BackgroundNo meta-analytical study has examined whether the quality of the studies examining psychotherapy for adult depression is associated with the effect sizes found. This study assesses this association.MethodWe used a database of 115 randomized controlled trials in which 178 psychotherapies for adult depression were compared to a control condition. Eight quality criteria were assessed by two independent coders: participants met diagnostic criteria for a depressive disorder, a treatment manual was used, the therapists were trained, treatment integrity was checked, intention-to-treat analyses were used, N⩾50, randomization was conducted by an independent party, and assessors of outcome were blinded.ResultsOnly 11 studies (16 comparisons) met the eight quality criteria. The standardized mean effect size found for the high-quality studies (d=0.22) was significantly smaller than in the other studies (d=0.74, p<0.001), even after restricting the sample to the subset of other studies that used the kind of care-as-usual or non-specific controls that tended to be used in the high-quality studies. Heterogeneity was zero in the group of high-quality studies. The numbers needed to be treated in the high-quality studies was 8, while it was 2 in the lower-quality studies.ConclusionsWe found strong evidence that the effects of psychotherapy for adult depression have been overestimated in meta-analytical studies. Although the effects of psychotherapy are significant, they are much smaller than was assumed until now, even after controlling for the type of control condition used.