This secondary analysis examined the processes of change from a randomized dismantling trial evaluating the Open (i.e., cognitive defusion, acceptance) and Engaged (i.e., values, committed action) components of acceptance and commitment therapy (ACT). Analyses were conducted with 161 distressed college students randomly assigned to a full online ACT program (Full n=40), online ACT targeting the Open components (Open n=41) or targeting the Engaged components (Engaged n=39), or a waitlist condition (Waitlist n=41). The intervention occurred over six weeks followed by a post-treatment assessment with mental health symptoms as the primary outcome. Consistent with predictions, pre- to post-treatment improvements in global psychological inflexibility, cognitive fusion, acceptance, values, and committed action all predicted pre- to post-treatment improvements in mental health, with most processes continuing to independently predict improvements when included in a single model. The relations between changes in psychological flexibility and mental health were generally equivalent between conditions. Each psychological flexibility process separately mediated improvements in mental health for Engaged versus waitlist and Full versus waitlist conditions. However, global inflexibility, committed action, and values progress did not mediate effects for Open versus waitlist. Overall, results indicate a range of acceptance, defusion, values, and committed action processes are functionally relevant for outcomes with the Engaged components of ACT and Full ACT, but values, committed action, and global psychological inflexibility processes may be more weakly related to the effects of the Open components of ACT alone.
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