Embrace the bumpy road and ACT your way: Examining transdiagnostic factors and the effectiveness of Acceptance and Commitment Therapy for transitional-age youth

Abstract

The general aim of this dissertation was to examine transdiagnostic factors and the effectiveness of Acceptance and Commitment Therapy (ACT) as a transdiagnostic intervention for transitional-age youth (TAY; ages 15 to 25). Research into transdiagnostic factors and interventions is essential to better understand and treat TAY with various psychological problems. The current dissertation had therefore three specific aims. First, I aimed to identify the most relevant transdiagnostic factors that are related to internalizing and externalizing symptoms in TAY. Second, I evaluated the effectiveness of ACT as a transdiagnostic intervention for TAY by conducting a meta-analysis and randomized controlled trial. Third, I explored for whom, under what circumstances, and through which transdiagnostic mechanisms ACT is (most) effective in improving mental health outcomes in TAY. The results of this dissertation support the adoption of a transdiagnostic approach in both research and clinical practice for TAY. Adaptive emotion regulation and psychological flexibility were found to be potential mechanisms of change in both ACT and TAU, and may therefore serve as valuable transdiagnostic intervention targets for this population. The findings of this dissertation also show that ACT is an effective transdiagnostic intervention for TAY. ACT was found to be as effective as other evidence-based interventions in reducing psychological problems (e.g., internalizing, externalizing and personality problems) and in improving ACT related processes (e.g., psychological flexibility and self-compassion), well-being (e.g., individual and societal functioning) and coping (e.g., emotion regulation). ACT seemed to work well in both individual and group formats, and across TAY with different ages, types and severities of problems and histories of earlier interventions. While more research is needed, especially in clinical samples, alternative forms of ACT (such as online or brief ACT interventions) show potential for TAY as well. I hope this dissertation inspires researchers and clinicians to adopt a more transdiagnostic perspective on mental health and to continue exploring ways to optimize the effectiveness of interventions for TAY, including ACT.