Existential isolation as a correlate of mental health problems, predictor of treatment outcome, and moderator of a patient-therapist match effect

Introduction: Ample research has established that interpersonal isolation-the objective separateness from others-is a correlate of maladaptive psychological outcomes. However, existential isolation (EI)-the feeling of aloneness in one's subjective experience-has received less empirical attention. From the limited existing research, higher EI has been associated with greater severity of mental health concerns and more negative beliefs about treatment. Yet, these investigations have largely been conducted with non-clinical samples. Further, virtually no studies have examined EI as a predictor of current psychotherapy outcomes. Moreover, given their risk of holding negative treatment beliefs, it is plausible that therapy would be most effective if more existentially isolated individuals experienced being well matched to their therapist. This study extended EI research to a clinical sample and provided a novel test of EI as a direct predictor of outcome and moderator of a patient-therapist match system in naturalistic psychotherapy. Method: Data derived from a randomized trial comparing the effectiveness of prospectively matching patients (N = 218) to therapists (N = 48) with empirical strengths in treating the patient's specific presenting problem(s) versus case assignment as usual (Constantino et al., 2021). Patients rated EI at baseline and mental health outcomes repeatedly through treatment. We conducted correlations to examine baseline EI-symptom associations and multilevel modeling to test EI as a predictor of symptom change and moderator of the known beneficial match effect on outcome. Results: As predicted, higher EI was associated with greater depression, anxiety, and distress at baseline, as well as poorer therapy outcome across and at the end of treatment. In the expected direction, though not to a statistically significant level, the positive match effect was stronger for patients with higher versus lower EI. Discussion: We discuss research and practice implications at the intersection of social and clinical psychology.