Immigration and globalization have spurred interest in the effects of ethnic diversity in Western societies. Most scholars focus on whether diversity undermines trust, social capital, and collective goods provision. However, the type of prosociality that helps heterogeneous societies function is different from the in-group solidarity that glues homogeneous communities together. Social cohesion in multiethnic societies depends on whether prosocial behavior extends beyond close-knit networks and in-group boundaries. We identify two features of modern societies—social differentiation and economic interdependence—that can set the stage for constructive interactions with dissimilar others. Whether societal adaptations to diversity lead toward integration or division depends on the positions occupied by minorities and immigrants in the social structure and economic system, along with the institutional arrangements that determine their political inclusion.