PROJECT SUMMARY/ABSTRACT Social isolation and its subjective counterpart loneliness—well established as risk factors for poor physical and mental health—have been rising at alarming rates in the US, especially among young adults. Mechanistic understanding of how best to build social connectedness to ameliorate social isolation is sorely needed to redirect life trajectories toward health and well-being. In creating this foundational knowledge, populations who face higher disease burden—Black/African American young adults, Latino/Hispanic young adults, and those with less socioeconomic privilege—merit special focus because persons in these at-risk populations often face unique challenges in initiating social interactions, including discrimination and economic inequality. The broad, overarching objective of this work is to conduct basic experimental research on social connectedness to test whether, how, where, and for whom health communication messages can motivate in-person interactions to reduce young adults’ social isolation and loneliness. Our multi-disciplinary team brings together expertise in social psychology, emotion science, communication science, and health disparities and will carry out a 6-week randomized controlled trial—the Keep Social RCT—using our innovative and ecologically valid simulated social media platform and a suite of rigorous repeated measures of social behavior, loneliness, and other health- relevant outcomes. This program of research is designed to meet three specific aims. SPECIFIC AIM 1 is to optimize health messages about the value of social connectedness for young adults (ages 18-29) from populations who face higher disease burden and then conduct the Keep Social RCT to build a rich empirical platform. This aim will be met using a human-centered process to design health communication messages that include peer imagery and stories for an online experiment with 500 at-risk young adults. Messages that receive highest ratings for encouraging in-person interactions in this online experiment will be selected for the Keep Social RCT, which is placebo controlled with behavioral, survey, and implicit assessments repeated over six weeks. SPECIFIC AIM 2 is to analyze theory-driven mechanisms through which health communication messages in the Keep Social RCT may reduce young adults’ social isolation and loneliness to identify intervention targets. This aim will be met with longitudinal statistical modeling to test whether and how the experimental health communication messages improve social connectedness. SPECIFIC AIM 3 is to extend data analyses of the Keep Social RCT to identify social structural and person-level moderators of reduced social isolation and loneliness to identify where and for whom effects are largest. This aim will be met with advanced statistical modeling to illuminate the conditions under which our health communication messages most effectively ameliorate social isolation and loneliness in young adults who fa...