Customary international law is understood to require that state practices be followed from a sense of legal obligation, though international lawyers have long puzzled over how those obligations come into being. Recent work applying rational choice theory suggests, unsettlingly, that the entire inquiry is misconceived: practices commonly attributed to obligations are merely behavioral regularities that arise from intersecting state interests, and the role of legal obligations is minimal at best. This Article attempts to explain how the rational choice critique and traditional doctrine may be reconciled. Rational choice theory, it is argued, responds to a genuine problem in the existing understanding of customary international law; if properly leavened, the approach provides a useful and insightful tool for analyzing the relevance of law to state practices-albeit one confirming suspicions that custom is not so widespread as may have been supposed, and may prove less resilient too. But expanding the range of games used to analyze state behavior, and understanding the interdependence of those games, also shows how the rational choice and traditional doctrine are not so very far apart-and how legal obligations may have an instrumental role to play in each.

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