By highlighting the many ways that constitutions vary, comparative constitutional law raises interesting and important causal questions: What explains cross-national constitutional variation, and what are the real-world consequences of different constitutional arrangements? But comparative constitutional law scholarship so far has done relatively little to address these issues of constitutional causes and consequences in a rigorous manner. In this paper, I argue that scholars have much to gain from taking causality seriously in comparative constitutional law, and I suggest that scholarship on comparative politics and comparative political economy provides useful insights about how this might be done. First, I provide an overview of recent comparative constitutional law scholarship to highlight the pervasive issues of causality that it raises. Second, I introduce some of the interesting work that political scientists and economists have done on comparative constitutional law. They are asking questions about the origins and consequences of constitutions that are similar to those raised in comparative constitutional law scholarship - but they are framing them in explicitly causal terms, developing positive theories about cause-and-effect relationships, and testing them empirically using social science methods of inference. Third, I illustrate one such method that can be used to address causal claims and causal questions in comparative constitutional law. Using regression analysis of cross-national data on constitutions, government spending, and other institutional, demographic and economic factors in 80 democracies, I test a series of hypotheses about the effects of different constitutional arrangements on government spending. I also show how multiplicative interaction terms can be used to model and empirically test for conditional relationships between constitutions and various political, social or economic outcomes. I conclude with a proposed agenda for empirical comparative constitutional law, outlining its theoretical, methodological and pedagogical implications.