Using session-by-session measurement to compare mechanisms of action for acceptance and commitment therapy and cognitive therapy

Debate continues about the extent to which postulated mechanisms of action of cognitive behavior therapies (CBT), including standard CBT (i.e., Beckian cognitive therapy CT]) and acceptance and commitment therapy (ACT) are supported by mediational analyses. Moreover, the distinctiveness of CT and ACT has been called into question. One contributor to ongoing uncertainty in this arena is the lack of time-varying process data. In this study, 174 patients presenting to a university clinic with anxiety or depression who had been randomly assigned to receive either ACT or CT completed an assessment of theorized mediators and outcomes before each session. Hierarchical linear modeling of session-by-session data revealed that increased utilization of cognitive and affective change strategies relative to utilization of psychological acceptance strategies mediated outcome for CT, whereas for ACT the mediation effect was in the opposite direction. Decreases in self-reported dysfunctional thinking, cognitive "defusion" (the ability to see one's thoughts as mental events rather than necessarily as representations of reality), and willingness to engage in behavioral activity despite unpleasant thoughts or emotions were equivalent mediators across treatments. These results have potential implications for the theoretical arguments behind, and distinctiveness of, CT and ACT.