Acceptance and commitment therapy (ACT) is evidence-based, transdiagnostic psychotherapy that seeks to increase values-consistent behavior and advances mental health by promoting psychological flexibility (PF) and diminishing psychological inflexibility (PI). PF implies a person's ability to cope with, accept, and adjust to challenging circumstances, whereas PI denotes the inflexible dominance of psychological responses in steering behavior over selected values and contingencies. The Multidimensional Psychological Flexibility Inventories (MPFI-60-24) have been established to cover all six factors of PF and six factors of PI at once. Still, these measures have yet to be validated in a Persian sample and this study seeks to validate the Persian versions of MPFI. We evaluated long and short versions of the MPFI in a Persian sample of 1270 (83% females, ages 18 through 57), divided into two groups, a healthy community group (n = 886) and a clinical group (n = 391). The MPFIs' psychometric properties, including criterion validity, construct validity, internal consistency, and measurement invariance, were investigated. Confirmatory factor analysis (CFA) revealed that a 12-factor model had a good fit to the data for the long version (CFI = 0.92) and a very good fit (CFI = 0.98) for the short version. Regarding higher-order models, two-factor second-order model indices for long-form were adequate (CFI = 0.92), and for short-form, they were good (CFI = 0.96). All 12 subscales demonstrated sufficient reliability. Measurement invariance for gender and mental health state was observed, as well as good internal consistency and adequate criterion validity with psychological distress markers (i.e., depression, anxiety, and stress). PI was more strongly related to psychological distress than PF. Persian versions of the MPFI are reliable instruments for indexing PF and PI in both genders as well as in community and clinical samples.
