Andrada D. Neacsiu

Emotion dysregulation (ED) is a key target for change among empirically supported treatments for emotional disorders, including dialectical behaviour therapy skills training (DBT‐ST), yet how treatments improve ED is poorly understood. Using data from a randomised trial of DBT‐ST versus supportive group therapy for transdiagnostic ED, we tested whether three mechanistic variables—behavioural skills use, mindfulness, and perceived control—explain variability in ED within people over time. We additionally explored the mediating roles of these variables between conditions. Adults with transdiagnostic ED ( N = 44) participated in weekly groups for 4 months, with assessments at pre‐, mid‐ and post‐treatment and at 2‐month follow‐up. As hypothesised, multilevel models disaggregating within‐ and between‐person effects indicated that skills use, mindfulness, and perceived control each had significant total and unique within‐person associations with ED at concurrent time points, net the effect of time. Unexpectedly, these within‐person relations were not significant for mechanistic variables predicting ED 2 months later. Further, unique between‐person variability in skills use, mindfulness, and perceived control did not significantly mediate the relationship between condition and ED improvements. The present study is an important step in clarifying ED mechanisms of change, both within and between persons.