PROJECT SUMMARY With widespread public concern about the increasing presence of smartphone and social media (SSM) technologies in the lives of American youth,1?3 there is a conspicuous need for investment in rigorous scientific work that will inform our understanding of the psychological precursors of SSM habits, and on the impacts that these habits may have on subsequent development and everyday functioning. While a foundational literature is starting to form, research on the origins and outcomes of SSM habits remains skeletal, and two basic questions are still unanswered: 1) Are the correlates of digital media habits stable across development, or do they vary with age? 2) In what direction do these relationships ensue ? do observed correlations reflect individual differences that presage subsequent variation in SSM habit formation/intensity, or, do they reflect the impacts of SSM habits on consequent brain/psychological development and everyday functioning? The present study aims to refine our answers to these key questions through a multi-methodological (behavior, neuroimaging, self-reports, ecological momentary assessment) cross-sequential investigation of the individual differences factors that prospectively predict SSM habit formation and of the outcomes associated with intensification of such habits across development. Through an initial cross-sectional assay of children, adolescents, and young adults, we will investigate whether the relations between psychological/brain functioning and SSM behaviors vary as a function of age. In a subsequent longitudinal phase, we will concurrently track trajectories of psychological/brain maturation, everyday functioning (e.g., academic outcomes, psychological and physical wellbeing), and SSM engagement, in order to shed light on the temporal chain of processes linking mental functioning and SSM usage. Our investigative team is extremely well positioned to execute the proposed research, which will draw heavily from the theoretical framing and methods that have propelled our previous work, and which will recruit from active developmental cohorts who have participated in related recent studies implemented in our labs. Based on guiding neurodevelopmental theories, we hypothesize that individual differences in self-regulatory control, reward and sensation seeking, reactivity to social inputs, and risk-taking propensity will be differentially predictive of usage patterns at varying points in development, and that these relationships will be shown to have bidirectional interactions with intensifying SSM behaviors, with important implications for everyday functioning.