We advance social identity theory by hypothesizing that the content of demographic attributes on which members differ, and not just their distribution, influences the relationship between a group’s composition and its performance. We test this theoretical logic, using both laboratory and field data, by investigating groups with different distributions of members (from the same or different nations) and cultural orientations (individualistic or collectivistic). We hypothesize that, because a collectivistic orientation promotes group identification, a focus on collective goals, and a sense of being an interchangeable exemplar of the group, it also reduces the polarizing effects of demographic heterogeneity and improves group performance. Using an experimental design, we find that subjects primed with a collectivistic rather than an individualistic orientation see less distinction between nationally homogeneous and heterogeneous groups, and expect the group to be more successful. We also analyze archival data representing 5,460 Himalayan climbing expeditions and find that expeditions characterized by higher levels of national heterogeneity are more likely to reach the summit if more members hail from collectivistic rather than individualistic countries. Simultaneously considering the distribution and content of attributes on which members differ will accelerate the evolution of a comprehensive theory of social identity processes and consequences.