Examining domains of psychological flexibility and inflexibility as treatment mechanisms in acceptance and commitment therapy: A comprehensive systematic and meta-analytic review

The current systematic and meta-analytic review sought to integrate a growing number of studies examining dimensions of psychological flexibility as treatment mechanisms for Acceptance and Commitment Therapy (ACT). Analyses of 77 records (67 unique studies; N_total = 9123 participants) from comprehensive searches of multiple databases suggested that ACT interventions led to reduced inflexibility (i.e., lowered global inflexibility, lack of present moment awareness, cognitive fusion, experiential avoidance, self-as-content, & inaction) and increased flexibility (i.e., committed action/contact with values, global flexibility/acceptance, & defusion). Those changes remained significant when ACT was compared with waitlist or active treatments and were significantly linked to corresponding drops in psychological distress, supporting their roles as ACT treatment mechanisms. Moderation analyses revealed that the use of student samples, exclusion of clinically symptomatic individuals, and comparisons of ACT with other active treatments weakened these effects whereas offering ACT as an individual therapy and excluding individuals in extreme crisis (i.e., with suicidal ideation) strengthened them. The meta-analytic findings and systematic review suggested specific recommendations for future clinical work and research on ACT mechanisms: (1) Evaluate both psychological flexibility and inflexibility as distinct treatment mechanisms, (2) Evaluate specific dimensions of psychological flexibility/inflexibility as mechanisms with multidimensional scales (CompACT, MPFI), (3) Broaden treatment outcomes to include forms of wellbeing (peace of mind, vitality, connectedness), (4) Assess mechanisms and outcomes repeatedly throughout treatment to model the process of therapeutic change, (5) Investigate non-specific factors (therapeutic alliance, treatment adherence) as mechanisms, and (6) Explore treatment mechanisms in effectiveness studies.