The Mentalisation Switch: Therapist Reflective Capacity and Alliance Dynamics in Digital MCT+ for Bipolar Disorder — A Longitudinal Quantitative Case Series

Objective: This study conducted a preliminary naturalistic effectiveness evaluation of Individualised Metacognitive Therapy (MCT+) delivered via videoconferencing for individuals with bipolar I disorder in a real-world clinical setting in Chile. It also explored how therapist characteristics—specifically mentalisation capacity—influence the therapeutic alliance in digital psychotherapy.Methods: A longitudinal quantitative case series design was implemented across 14 therapist–patient dyads. Patients received 12 weekly sessions of MCT+ online. Standardised measures assessed anxiety (GAD-7), depression (PHQ-9), metacognitive beliefs (MCQ-30), and quality of life (WHOQOL-BREF). Therapeutic Alliance was tracked session-by-session (WAI-S). Therapist mentalisation and attachment were evaluated at baseline (MASC-SP, RFQ-8, ECR-12). Changes in outcome measures were analysed using paired t-tests, effect sizes (Cohen’s d), correlations, Reliable Change Indices (RCI), and hierarchical linear modelling. Results: Across the intervention, there were significant decreases in anxiety (d = 0.64) and improvements in metacognitive beliefs (d = 0.37). Depression showed a modest improvement (d = 0.34), while changes in quality of life were negligible (d = –0.21). Hierarchical modelling indicated a significant interaction between automatic and controlled mentalisation (b = –0.45, p = .008), suggesting that flexible adjustment supported therapeutic alliance development. Attachment style showed no significant associations with the alliance (largest unadjusted effect: ρ = –.54, p = .073; all adjusted ps > .99).Conclusions: Digital MCT+ showed preliminary effectiveness in reducing anxiety and maladaptive metacognitive beliefs among individuals with bipolar disorder, with more limited effects on depression and quality of life. Importantly, therapist mentalisation flexibility—the capacity to shift between automatic and controlled modes, or the mentalisation switch—emerged as a central mechanism for alliance-building and engagement in digital contexts, highlighting a key target for clinical training and future research.Keywords: therapeutic alliance, bipolar disorder, digital psychotherapy, MCT+, therapist mentalisation, reflective functioning, mentalisation, metacognitive therapy