Psychological Flexibility and Mental Health: Theoretical Foundations, Empirical Support, and Applications Across...

A key transdiagnostic process in modern mental health science is psychological flexibility, defined as the ability to remain freely engaged with present-moment experience while pursuing valued goals, even in the context of suffering. The construct, which is based on Relational Frame Theory and operationalised within the framework of Acceptance and Commitment Therapy (ACT), consists of six interconnected processes: self-as-context, values clarity, acceptance, cognitive defusion, present-moment awareness, and committed action. This narrative review, based on systematic searches of PubMed, PsycINFO, and Web of Science covering publications from 1987 to 2024, synthesizes theoretical foundations, measurement approaches, mechanistic evidence, and clinical applications of psychological flexibility across diverse populations and conditions. Meta-analytic findings consistently demonstrate small-to-medium effects of flexibility-based interventions on anxiety, depression, chronic pain, substance use disorders, and workplace functioning, with converging evidence that psychological flexibility mediates treatment outcomes. The Acceptance and Action Questionnaire-II (AAQ-II) remains the most widely used assessment tool; however, concerns persist regarding its discriminant validity from general negative affect. Key limitations include predominantly cross-sectional research designs, cultural homogeneity in study samples, and methodological weaknesses in mediation research that constrain causal inference. Ten research priorities are proposed, with particular urgency around prospective longitudinal designs to establish causal precedence, the development of culturally sensitive assessment instruments, and systematic cross-cultural adaptation research. Current evidence supports psychological flexibility as a clinically accessible, theoretically coherent construct with demonstrated utility across a wide range of conditions. For practitioners, this translates into a process-based intervention target that is applicable across diagnostic categories and that does not require disorder-specific protocol switching. It can be reduced to specific clinical techniques like acceptance exercises, defusion strategies, and values-guided behavioural activation.