Development of the Flexibility in Daily Life scale to measure multidimensional cognitive and behavioural flexibility in health and disease

Objectives: Inflexibility of thought and behaviour is a transdiagnostic feature of many neuropsychiatric disorders and presents several empirical measurement challenges. Here, we developed and validated the Flexibility in Daily Life scale (FIDL); a novel, self- report questionnaire, which captures expressions of cognitive and behavioural flexibility in daily life and is sensitive to natural shifts in these processes across the adult lifespan. Methods: The FIDL was developed using a deductive scale development approach, which aimed to capture common themes within the flexibility literature and across diagnoses (e.g. insistence on sameness, preference for routines). Following multidisciplinary consensus, an initial 37- item questionnaire was submitted for validation in an online sample of 295 healthy adult participants (19-78 years). Results: Exploratory factor analysis produced a revised 21- item version comprising five factors, labelled: Repetition, , Switching, , Predictability/Control, , Routine, , and Thoughts/Beliefs. . Internal consistency reliability was good- to- strong for the total FIDL score and moderate- to- strong for individual subscales. Convergent validity was established between the FIDL and an existing measure of cognitive flexibility. Critically, the FIDL total score evinced a U- shaped relationship with age, whereby flexibility was lower at the younger and older tails of the lifespan and greater in middle age. The same U- shaped trajectory emerged for the Repetition, , Routine, , and Thoughts/Beliefs factors. Conclusions: Overall, the FIDL is a valid and reliable multidimensional measure of flexibility, which upholds a clearly defined factor structure and good psychometric properties. It promises to be a valuable clinical and research tool to assess the natural fluctuations in flexibility across the lifespan and departures thereof.