Individuals with strong broad social motives (BSM) place high personal value on social activities and are at increased risk for heavy drinking. Those with strong BSM who also perceive that college students are heavy drinkers (high descriptive norms) might be particularly susceptible to increases in alcohol use during the transition to college, and may be likely to drink for social facilitation (social drinking motives). To test these hypotheses, we examined a mediated moderation model testing interactive effects of BSM and descriptive norms (perceptions of drinking in college) on drinking behavior, mediated by social drinking motives. Data were from 426 incoming college students and analyses were based on a subsample of 218 who reported drinking behavior at either time point. Results indicated that BSM interacted with descriptive norms to predict increases in social drinking motives from high school to college and social drinking motives in turn predicted increases in alcohol use. Probing this interaction revealed that those with high BSM and high descriptive norms experienced greater increases in social drinking motives than those with low descriptive norms. Tests of moderated indirect effects indicated that effects of BSM on drinking behavior through social drinking motives were also stronger among those with high descriptive norms. These results identify a particularly high risk group for changes in alcohol use during an important developmental period. This may have important implications for prevention and intervention methods which can attenuate college alcohol use by targeting individuals with strong BSM who perceive heavy drinking among college peers.