Defining and measuring "psychological flexibility": A narrative scoping review of diverse flexibility and rigidity constructs and perspectives.

Psychological flexibility (PF) is a popular construct in clinical psychology. However, similar constructs have existed since the mid-20th century, resulting in different terms, definitions and measures of flexibility, hindering the advancement of the field. The main measure of PF - the Acceptance and Action Questionnaire (AAQ-II; Bond et al., 2011) - has also been heavily criticized. To move towards definitional consensus and improved measurement, we surveyed the literature to map PF and related-terms, examine definitional overlaps, and assessthe psychometric quality of prominent flexibility measures. A scoping review was conducted in two databases (PsycNET and SCOPUS). Twenty-three flexibility constructs appeared across 220 articles, and twelve measures were included and rated for quality. PF, psychological inflexibility (PI), and cognitive flexibility (CF) were most prominent. Definitional similarities among prominent flexibility constructs emerged, namely handling distress or interference, taking action, and meeting goals or values. The Personalized Psychological Flexibility Index (PPFI; Kashdan, Disabato, Goodman, Doorley, & McKnight, 2020) appears to be the best measure available to assess PF. Problems with the current use of the AAQ-II were apparent, hindering current knowledge of PF. Definitional consensus and measurement development are vital to advance the field. To this end, recommendations and next steps for researchers and practitioners are outlined.