What do 30 years of randomized trials tell us about how psychotherapy improves youth depression? A systematic review of candidate mediators

Psychotherapy for youth depression is only modestly efficacious. Investigating treatment mediators may guide improvement. Among 46 randomized trials of cognitive behavioral therapy and interpersonal psychotherapy with depressed youths, 74% measured candidate mediators (CMs), but only 17% analyzed CMs as mediators. Although four significant CMs emerged, findings were sparse, conflicting, and clouded by methodological issues. We see enormous untapped opportunity in the ample data collected but not yet analyzed using recommended methods. We propose exploiting existing data using mediation meta-analysis, broadening the range of CMs and the rigor of assessments, making measurement longitudinal and idiographic to clarify temporal precedence of CMs over outcomes and detect individual differences, and markedly altering journal and funder policies to promote change process and mechanism research.