Therapist personality traits as predictors of psychological treatment outcomes.

Aim: To investigate if therapists' personality influences their patients' treatment outcomes. Methods: N = 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. Results: 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 between therapists was around 6%, such that the best-performing therapists outperformed average therapists by a margin of moderate to large effects (g = .57-1.10). Clinical experience, technical competence and reflective ability were unrelated to treatment outcomes. Patients treated by PWPs with above-average agreeableness scores and CBTs with above-average openness to experience scores had poorer treatment outcomes. Conclusions: Therapist effects may be partly explained by the influence of their personality on their work with anxious and depressed patients.