Constitutional political economy addresses four questions: (1) the causal question: What explains the constitutional institutions we observe? (2) the consequential question: What consequences do constitutional institutional have? (3) the ideal question: What constitutional institutions does justice require? and (4) the design question: What constitutional institutions are best for a polity given the constraints imposed by its current situation? Answers to the ideal and design questions require a theory of behavior that predicts how individuals will behave within constitutional institutions. Analysts usually assume that this theory of behavior corresponds to the explanatory theory developed to answer the second, consequential question. This essay argues that the assumption of rational self-interested behavior as the basis for a behavioral theory is not justified.