Objectives: To determine the proportion of medical journals requiring trial registration and to understand their reasons for adopting (or not adopting) such policies and other measures designed to reduce publication bias.
Design: Quantitative study of journals' instructions to authors (in June 2012) and qualitative study of editors' and publishers' views on trial registration and publication bias (carried out in Autumn 2012).
Setting: Random selection of 200 medical journals publishing clinical trials identified from the Cochrane CENTRAL database.
Participants: Editors (n=13) and publishers (n=3) of journals with different policies on trial registration (and with recently changed policies) identified from the survey of their instructions to authors.
Results: Only 55/200 journals (28%) required trial registration according to their instructions and a further three (2%) encouraged it. The editors and publishers interviewed explained their journals' reluctance to require registration in terms of not wanting to lose out to rival journals, not wanting to reject otherwise sound articles or submissions from developing countries, and perceptions that such policies were not relevant to all journals. Some interviewees considered that registration was unnecessary for small or exploratory studies.
Conclusions: Although many major medical journals state that they will only publish clinical trials that have been prospectively registered, and such policies have been associated with a dramatic increase in the number of trials being registered, most smaller journals have not adopted such policies. Editors and publishers may be reluctant to require registration because they do not understand its benefits or because they fear that adopting such a policy would put their journal at a disadvantage to competitors.