This prospective, longitudinal functional near-infrared spectroscopy (fNIRS) study aims to characterize the very early development of brain mechanisms for multimodal social perception and cognition (i.e., the "social brain") in infants at high-risk for developing autism. High-risk infants are characterized as such by virtue of having an older brother or sister already diagnosed with an autism spectrum disorder (ASD). The goal of this work is to apply the research team’s recent discovery of neuroendophenotypes of ASD (Kaiser et al., 2010, PNAS) to identify early biomarkers of ASD risk. While other groups have been using a variety of methods in attempts to identify early behavioral markers of ASD, these investigators are uniquely situated to apply a known neurobiological marker of this developmental disorder to a highly innovative, prospective, longitudinal neuroimaging study. The research team’s recent finding of neuroendophenotypes of ASD was heralded by the scientific community as a major advance and was selected by the Director of the National Institute of Mental Health as one of the top ten research events in 2010 and by the Interagency Autism Coordinating Committee as one of the top papers in the field of autism in 2010. Importantly, in contrast to other research groups that claim to have identified a biomarker of autism, the investigators included unaffected siblings in their sample to identify a true endophenotype of this neurodevelopmental disorder. This allowed us to identify unique compensatory mechanisms of brain development in unaffected siblings of children with ASD. The study includes a longitudinal neuroimaging study which will employ fNIRS to (1) identify the emergence and developmental trajectory of neuroendophenotypes in infants at high-risk to develop ASD and (2) extend the findings of disrupted brain mechanisms for social perception in high-risk infants beyond the visual domain to the perception of affective touch. In addition, the investigators aim to (3) identify early developmental trajectories of the state, trait and compensatory neural signatures - the trajectories in brain function that differentiate children who develop ASD from those who do not. The innovative, longitudinal study of the developing social brain will inform early detection of ASD risk, potential treatment approaches, and the search for genetic mechanisms.