Therapist's warmth and competence increased positive outcome expectations and alliance in an analogue experiment.

The quality of the therapeutic alliance, treatment motivation, outcome expectations (OE), and specific health behaviour predicts psychotherapy success. Therapists should be able to improve these factors to optimize outcomes. This study investigated the therapist's interpersonal behaviour to optimize alliance, motivation, OE, and health behaviour. A stressed study sample (= 465) completed an online analogue experiment. We gave participants positive information about psychotherapy effectiveness and varied the therapist's interpersonal behaviour along the dimensions of warmth and competence. High (vs. low) competence and high (vs. low) warmth increased alliance, OE, and help-seeking scores, while high (vs. low) competence increased motivation to do psychotherapy. We found no effects on health behaviour. In contrast to previous correlational analyses, our experimental study supports the causal role of the therapist's warmth and competence and its impact on alliance, motivation, and OE. We discuss approaches for future studies and clinical implications.