PROJECT SUMMARY Young adulthood is a critical period for alcohol initiation and changes in alcohol use patterns, including increased risk for alcohol misuse and alcohol use disorders (AUD). Among young adults, social contexts are strong determinants of alcohol consumption and alcohol-related consequences. As such, it is critical for research studies to investigate social factors as mechanisms to understand future AUD risk in young adults. Rigorous prior research has shown that an individual’s response to alcohol is greater when they are drinking with another person compared to when they are drinking alone. We refer to this phenomenon as “social facilitation of alcohol effects”. We propose social facilitation of alcohol effects may represent the rewarding aspects of social contexts combined with alcohol-related rewarding effects to produce an additive rewarding experience. Social facilitation of alcohol effects may be relevant to young adults, as this developmental period is marked by lifetime peaks in alcohol use and heightened sensitivity to social factors. In Study 1, participants will complete 4 counterbalanced lab sessions: alcohol (0.0 g/kg vs 0.8 g/kg) × social context (alone vs with friend). During each session, participants will complete measures of affect and subjective alcohol effect prior to and following beverage consumption. During social sessions, participants will also complete a standardized conversation task with behavioral coding. The goal of Study 1 is to extend prior research characterizing the phenomenon of social facilitation alcohol effects to investigate person-level predisposing factors to social facilitation alcohol effects and mechanisms of social facilitation alcohol effects. Based on prior studies demonstrating that extraversion and social alcohol expectancies may associate with greater social reward and positive alcohol effects, we propose these as predisposing factors to social facilitation alcohol effects. Based on prior research demonstrating that alcohol consumed in social settings increases affiliative behavior and emotional contagion (i.e., individual’s affect can alter the affect of others in a coordinated manner), we propose increased agreeableness, emotional responsiveness, and emotional contagion as mechanisms of social facilitation of alcohol effects. The same participants will then complete Study 2, which includes 28-days of Ecological Momentary Assessment (EMA) and 6- and 12-month follow ups. EMA will capture social context and EMA and follow-up will assess naturalistic alcohol use outcomes (i.e., alcohol use, binge alcohol use, alcohol consequences). Study 2 will extend findings by investigating the role of acute social alcohol response (i.e., social facilitation of alcohol effects measured in Study 1) on future alcohol use and consequences. The combined EMA/longitudinal follow-up will allow for granular assessment (including social context) of real-time alcohol use uncontaminated by retrospective recal...