Autism spectrum disorder (ASD) affects 1 in 68 children in the United States. Rising along with the prevalence of ASD is the rate of being a sibling of a child with ASD. Multiple previous studies have demonstrated that many first-degree relatives of individuals with ASD experience their own challenges, including not only higher rates of ASD, but also milder social and communication difficulties, attention problems, and mood and anxiety symptoms. In the past decade, multiple investigations, including our own, have followed longitudinally later- born siblings who were enrolled in infancy, before parent concerns about their development were likely. This provides an ideal sample in which to study the development of childhood psychopathology, since it is not only relatively unbiased by selective enrollment, but is also characterized by a high rate of suboptimal outcomes. Rates of atypical development in later-born siblings of children with ASD are high (up to 30%) at age 3, but very few studies have followed them to school-age, making it difficult to determine whether early differences persist and/or new difficulties emerge with age. What is imperative to know, as the prevalence of ASD continues to increase, is what the longer-term developmental effects on siblings of children with ASD are, and whether this growing population requires more intensive early surveillance, screening, and intervention. The proposed project has three aims, each of which examines developmental and mental health functioning of school-aged siblings of children with ASD. All three aims leverage previous funding, following longitudinally 3 cohorts of younger siblings of children with ASD (n=166) or typical development (n=134) ascertained in infancy and assessed regularly, at up to 7 ages, from birth to 36 months. The sample is currently aged 6 to 16 years and we now propose to assess them twice more, approximately two years apart, to examine multiple dimensional measures of social-communication, attention, anxiety, and cognitive control processes. Analyses will examine group differences in current levels of performance in these domains, as well as infant predictors of later functioning. We will also employ multiple measures of functional impairment to explore how individual variations in social-communication processes, attention, and cognitive control are related to later adjustment in home, peer, and school contexts. This project, by studying longitudinally a sample at high risk for multiple suboptimal developmental outcomes, provides the opportunity to study very early predictors of later psychopathology and impairment. This will enhance both early identification and treatment development efforts, as well as set the stage for future genomic and neurobiological studies of individuals at risk for a wide range of neurodevelopmental and mental health conditions.