The focus of this renewal application is to build upon the findings from the Influence of Social Networks on Infant Care and Disparities in Postneonatal Infant Mortality (SONIC) study by conducting a 4-armed randomized controlled trial (RCT) using online social networks (OSNs) to promote safe infant care practices and reduce Black/White (B/W) disparities in adherence to these practices. In SONIC, we conducted social network analysis on 402 new and experienced mothers of young infants. We learned that there are 2 social network (i.e., family, friends, and peers) types: exclusive (kin-based) and expansive (not kin-based); that network social norms (group rules defining appropriate behaviors, values, and attitudes) are highly predictive of infant sleep practices (sleep position, bedsharing, soft bedding use); that mothers are more likely to change from safe to unsafe practices if their network members espouse unsafe practices; and that Black mothers are more likely to have exclusive networks, which are more likely to have unsafe practices as the norm. Safe sleep practices are associated with lower rates of sudden infant death syndrome (SIDS) and unintentional injury- related deaths (e.g. accidental suffocation) associated with the sleep environment. These sleep-related deaths (SRDs) remain the leading cause of postneonatal death in the US, with ~3600 deaths/year; Black infants die at more than double national rates. Thus, differences in social networks and norms contribute to the Black/White (B/W) disparity seen in safe sleep practices, which in turn contribute to the B/W disparity in postneonatal infant deaths; the influence of social networks and norms, if contrary to recommended infant care practices, is a major barrier to acceptance of these practices. OSNs have become a powerful tool for establishing social norms and influencing behavior, and data suggest that this is equally or more so for Blacks. Thus, our proposed SUPERSONIC (Support via Online Social Networks to promote Safe Infant Care practices Toward Reducing Racial Disparities in Infant Mortality) study is the natural next step after SONIC. We plan to test, through a 4-armed pragmatic RCT, an intervention strategy that uses OSNs (private Facebook groups) to change social norms among WIC recipient mothers, who have a disproportionate burden of SRDs and high proportion of Blacks. Mothers in each study arm will participate in an OSN and receive one of 4 messaging interventions: 1) safe sleep alone, 2) breastfeeding alone, 3) both safe sleep and breastfeeding, and 4) routine health messages (attention-matched control). Beginning prenatally, we will disseminate via OSNs videos and messages that address common myths and misconceptions to ultimately impact attitudes and social norms regarding infant sleep practices and breastfeeding. If utilization of OSNs is effective in changing network norms and maternal attitudes, and improving adherence, this approach is easily scalable. Moreover, it coul...