A cross-cultural incremental effects meta-analysis of acceptance and commitment therapy for depression: does targeting depression even matter?

Acceptance and commitment therapy (ACT) is a transdiagnostic psychotherapy intervention. We sought to demonstrate the degree to which ACT interventions incrementally reduce depression symptoms when they are primary targets, as well as secondary features of other conditions. We reviewed controlled trials for ACT interventions with depression outcome measures. Studies were identified from the comprehensive repository of ACT controlled trials. Analyses included data from 21,830 participants (k = 263 studies). Overall, ACT interventions resulted in a significant reduction in depression symptoms (g = 0.88). ACT interventions targeting depression against passive controls (waitlists) demonstrated strong incremental effects (g = 1.10). ACT interventions against passive controls where depression was a secondary outcome demonstrated moderate incremental effects (g = 0.61). ACT interventions that targeted depression against active controls demonstrated weak effects (g = 0.35). Publication year did not influence outcomes. ACT was only modestly superior to controls in trials conducted in the United States/Canada, relative to trials from other world regions. Higher quality studies were associated with weaker effects. Altogether, ACT is associated with a significant reduction in depression regardless of whether it is targeted. The degree to which ACT is superior to other interventions is small and likely dependent on contextual factors.