Social identity complexity refers to the way in which individuals subjectively represent the relationships among their multiple ingroup memberships. More specifically, individuals with low social identity complexity see their ingroups as highly overlapping and convergent, whereas those with high complexity see their different ingroups as distinct and cross-cutting membership groups. The present study tested the hypothesis that perceived overlap among ingroup memberships would be negatively related to ingroup inclusiveness and tolerance for outgroups, such that individuals with high overlap (low complexity) would be less tolerant and accepting of outgroups in general than those with low overlap (high complexity). Results from a telephone interview survey of adult residents of the state of Ohio supported this hypothesis. Individual differences in complexity of perception of their national, religious, occupational, political, and recreational social identities were systematically related to their attitudes toward ethnic outgroups and diversity.