Elucidating therapist differences: Therapists' interpersonal skills and their effect on treatment outcome

Therapists differ in their average treatment outcomes. However, it remains unclear which characteristics differentiate more from less effective therapists. This study examined the association between therapist interpersonal skills and treatment outcome as well as the moderating effect of initial impairment.

Interpersonal skills were assessed with the Therapy-Related Interpersonal Behaviors (TRIB) scale, a group-discussion based rating system, in 99 incoming therapy trainees. The trainees treated n = 1031 outpatients with psychological therapies, whose treatment outcomes were assessed with the Symptom-Checklist 90 Revised (SCL-90-R). Linear mixed models were conducted to predict outcome by therapists' interpersonal skills beyond initial impairment, number of sessions, therapist age, gender, and theoretical orientation. The moderating effect of initial impairment was calculated as cross-level interaction.

The therapist effect (TE) in this sample was 5.6%. Interpersonal skills were a significant predictor of outcome (b = -0.124, p < .001) and explained 1.3% of variance beyond all control variables. The TE in the final model was VPC = .036 indicating that 26.79% of the TE were attributable to interpersonal skills. The impairment-skills interaction was significant (b = -0.172, p < .001). The effect of interpersonal skills on outcome increased with more severe initial impairment. Results were replicated in a second outcome measure (Outcome Questionnaire 30).

Interpersonal skills were found to be important characteristics to differentiate between more and less effective therapists, especially when treating severely distressed patients. Considering them in therapist selection and matching, outcome prediction, and clinical training could improve the effectiveness of psychological therapies.