Establishing construct validity for dynamic measures of behavior using naturalistic study designs.

There has been a recent surge of naturalistic methodology to assess complex topics in psychology and neuroscience. Such methods are lauded for their increased ecological validity, aiming to bridge a gap between highly controlled experimental design and purely observational studies. However, these measures present challenges in establishing construct validity. One domain in which this has emerged is research on theory of mind: the ability to infer others' thoughts and emotions. Traditional measures utilize rigid methodology which suffer from ceiling effects and may fail to fully capture how individuals engage theory of mind in everyday interactions. In the present study, we validate and test a novel approach utilizing a naturalistic task to assess theory of mind. Participants watched a mockumentary-style show while using a joystick to provide continuous, real-time theory of mind judgments. A baseline sample's ratings were used to establish a "ground truth" for the judgments. Ratings from separate young and older adult samples were compared against the ground truth to create similarity scores. This similarity score was compared against two independent tasks to assess construct validity: an explicit judgment performance-based paradigm, and a neuroimaging paradigm assessing response to a static measure of theory of mind. The similarity metric did not have ceiling effects and was significantly positively related to both the performance-based and neural measures. It also replicated age effects that other theory of mind measures demonstrate. Together, our multimodal approach provided convergent evidence that dynamic measures of behavior can yield robust and rigorous assessments of complex psychological processes.