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Assumptions not often assessed or satisfied in published mediation analyses in psychology and psychiatry.

Mediation analysis aims to investigate the "mechanisms of action" behind the effects of interventions or treatments. Originally developed in psychology, a robust set of mediation methods are now used across a range of fields. Given the history and common use of mediation in mental health research, this review aimed to understand how mediation analysis is implemented in psychology and psychiatry and whether analyses adhere to, address, or justify the key underlying assumptions of their approaches. All articles (N=206) came from top academic psychiatry or psychology journals in the PsycInfo database, published in the English language from 2013-2018. Information extracted from each article related to: study design, covariates adjusted for in the analysis, temporal ordering of variables, and the specific method used to perform the mediation analysis. Most papers did not adhere to their underlying assumptions. Only approximately 20% of papers had full temporal ordering of exposure, mediator, and outcome. Fewer than half of the papers controlled for confounding of the exposure-mediator and/or mediator-outcome relationships. Almost none discussed the underlying assumptions of their approaches or used causal mediation methods. These results provide insights for how methodologists should aim to communicate methods, and motivation for more outreach to the research community on best practices for mediation analysis. Mediation analysis is an appealing method in many areas within epidemiology. However, mediation analysis is complex, and it is unclear how well the underlying assumptions and alternative analytic approaches have been disseminated and understood.