance or low agreement.
Results: 551/890 (62%) reviewers responded. Factors rated most highly in importance for the decision to accept to review a paper included contribution of the paper to subject area (mean 3.67 (standard deviation (SD) 86)), relevance of topic to own work (mean 3.46 (SD 0.99)) and opportunity to learn something new (mean 3.41 (SD 0.96)). The most highly rated factor important in the decision to decline to review was conflict with other workload (mean 4.06 (SD 1.31)). Most respondents agreed that financial incentives would not be effective when time constraints are prohibitive (mean 3.59 (SD 1.01)). However, reviewers agreed that non-financial incentives might encourage reviewers to accept requests to review: free subscription to journal content (mean 3.72 (SD 1.04)), annual acknowledgement on the journal's website (mean 3.64 (SD 0.90)), more feedback about the outcome of the submission (mean 3.62 (SD 0.88)) and quality of the review (mean 3.60 (SD 0.89), and appointment of reviewers to the journal's editorial board (mean 3.57 (SD 0.99)).
Conclusion: Reviewers are more likely to accept to review a manuscript when it is relevant to their area of interest. Lack of time is the principal factor in the decision to decline. Reviewing should be formally recognised by academic institutions and journals should acknowledge reviewers' work.