The prospective study of younger siblings of children with ASD offers an ideal opportunity to learn about the earliest signs of autism as well as the development, manifestation, and boundaries of the broader autism phenotype. This cross-site, project will focus specifically on the domains of attention coordination and positive affective competencies, and examine their development across the first two years of postnatal life (i.e., from 6 to 18 months) and their relation to later social and communicative outcomes at age 24 and 36 months. The overarching hypotheses are that there will be developmental continuity in these domains across the first two years, that early differences in these domains will be evident between groups at high and low-risk for ASD, and that early impairments will predict individual differences in social and communicative outcomes observed in later-born ASD siblings.  This information is critical for understanding the causes of autism and for developing more effective and targeted treatments.