Autism is currently widely viewed as a spectrum disorder with affected individuals having varying degrees of social and emotional disturbances. This project entails a new conceptualization of autism in order to account for some of the heterogeneity. For this research, a neuroendocrine spectrum model is proposed in which aspects of emotional reactivity, social responsiveness and biological indices intersect to define distinct subtypes. The investigation involves three interrelated studies of socioemotional functioning utilizing several methods of analysis, including the assessment of biological markers of emotional arousal and stress, sophisticated behavioral observational techniques, and functional neuroimaging, in order to carefully explore the psychobiological profiles of children with and without autism. Specifically, during peer interactions of a playground activity, sophisticated behavioral observation will be obtained to evaluate the frequency, duration and interactive sequences exhibited by children with autism. These data will be analyzed with stress hormones to uncover relationships between behavioral and biological symptom profiles. In addition, brain activation patterns in these children will be explored while they play games with different computer and human partners. It is anticipated that this comprehensive study of "real world" social interactions will elucidate meaningful associations between the child's behavior, biological profiles and brain activity before, during and after play with peers. The ultimate goal is that these results will better inform understanding of autism to provide individualized biological and behavioral treatments.