In many parts of the world, fertility has declined in important ways in the past century. What are the consequences of this demographic change? Our study expands the empirical basis for understanding the relationship between number of siblings in childhood and social outcomes among adults. An important recent study found that for each additional sibling an individual grows up with, the likelihood of divorce as an adult declines by 3%. We expand this work by (a) determining whether the original pattern replicates in the National Longitudinal Study of Adolescent to Adult Health and (b) extending the analysis beyond divorce to consider whether growing up with siblings is related to prosocial adult behaviors (relationships with parents, friends, and views on conflict management with one’s partner). Our results confirm a negative association between number of siblings and divorce in adulthood. We find mixed results related to other prosocial adult behaviors.