The therapist effect in adult psychotherapy: a systematic scoping review of reviews

INTRODUCTION: The therapist effect, i.e., the variation in psychotherapy outcomes attributable to individual therapists, is consistently estimated at 5-8%, yet a deeper understanding of the mechanisms remains poorly understood. This scoping review synthesizes systematic reviews and meta-analyses to clarify conceptualizations of therapist effects, assess methodological strengths and limitations, and summarize evidence in adult psychotherapy. METHODS: Following PRISMA-ScR guidelines, searches were conducted in PubMed, Web of Science, and PsycINFO. Eligible studies were peer-reviewed systematic reviews or meta-analyses examining therapist-related factors and adult psychotherapy outcomes. Twenty-nine reviews met the inclusion criteria. Data on study aims, methods, main results, and conclusions were retrieved. Review quality was assessed using the AMSTAR 2. RESULTS: Two reviews estimated overall therapist effects without examining explanatory variables, while 27 focused on specific therapist traits or competencies. Methodological limitations were common, including limited preregistration, inadequate risk-of-bias assessment, and weak statistical approaches. Contextual moderators were rarely examined. Evidence for specific therapist characteristics was mixed, though moderate support emerged for interpersonal qualities and skills. CONCLUSIONS: Therapist effects appear multifaceted, involving therapist, patient, and contextual factors. Interpersonal skills show the most consistent associations with outcomes, but methodological weaknesses and limited contextual analysis constrain progress. Suggestions for future research include more rigorous study designs, including multi-level analyses.