Specific and non-specific effects of psychotherapeutic interventions for depression: Results from a meta-analysis of 84 studies

There is a long-standing and very active debate regarding which psychotherapeutic intervention should be used in depressive disorders. However, the effects of psychotherapies may result majorly from non-specific factors rather than from specific factors related to the type of psychotherapeutic intervention. We performed a systematic review and meta-analysis on aggregated data to understand how the effects of different psychotherapies are impacted by non-specific factors. We included randomized controlled trials that assessed the efficacy of psychotherapeutic interventions in the treatment of adult depressive disorders. The primary outcome was the change in depression score from baseline to the latest follow-up visit (i.e. response). A meta-regression was performed to predict response according to the type of intervention and non-specific factors (e.g. number of treatment sessions, length of follow-up, therapeutic allegiance of the investigator). The main analysis included 214 study arms from 84 trials. The effects of psychotherapies compared to the waiting list control condition failed to remain significant after adjusting for non-specific factors. Response increased with the number of treatment sessions (β = 0.03, 95% CI [0.01; 0.04]) and the length of follow-up (β = 0.01, 95% CI [0.00; 0.02]). Response also improved in case of presumed therapeutic allegiances among investigators (β = 0.29, 95% CI [0.07; 0.52]). Response to psychotherapies seems to be closely related to non-specific effects. The development of a well-designed trial that controls for non-specific factors might help disentangle the effects of psychotherapies.