Despite the historical importance of ideology-based, economically inhibitive laws, we know little about the economic factors underlying their origin. This paper accounts for the historical emergence of one such law: the Christian ban on taking interest-a doctrine that shaped the evolution of numerous financial contracts and related organizational forms. A game-theoretic analysis and historical evidence suggest that the Church's commitment to providing social insurance for its poorest constituents encouraged risky borrowing, which the Church attempted to limit by banning interest. The analysis highlights the applicability of the rational choice framework to seemingly irrational actions and laws, the role of nonmonetary sanctions in circumventing commitment problems, and the importance of economic forces vis-à-vis ideology. (c) 2009 by The University of Chicago. All rights reserved..