PROJECT SUMMARY Few effective interventions exist for neurodevelopmental disorders (NDDs), which are associated with significant impairment, high rates of disability, and substantial public health costs. The prevalence of NDDs has increased considerably over the past two decades, and children with NDDs make up the largest proportion of youth that seek mental health treatment. Thus, novel approaches for understanding NDDs and identifying potential treatment targets for these disorders are needed. NDDs often have comorbid presentations and shared symptomatology (e.g., impaired social functioning), which suggests that using a transdiagnostic approach to study NDDs may be informative. This approach could reveal modifiable factors and underlying neurobiological mechanisms that could be targeted through intervention to reduce neurodevelopmental symptomatology. Social connectedness (i.e., extracurricular involvement, family dynamics, and relationships with peers and parents) may be a salient protective factor for youth with neurodevelopmental symptomatology. An important neural correlate for social functioning appears to be functional connectivity within the salience network (SN), which is involved in detection of relevant stimuli (e.g., changes in other's emotional expressions).Thus, social connectedness and related connectivity within the SN may be predictive of neurodevelopmental outcomes. The potential protective role of these factors may be particularly salient during adolescence, which is a vulnerable period for many of the negative outcomes associated with NDDs. The proposed longitudinal project will capitalize on the multi-level framework of the Adolescent Brain Cognitive Development (ABCD) study by using psychosocial, functional neuroimaging, and cognitive data from two timepoints (i.e., baseline and one-year follow-up) to assess neurodevelopmental functioning in a sample of >11,500 youth in early adolescence. The project will be conducted at the Laureate Institute for Brain Research, which is an ABCD study site. The aims of the proposed project are threefold: to determine whether 1) social connectedness is associated with SN functional connectivity cross-sectionally; 2) social connectedness predicts future neurodevelopmental function; and 3) SN functional connectivity predicts future neurodevelopmental function. These aims will be assessed using linear mixed effects models with relevant covariates included (e.g., baseline neurodevelopmental function, youth ethnicity, youth gender, and family income). The proposed study has an exploratory aim of utilizing a data-driven, machine learning approach to identify additional modifiable factors (e.g., sleep, physical activity, screen time) that predict future neurodevelopmental function. The project is novel in that it takes a transdiagnostic approach to studying NDDs, it is longitudinal, and it has the potential for replication and extension as future data are released. Results from the project could have meaningful clinical relevance with the potential for identifying treatment targets for future interventions aiming to improve neurodevelopmental symptomatology in early adolescence.