Psychological Inflexibility, Cognitive Fusion, and Thought–Action Fusion as a Transdiagnostic Construct: Direct Comparisons Among Major Depressive Disorder, Obsessive–Compulsive Disorder, and Healthy Controls

Objective: Evidence suggests that acceptance and commitment therapy (ACT) processes function as transdiagnostic factors in both major depressive disorder (MDD) and obsessive-compulsive disorder (OCD) individually. However, few studies have directly compared these two clinical disorders. Therefore, this study aimed to identify potential transdiagnostic factors associated with ACT across MDD, OCD, and healthy control (HC) groups. Methods: A total of 34 MDD patients, 43 OCD patients, and 46 HCs were recruited through subway advertisements and outpatient services at a university hospital. Participants completed the Acceptance and Action Questionnaire-II, Cognitive Fusion Questionnaire, thought-action fusion (TAF) scale, and symptom severity scales. Results: Direct group comparisons revealed higher psychological inflexibility, cognitive fusion, and likelihood of TAF in the clinical groups compared to the HCs, with no differences between the MDD and OCD groups. These three transdiagnostic factors were variously correlated with both depressive and obsessive–compulsive (OC) symptoms in all groups. Regression analyses demonstrated that the three transdiagnostic factors accounted for 26% of the depressive symptoms in the MDD group (R2=0.26, p=0.028) and 27% of the OC symptoms in the OCD group (R2=0.27, p=0.014). Conclusion: These findings from the direct group comparisons of the three groups confirmed that psychological inflexibility, cognitive fusion, and likelihood of TAF are potential transdiagnostic factors that moderately contribute to the primary symptoms of both MDD and OCD. From another perspective, these results also highlight the need to consider how ACT addresses disorder-specific variations beyond what is explained by these transdiagnostic factors in the future.