The goal of this clinical trial is to compare etc. in patients with borderline personality disorder pure individual schema therapy to combined individual-group schema therapy. The main questions it aims to answer are:

• is there a difference in effectiveness?
• is there a difference in (early) treatment dropout?

Participants will receive either

• individual schema therapy or
• combined individual-group schema therapy.

Researchers will compare individual to combined individual-group schema therapy see if there is a difference in effects and/or a difference in dropout from treatment.

Schema Therapy (ST) for Borderline Personality Disorder (BPD) was originally developed and tested as an individual treatment. Since 2009 group-ST became popular as an supposedly efficient alternative. A RCT found combined individual-group ST to be superior to predominantly group-ST. Thus, individual ST is important. The question arises how combined individual-group ST compares to purely individual ST. This study compares the two formats of ST in a multicenter randomized clinical trial (RCT). The primary outcomes are severity of BPD and (early) treatment dropout. Secondary outcomes include: measures of specific BPD-symptoms; schemas and schema-modes; general mental symptoms; general functioning; and happiness. Moderators hypothesized to predict what patient characteristics are related to differences in effectiveness between the 2 formats are also tested. At least 132 BPD patients from at least 2 sites will be randomized to combined individual-group ST or individual ST. Treatments will take 2 years. Assessments take place every 6 months, and 1 year after end of treatment.

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.

Given Dutch and European Union legal data protection regulations sharing individual data with other researchers can only be considered if other researchers can guarantee meeting strict conditions.

Assessors and investigator are blind to treatment arm up to and including the last follow-up assessment.