Does socio-demographic diversity within undergraduate student teams affect learning outcomes? Based on an experiment among undergraduate student teams, we found some evidence of the aggregate of a team's socio-demographic characteristics influencing its performance, no such association was found concerning intra-team diversity. Remarkably, though, the greater degree of familiarity among the team members in self-selected as opposed to randomly assigned teams is associated with lower team performance. This result may be worth bearing in mind for students and instructors alike, not least in the context of peer instruction.