Empathy in psychotherapy: subjective ratings versus remote biosensing of interpersonal heart rate synchrony as outcome predictors

1The therapeutic alliance is widely recognized as a key predictor of psychotherapy outcome, though it is predominantly assessed through subjective self-reports. This proof-of-concept study investigated whether interpersonal movement and heart rate synchrony can serve as objective biomarkers for therapy effectiveness within naturalistic cognitive behavioral therapy (CBT) settings. Twelve patient-therapist dyads were analyzed in a short-term follow-up setting. Physiological signals were continuously recorded using wrist-worn wearable sensors, while head and body movements were captured via video recordings. After each session, both dyad members completed alliance questionnaires. Patients also provided standardized symptom ratings. Interpersonal synchrony was quantified using lagged-windowed cross-correlations across both modalities. Results confirmed the presence of synchrony in movement and heart rate during CBT sessions. Notably, in-phase heart rate synchrony measured in the initial session significantly predicted changes in both, patients self-rated global severity index and depressive symptom severity over time, independent of baseline symptom burden. Among subjective measures, only therapists ratings of the therapeutic alliance significantly predicted short-term therapy outcome. These findings highlight interpersonal physiological synchrony as a meaningful and unobtrusive objective marker of therapeutic engagement and effectiveness. Incorporating wearable biosensing into psychotherapy research might offer a scalable, objective approach to understanding and enhancing treatment effectiveness. Based on this proof-of-concept study, future studies with larger samples are warranted to refine synchrony-based predictive models and explore their potential for personalizing treatment choices for improving patient-therapist matching in evidence-based psychotherapy.