What are the political origins of redistributive welfare policies? What explains why policies vary over time and across countries? Hypotheses regarding the causal influence of various factors, such as political regime type, the mobilization of the organized working class, the structure and capacity of the state, patterns of economic development, and the influence of international organizations, will be tested in both a Mexican case study and a cross-national statistical analysis. Mexican social insurance does not seem to fit the standard explanations presented in the theoretical debates regarding the economic and political genesis of the welfare state, making it a case which deserves closer scrutiny. My dissertation will explain this paradoxical case by combining qualitative, comparative historical and quantitative, multivariate timeseries analyses of Mexican social insurance since the Revolution. A pooled, cross-sectional time-series analysis of social insurance in twelve Latin American countries since the 1960s will be used to explain social policy variation across countries and time. By combining quantitative and qualitative methods with historical, intra-regional, and cross-regional comparisons, this research will address an important scholarly and policy debate which has largely ignored the developing world.