Exploring the evolution of nonverbal synchrony in psychotherapy: The idiographic perspective provides a different picture.

Objectives: A methodological obstacle for the assessment of psychotherapy process concerns the compatibility between nomothetic versus idiographic approaches. Using quantitative methodology at an idiographic level, we seek to narrow this gap for the special case of nonverbal synchrony in psychotherapy dyads.Methods: Using quantitative idiographic process analysis (QUIPA), we combined the assessments of nonverbal synchrony-the coordination of patient's and therapist's body-movement-quantified with motion energy analysis (MEA), patient- and therapist session-reports, and patient's self-reports of symptomatology into multivariate time-series of psychotherapy process across the course of N = 12 dyadic psychotherapies with a total of N = 150 sessions. Adopting both mixed models as well as vector auto regressive models (VAR) to time-series, we aimed to extend previous results assessed in a randomized sample by exploring the evolution of nonverbal synchrony in psychotherapy dyads across time.Results: Aggregated idiographic data revealed that nonverbal synchrony was different from findings based on a previously reported nomothetic account: Associations with session-level outcome were weaker and did not replicate cross-sectional analyses.Conclusions: The application of idiographic assessments may help broadening our conception of synchrony in psychotherapy and also reminds us that ergodicity is not necessarily the usual case, but one of many possibilities.