Cortisol interdependence during psychotherapy in major depressive disorder.

The current study explored cortisol interdependence between patients and therapists during psychotherapy, the possible moderating effect of patient alliance ratings on this interdependence, and the associations between cortisol interdependence and treatment outcome. While cortisol interdependence was explored in other interpersonal contexts, its presence in psychotherapy has remained unexplored. We hypothesized that (a) patients' and therapists' cortisol levels at pre-session will predict their own and their partner's subsequent cortisol levels at post-session, (b) patient ratings of their relationship with their therapists will moderate these partner effects, and (c) cortisol interdependence will be associated with better treatment outcome. Fifty dyads undergoing 16 weeks of psychodynamic treatment for major depressive disorder participated in this study. Patient-therapist salivary cortisol samples were collected at eight time points, alongside a post-session patient-rated alliance questionnaire and a symptom severity interview. For analyses we employed the actor-partner interdependence model. Results revealed that (a) patients' and therapists' cortisol levels before sessions predicted their own post-session cortisol changes. However, significant cortisol interdependence was observed in patients' pre-session cortisol levels predicting therapists' post-session cortisol levels. Furthermore, (b) poorer alliance ratings associated with more pronounced cortisol interdependence, and (c) in dyads where patient pre-session cortisol predicted therapist's post-session cortisol, a better treatment outcome was found. This study found novel evidence of cortisol interdependence in psychotherapy and is partially in line with other studies inspecting cortisol interdependence in adjacent research fields. These findings emphasize the intricate psychophysiological interactions within therapeutic relationships and their associations with treatment outcome.