This project sought to examine 2 competing hypotheses: first, that twins are likely to be more prosocial by virtue of growing up with another same-age peer in the home, or second, that they are less prosocial because they have become more comfortable interacting with a same-age peer who is both genetically and environmentally similar to themselves and therefore they are less comfortable with other children who are dissimilar. Two studies were conducted to compare twins to singletons on measures of prosocial and aggressive behaviors. In Study 1, 5-year-olds (N = 91 twins and N = 152 singletons) engaged in a peer play situation with an unfamiliar, same-age, same-sex peer, and they were rated on items assessing prosocial and aggressive behaviors. Results showed that twins were less prosocial but not more aggressive than were singletons. In Study 2, which was a supplemented follow-up study of twins in Study 1, 10- to 15-year-old twins (N = 98) and singletons (N = 84) were rated by their parents on prosocial and aggressive behaviors. No significant differences were found between the groups on prosocial behavior, but twins were rated as more aggressive than singletons. Thus, in early childhood twins appear to exhibit fewer prosocial behaviors with unfamiliar peers, but this prosocial deficit was not aligned with parent-reported prosocial behaviors in adolescence. In adolescence, twins were rated by parents as more aggressive. These studies suggest that twins may be at risk for poorer social interactions in early and middle childhood.