ABSTRACT The rapid adoption of mobile and interactive technologies by American families has outpaced research on their potential effects on child development and health. Much prior research on television (TV) and older forms of media relied upon parent recall of global constructs such as “screen time,” which may not be a complete representation of family media use now that parents and children use mobile and interactive devices in an intermittent, on-demand manner throughout the day. As highlighted at the 2018 NICHD scientific workshop on Media Exposure and Early Child Development, the design affordances of mobile and interactive media differ from TV in several important ways, and deserve novel scientific paradigms to describe how children and families use media. The proposed project will fill these gaps in scientific knowledge by testing a conceptual framework that examines how parent and child mobile media use influence the development of child socio- emotional skills through mechanisms involving parent and child cognitive, emotional, and behavioral responses states. We will also test whether effects are moderated by child temperament, parent executive functioning, and family psychosocial stress. The proposed research leverages an ongoing longitudinal cohort study of diverse toddlers and their parents (NICHD 1R01HD102370, PI: Radesky) that conducts home visit-based data collection at ages 2, 3, and 4 years, adding virtual data collection waves at 2.5, 3.5, and 4.5 years with the CAFE Consortium data collection toolkit. Our approach will be synergistic with the other CAFE investigators on this P01 Proposal and include a program aim of efficiently classifying children’s content as positive and child- centered vs. engagement-prolonging and exploitative. In specific aim 1, we will use structural equation models to examine bidirectional, longitudinal associations between parent mobile media use duration, frequency of interruptions of family activities, and media response states (cognitive load related to media use) with early childhood social competence, emotion regulation, and problematic media use. In specific aim 2, we will examine bidirectional longitudinal associations between child mobile media use for regulatory purposes, content quality (exploitative vs. positive), and media response states (difficulty transitioning from media) with early childhood social competence, emotion regulation, and problematic media use. In specific aim 3, Examine differential susceptibility to media effects by testing moderation of associations in SA1 and SA2 by child temperament, parent executive functioning, and psychosocial stressors. Our findings will contribute to the evidence base upon which digital media guidelines are based, as well as digital design approaches that reflect the needs of families with young children. In addition, our attention to modeling bidirectional associations and articulating individual child, parent, and design affordance factors in t...