Therapist personality traits as predictors of psychological treatment outcomes.

To investigate if therapists' personality influences their patients' treatment outcomes.  = 4,052 patients were treated by 69 therapists, including 36 Psychological Wellbeing Practitioners (PWPs) and 33 Cognitive Behavioural Therapists (CBTs). Therapists completed the NEO-PI-R personality inventory, they reported years of clinical experience, and expert assessors rated their clinical competence and reflective abilities. Their patients completed pre and post-treatment measures of depression (PHQ-9) and anxiety (GAD-7). Associations between therapist personality traits and patient treatment outcomes were examined using multilevel modelling, controlling for therapist demographics, clinical experience, technical competence and reflective ability. Relative to other sources of variability, therapists accounted for 1% to 3% of overall variability in treatment outcomes. However, the magnitude of systematic heterogeneity in performance was around 6%, such that the best-performing therapists outperformed average therapists by a margin of moderate to large effects ( = .57-1.10). Clinical experience, technical competence and reflective ability were unrelated to treatment outcomes. Patients treated by PWPs with above-average scores and CBTs with above-average scores had poorer treatment outcomes. Therapist effects may be partly explained by the influence of their personality on their work with anxious and depressed patients.