Immigration is the driving force behind the rapidly growing ethnic, racial, and religious diversity of Western welfare states. This essay reviews research on the effects of immigration and diversity on the political support for redistributive social programs. A growing body of scholarship finds negative relationships between indicators of ethnic heterogeneity and measures of immigration, on the one hand, and attitudes towards welfare measures and state expenditures on welfare, on the other. Responding to these results, scholars have searched for mediating factors that slow or reverse these effects. As empirical studies multiply it is important to develop theories to make sense of the data. I discuss in-group and out-group theory, reciprocal altruism, and the neoDarwinian theory of ethnic nepotism. I argue that while no theory has been shown to be clearly superior to the others, neoDarwinism is the most comprehensive and should not be dismissed because of its pessimistic implications for immigration, diversity, and welfare states.