ABSTRACT
OBJECTIVES: Dialectical behaviour therapy (DBT) and mentalization-based therapy (MBT) are widely used evidence-based psychological treatments for borderline personality disorder (BPD). The study aimed to establish evidence on common and unique, and helpful and unhelpful, treatment processes. DESIGN: Mixed-methods. METHODS: In-depth qualitative interview data on patient experiences during treatment were combined with quantitative outcome measures in 73 patients diagnosed with a personality disorder and receiving DBT or MBT. RESULTS: Across both DBT and MBT, accounts of learning not to react impulsively, learning to question one's thoughts and assumptions, learning to communicate more effectively, and exposure to painful emotions that may previously have been avoided were each associated with less baseline-adjusted self-harm at the end of treatment. Difficulties in interacting with other group members were more likely to be described by patients receiving MBT than DBT, whilst difficulties in the therapeutic relationship were equally common. Both of these types of difficult experience were associated with higher baseline-adjusted levels of BPD traits and emotional dysregulation, at the end of the 12-month study period. CONCLUSIONS: The findings identify novel evidence of common therapeutic processes across DBT and MBT that may help to reduce self-harm. The findings also highlight the potential iatrogenic effect of difficulties in the alliance with therapists or with other group members. This underscores the importance of listening to patients' voices about what they are finding difficult during therapy and working to address these relational challenges, so that the patient is able to progress and make best use of the treatment. PRACTITIONER POINTS: Regardless of whether dialectical behaviour therapy or mentalization based therapy is used, helping service-users to learn not to react impulsively, to question their thoughts and assumptions, and to communicate more effectively, may be beneficial for reducing self-harm. Across both types of therapy, exposure to painful emotions is a difficult experience for service-users, but may also be beneficial for reducing self-harm, if carefully managed. Whilst service-users' experiences across both types of therapy have much in common, accounts of mentalization based therapy stand out in more often describing both helpful and unhelpful experiences of interactions with therapy group members. Service-users across both types of therapy report the benefits of learning intrapersonal mentalization skills, whilst recipients of mentalization based therapy uniquely extend this to learning interpersonal mentalization. Ruptures in the therapeutic alliance, and distressing interactions with group members, may be iatrogenic and must be carefully managed.