This research will evaluate dynamic pupillary light reflex (PLR) as a potential biomarker for autism. PLR refers to the involuntary response whereby the pupil size changes in response to a short flash of light, and it can serve as a simple objective measure of neurological functions. Preliminary studies revealed that individuals with autism showed significantly longer constriction latencies and smaller constriction amplitudes in their PLR responses. The proposed research will validate these preliminary results in a large heterogeneous group of participants and will clarify whether the atypical PLR parameters are specific to autism spectrum disorder (ASD) or are associated with autonomic nervous system dysfunction in general. These studies will allow for further evaluation of the specificity and generalizability of PLR as a potential biomarker for ASD. The PLR testing procedure is low-cost, non-invasive, objective, and has the potential to be implemented for very young children.