This study aims to better understand the shared and divergent neurologic underpinnings of fragile X syndrome (FXS) and autism spectrum disorder (ASD) as well as to expand knowledge about the psychophysiology of these disorders. While both groups demonstrate impairments or alterations in the use of social behaviors, such as eye gaze, it is unclear if these behaviors share the same underlying neural mechanisms. This project will investigate alterations in neural responses by measuring event-related potentials (ERPs), the electrophysiological brain responses that are the direct result of thoughts or perceptions. The study will determine if altered ERPs underlie the behavioral phenotype of abnormal attention to and use of eye gaze in individuals with ASD and FXS. It will also examine whether these individuals can use eye gaze cues to facilitate behavior in a task with low social demand and determine whether the speed of neural response to faces is related to behavioral use of eye gaze in a task that has high social demand. Long term goals for this project include investigating how eye gaze behaviors are related to the development of social deficits in these disorders and identifying links between ERP responses to social stimuli and the behavioral phenotype and biological markers of FXS and ASD.