Which therapy format works best for borderline personality disorder?

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * BPD as defined by the Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5), assessed with the Structured Clinical Interview for the DSM-5 Personality Disorders (SCID-5-PD), as primary diagnosis * BPDSI total score ≥ 20 (i.e, severity in the BPD range, Giesen-Bloo et al., 2010) * Ability to understand, read, write and speak Dutch, or English if the site has research assistants and therapists of both conditions that are sufficiently fluent in English, as well as an English ST group (as at ACTP). Exclusion Criteria: * DSM-5 substance use disorder that needs clinical detox according to the clinical staff (after 6 weeks of abstinence participation is possible). * Comorbid psychotic disorder (patients with temporary psychotic problems falling under BPD criterion 9 are not excluded) * DSM-5 Bipolar disorder, type 1 (current or past) * Acute suicide risk * Intelligence Quotient (IQ) \< 80 * Serious neurological problems such as dementia * Patients should not start with any form of psychological treatment during screening or during the study's waitlist or treatment. (Low frequency supportive treatment may be continued during wait and screening, but not during the study treatment.) * Not able or willing to plan participation in the treatment sessions. Note 1. In case of suspicion of exclusion criteria that need specific assessment, like IQ and dementia, a formal diagnostic assessment needs to be done before in/exclusion can be decided. Note 2. No other psychological treatment during the study follow-up period is provided, unless this is necessary (clinical judgment). The 1 year treatment-free follow-up period is part of the treatment protocol, to help patients gain experience with life without mental health care. After 1 year further treatment can be offered, if indicated.