This project is focused on the development, validation, and standardization of quantitative methods to measure core components of variation in the social development of young children. This is critical for advancing understanding of autism spectrum disorder (ASD), and will inform the development and evaluation of early interventions for children with ASD and related disorders. This study will examine the general population distribution and longitudinal course of quantitative variation in reciprocal social behavior (and related indices of social behavioral development) in an ethnically diverse sample of preschool age twins, assessed prospectively at key ages from 18-48 months. The project will also examine the stability and change in the same set of quantitative measures when implemented among children with ASD first diagnosed at 18-24 months, and will clarify the relationship between these measures and traditional diagnostic assessments for ASD. Finally, the variation in visual social engagement (VSE)-a neurodevelopment capacity compromised in children with ASD-among preschool children will be studied in children with ASD and typically developing children.