Introduction: Dynamic Interpersonal Therapy (DIT), a psychodynamic and mentalization-based treatment, demonstrates efficacy for depression through theoretically enhancing mentalizing—the capacity to understand behaviours through intentional mental states. While mentalizing is considered central to DIT's theoretical framework, empirical evidence for its specific therapeutic mechanisms remains limited.
Purpose: This study aimed to explore how mentalizing contributes to therapeutic change in DIT for depression.
Method: Data derived from an RCT comparing DIT (n = 73), LIT (Low-Intensity Therapy, n = 54), and CBT (Cognitive-Behavioural Therapy, n =20). Patients were assessed at baseline, mid-treatment and post-treatment using the Hamilton Rating Scale for Depression (HRSD-17), Beck Depression Inventory-II (BDI-II), and Reflective Functioning Questionnaire (RFQ: certainty/uncertainty dimensions). Mixed-effect Linear Models, mediation analysis (DIT vs LIT and DIT only), and individual slope extraction examining change-to-change relationships were conducted. In the cases of mediation and slope analysis, CBT was excluded due to a small sample size.
Results: Mentalizing uncertainty showed significant treatment-dependent effects in DIT (β = 0.59, p = .003; LIT vs DIT: β = -0.97, p = .002). Only within-DIT mediation showed significant mentalizing uncertainty-depression Pathways (RFQ-Uncertainty → HRSD-17: β = 0.615, p = .032; BDI-II: β = 1.62, p = .003). Individual slope analysis revealed distinct mentalizing-depression relationships: in DIT, depression improvement was significantly predicted by mentalizing uncertainty (BDI-II: β = 1.099, p < .001; HRSD: β = 0.358, p = .042) and marginally by certainty (BDI-II: β = 0.871, p =.059), without significant associations in LIT.
Conclusions: Mentalizing functions as a therapeutic factor in DIT, with enhanced clinical associations compared to control therapy. Findings challenge simple mediation models, suggesting DIT's effectiveness derives from amplifying mentalizing's regulatory power for depression outcomes rather than solely building mentalizing capacity. These insights inform therapeutic training, highlighting the importance of deepening mentalizing in psychodynamic treatments.