PROJECT SUMMARY/ABSTRACT Autism spectrum disorder (ASD) affects 1 in 54 children in the United States and is characterized by difficulties in initiating and maintaining social interactions, leading to negative impacts on real-world social functioning and mental health. Therefore, an important target of intervention in ASD is improving social interaction outcomes, particularly peer interactions in the sensitive time period of early adolescence. During early adolescence, children with ASD lag behind their peers in social development, yet peer relationships are an increasingly important determinant of mental health. One potential target of intervention for improving social interaction outcomes in ASD is alignment, the degree to which two or more individuals are aligned in their neural signals, behaviors, and emotions in time. Alignment includes neural similarity (the pre-existing similarity of individuals' neural response to stimuli) and interactional synchrony (the dynamic adjustments in body position and facial expressions that occur between interacting individuals). Alignment has been established as an important predictor of social interaction success in typically developing individuals, and some preliminary work has suggested that alignment may be atypical in interactions including individuals with ASD. However, alignment and its associations with social interaction outcomes have not yet been examined in early adolescent peer relationships in ASD. Furthermore, a promising mechanism of atypical interactional synchrony in ASD, atypical social attention, has not been directly tested as a mediator between diagnosis and atypical synchrony. By using ecologically valid methods to examine live social interaction behaviors and outcomes between dyads of novel peers, this proposal will test alignment as a mechanism of peer interaction challenges in ASD in real- world contexts. Two aspects of alignment, neural similarity (Aim 1) and interactional synchrony (Aim 2), will be tested as predictors of social interaction challenges in dyads including an individual with ASD. Aim 3 will test the hypothesis that atypical social attention mediates the relation between ASD diagnosis and atypical interactional synchrony. Collectively, these experiments will not only examine diagnostic group differences in alignment, but also quantify the impact of alignment on real-world social interaction success, and investigate a potential mediator of relations between diagnosis and interactional synchrony. Peer interaction success in early adolescence is a key determinant of mental health, and the identification of atypical alignment as a mechanism of peer interaction challenges could lead to new targets of intervention to support social functioning in ASD. This proposal is supported by an integrated training plan developed with an expert mentorship team. Three training goals will ensure the applicant is well prepared to transition to a future independent research career investigat...