Reducing nonsuicidal self-injury in depressed adolescents: The role of cognitive defusion as a behavioral mechanism

OBJECTIVE: Nonsuicidal self-injury (NSSI) is prevalent among adolescents with depression, yet the mechanisms through which psychotherapy reduces NSSI remain unclear. This study evaluated the efficacy of Acceptance and Commitment Therapy (ACT) and examined whether reductions in NSSI were associated with changes in specific psychological flexibility (PF) processes. METHODS: In this randomized controlled trial, 120 adolescents with comorbid depression and NSSI were assigned to ACT or treatment as usual (TAU). PF, depressive symptoms, and NSSI frequency were assessed at baseline and post-treatment. Network intervention analysis (NIA) examined treatment-related changes within the PF system. A parallel change-score structural equation model estimated indirect effects of PF processes on NSSI reduction. RESULTS: Compared with TAU, ACT was associated with greater improvements in PF and larger reductions in depressive symptoms and NSSI. NIA indicated that treatment exposure was most strongly linked to changes in cognitive defusion and contact with the present moment. Structural modeling suggested a significant indirect effect of PF on NSSI reduction, with cognitive defusion emerging as the only specific process significantly associated with change in NSSI. CONCLUSION: Findings support a process-based account of ACT and identify cognitive defusion as a central mechanism of change in adolescent NSSI.