LAUSR

Abstract Objective: To test whether adherence to mentalization-based treatment (MBT) principles predict better patient in-session mentalizing. Methods: Two sessions for each of 15 patients with borderline personality disorder and comorbid substance abuse disorder were rated for MBT adherence and competence. Individual patient statements were rated for Reflective Functioning (RF), therapist statements were rated as demanding RF or not. Data were analysed using multilevel modelling. Results: MBT adherence and competence predicted higher session RF (β = .58–.75), even while controlling for pre-treatment RF. In addition, therapist interventions directed toward exploring mental states predicted higher RF of subsequent patient responses (β = .11–.12). Conclusions: MBT adherence and competence were significantly related to patient in-session mentalizing, supporting the validity of MBT principles. Results point to the importance of supervision for therapists to become adherent to MBT principles. The small number of patients and sessions limits generalizability of results.