This dissertation explores the relationship between contemporary immigration-generated ethnic diversity and popular support for American and European welfare states. Drawing from theoretical and empirical research from across the social sciences, I derive the hypothesis that recent immigration negatively affects attitudes towards national welfare states. I test this "diversity-altruism hypothesis" in countries that vary across two important dimensions: contemporary welfare state institutions and historical institutions that impacted ethnic diversity. This project relies primarily on quantitative methods and employs a robust comparative strategy. Using country-specific data collected by scholars in each country and regional-level census data, I perform multilevel analyses of attitudes in Sweden (1986-2002), the Netherlands (1996-2006), and the United States (1980-2000). In addition, I use attitudinal data from 13 Western European countries, collected by the European Social Survey (2002-2004), and merge these data with institutional measures from the OECD, United Nations, and other sources. Taken together, the analyses in this dissertation provide empirical support for the diversity-altruism hypothesis. The comparative strategy reveals that immigration-generated diversity depresses support for welfare state attitudes regardless of a country's institutional features. However, these analyses also demonstrate that the relationship between diversity and altruism manifests itself in country- specific ways. Results suggest that countries' historical institutions and experiences with ethnic diversity play a more important role than contemporary national institutions in how diversity affects attitudes. This research contributes to a growing literature in sociology, political science, and economics, and its findings elucidate the challenges facing multiethnic societies.