The (Cost-)Effectiveness of Generalist Versus Specialist Treatment for Severe Personality Disorders'

The goal of this clinical trial is to investigate if generalist treatment (GIT-PD) is non-inferior in improving level op personality functioning compared to specialist treatment (MBT/ST) in patients with severe personality disorders. The main questions it aims to answer are: 1. Is generalist treatment (GIT-PD) non-inferior in improving level of personality functioning compared to specialist treatment (MBT/ST) in patients suffering from severe personality disorders (PD)? 2. What patient characteristics predict which patients will benefit more from specialist or generalist treatment? Participants will be randomized to either specialist or generalist treatment. Interventions included in specialist treatment are Mentalization-Based Treatment (MBT) and Schema Therapy (ST). Generalist treatment will be offererd following the principles of the Guideline-Informed Treatment for Personality Disorders (GIT-PD), a principle-driven treatment framework based upon common factors and allowing personalized treatment according to patients' needs. All participating institutions offer GIT-PD and MBT and/or ST. Before, during and after following this treatment participants will complete measurements at 8 distinct time points. Participants will be asked to: * complete a range of instruments at baseline to enable the construction of a personalized advantage index to predict treatment response based on patient characteristics * complete questionnaires at all 7 post-randomization time points * undergo a semi-structured interview at 3 time points * follow treatment for personality disorders (either GIT-PD or MBT/ST)