Measuring the Shadow of the Future: An Introduction to the Game Theory of Customary International Law

Compliance remains one of the most contested issues in interna-tional law. In this article, Professors Norman and Trachtman present a game theoretic model to identify the circumstances under which states have a rational incentive to comply with customary international law (CIL). One common explanation for compliance is the “shadow of the future,” in which a state is understood to comply out of fear that failure to do so will prevent it from using international law at some later time. A violator, the logic goes, will eventually be in a position where it wishes to invoke international law, and having failed to comply at an earlier time, will not be able credibly to invoke international law at a later time. Professors Norman and Trachtman demonstrate how game theorists measure the magnitude of the “shadow of the future,” and how this magnitude may be modified in order to increase the likelihood of compliance. The article thus shows what facts or parameters would be relevant in a rational preference-maximizing model of a state’s decision to comply.

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