RESUMEN
Dialectical Behaviour Therapy (DBT) is an evidence-based treatment for borderline personality disorder (BPD), with findings demonstrating improvements in various BPD features and related behaviours, such as nonsuicidal self-injury (NSSI). Theory and research suggest that reductions in emotion dysregulation and interpersonal dysfunction could account for at least some of the reduction in NSSI observed during the course of DBT. The current research investigated: 1) the trajectory of changes in emotion dysregulation, interpersonal dysfunction, and NSSI over the course of DBT, and 2) whether changes in emotion dysregulation mediate the relationship between changes in interpersonal dysfunction and changes in NSSI over treatment. One hundred and twenty individuals with BPD enrolled in a multi-site randomized-clinical trial were assessed at five timepoints over 12 months of standard DBT. Results indicated that interpersonal dysfunction and NSSI decreased over the course of DBT. Emotion dysregulation decreased in a quadratic manner such that most of the gains in emotion dysregulation occurred in earlier phases of DBT. Although changes in interpersonal dysfunction predicted changes in emotion dysregulation, changes in emotion dysregulation did not mediate the relationship between changes in interpersonal dysfunction and changes in NSSI.