Parsing the existential isolation-outcome association into its within- and between-patient components in naturalistic psychotherapy.

Complementing the oft-studied construct of interpersonal isolation, research has increasingly focused on existential isolation (EI), or the subjective feeling of separateness in one's experience. In the clinical realm, several studies have demonstrated that higher EI is associated with more severe mental health problems at a single cross-section of time. Moreover, one study showed that higher pretreatment EI predicted worse psychotherapy outcomes. However, it remains unknown whether an average level of EI across all of psychotherapy (a type of during-treatment "trait") and/or fluctuations in EI during psychotherapy (a type of during-treatment "state") relate to broader treatment outcomes. Addressing EI in this more nuanced and complex manner, the present study parsed the EI-outcome association in its between- (trait) and within-patient (state) components in the context of naturalistic outpatient psychotherapy (Constantino et al., 2021). Participants were 46 therapists treating 144 patients who provided enough EI and outcome data to establish average, longitudinal, and temporal associations across treatment. Using a random intercept cross-lagged panel model, the between-patient results showed that higher average EI was associated with worse average outcomes (standardized association = .60, p < .001). However, although within-patient EI demonstrated significant variability over time, such fluctuations were unexpectedly unrelated to subsequent changes in outcome-when accounting for prior EI and outcome changes (p = .617). The findings suggest that although EI seems to possess both traitlike and statelike qualities, the former component may have the greatest influence on treatment outcomes, whereas the latter may be more of an outcome in itself. (PsycInfo Database Record (c) 2025 APA, all rights reserved).