The influence of partisan politics on public policy is a much debated issue of political science. With respect to foreign policy, often considered as above parties, the question appears even more problematic. This comparison of foreign aid policies in 16 OECD countries develops a structural equation model and uses LISREL analysis to demonstrate that parties do matter, even in international affairs. Social-democratic parties have an effect on a country's level of development assistance. This effect, however, is neither immediate nor direct. First, it appears only in the long run. Second, the relationship between leftist partisan strength and foreign aid works through welfare state institutions and social spending. Our findings indicate how domestic politics shapes foreign conduct. We confirm the empirical relevance of cumulative partisan scores and show how the influence of parties is mediated by other political determinants.