The effects of clinical supervision on mental health professionals and their clients: a meta-analysis

Objective: Supervision is a key tool for both novice and experienced mental health professionals, yet evidence on its effectiveness remains inconclusive—especially regarding supervisee competence, mental health, client outcomes, and long-term impact. This meta-analysis aims to systematically synthesize empirical findings on the effects of supervision across professional backgrounds, its impact on client outcomes, long-term stability, and implementation features influencing its effectiveness. Method: A comprehensive search based on PRISMA guidelines (Page et al., 2021) identified 44 controlled evaluation studies, comprising 52 comparisons, 261 effect sizes, and 2,353 participants. Results: Supervision showed a moderate effect (g = 0.40, 95%-CI [0.25; 0.54]) and a small effect on client outcomes (g = 0.20, 95%-CI [0.02; 0.37]). No significant change was found between post and follow-up assessments (gchange = -0.027, p = .676), suggesting sustained effects over three to seven months. Regarding implementation measures, we detected a declining effect of supervision when specific implementation measures were applied. Study quality varied considerably, though trends suggest growing international research, increased use of randomized controlled trials, observational tools, and transparency around implementation issues. However, there was a notable lack of descriptive data on supervisee- and supervisor-related variables and possible dependencies between researchers, supervisors, and outcome assessors. Conclusion: Our meta-analysis indicates that supervision has a positive effect on supervisees’ skill development, reflection, mental health, and—albeit smaller—on client outcomes, with moderate stability over time. Future studies should examine client-related variables and provide detailed analyses of supervision quality and processes, especially the interpersonal dynamics between supervisor and supervisee.