University of Chicago Law School Chicago Unbound Coase-Sandor Working Paper Series in Law and Coase-Sandor Institute for Law and Economics Economics 1998 A Theory of Customary International Law Eric A. Posner Jack L. Goldsmith Follow this and additional works at: https://chicagounbound.uchicago.edu/law_and_economics Part of the Law Commons Recommended Citation Eric Posner & Jack L. Goldsmith, "A Theory of Customary International Law" (John M. Olin Program in Law and Economics Working Paper No. 63, 1998). This Working Paper is brought to you for free and open access by the Coase-Sandor Institute for Law and Economics at Chicago Unbound. It has been accepted for inclusion in Coase-Sandor Working Paper Series in Law and Economics by an authorized administrator of Chicago Unbound. For more information, please contact [email protected]. CHICAGO JOHN M. OLIN LAW & ECONOMICS WORKING PAPER NO. 63 (2D SERIES) A THEORY OF CUSTOMARY INTERNATIONAL LAW Jack L. Goldsmith and Eric A. Posner THE LAW SCHOOL THE UNIVERSITY OF CHICAGO This paper can be downloaded without charge at: The Chicago Working Paper Series Index: http://www.law.uchicago.edu/Lawecon/workingpapers.html The Social Science Research Network Electronic Paper Collection: http://papers.ssrn.com/paper.taf?abstract_id=145972 cil\draftnov4 A Theory of Customary International Law Jack L. Goldsmith1 & Eric A. Posner2 Customary international law (“CIL”) is one of two primary forms of international law, the other being the treaty. CIL is typically defined as a “customary practice of states followed from a sense of legal obligation.”3 Conventional wisdom views CIL as a unitary phenomenon that pervades international law and international relations. Governments take care to comply with CIL, and often incorporate its norms into domestic statutes. National courts apply CIL as a rule of decision, or a defense, or a canon of statutory construction. Nations argue about whether certain acts violate CIL. Violations of CIL are grounds for war or an international claim. Legal commentators view CIL to be at the core of the study of international law. And yet CIL remains an enigma.4 It lacks a centralized lawmaker, a centralized executive enforcer, and a centralized, authoritative decision-maker. The content of CIL seems to track the interests of powerful nations. The origin of CIL rules is not understood. We do not know why nations comply with CIL, or even what it means for a nation to comply with CIL. And we lack an explanation for the many changes in CIL rules over time. Both parts of CIL’s standard definition raise perennial, and unanswered, questions. It is unclear which state acts count as evidence of a custom, or how broad or consistent state practice must be to satisfy the custom requirement. It is also unclear what it means for a nation to follow a custom from a sense of legal obligation, or how one determines whether such an obligation exists. 1 Associate Professor of Law, University of Chicago. 2 Professor of Law, University of Chicago. Thanks to Jaqueline Bhabha, Richard Epstein, Tracey Meares, Richard Ross, Cass Sunstein, Doug Sylvester, Adrian Vermeule, and participants at a workshop at the University of Chicago Law School for comments, and to Christopher Chow, Kyle Gehrmann and Kathryn Walsh for research assistance. 3 Restatement (Third) of the Foreign Relations Law of the United States § 102(2) (1986). 4 See G.J.H. van Hoof, Rethinking the Sources of International Law 176-178 (1983) (“confusion and divergence of opinion . reign supreme as far as [CIL] is concerned”); David P. Fidler, Challenging the Classical Concept of Custom: Perspectives on the Future of Customary International Law, 39 Ger. Y.B. Int’l L. 198, 198 (1997) (“CIL stands at the heart of modern international law while generating frustration and frictions in its identification and application. CIL appears indispensable and incomprehensible.”). This article presents a theory of CIL that seeks to sort out these and many other difficulties with the standard account of CIL. The theory uses simple game theoretical concepts to explain how what we call CIL arises, why nations “comply” with CIL as commonly understood, and how CIL changes.5 After briefly describing conventional wisdom about CIL in Section I, Section II presents the theory. This theory views the behaviors that are traditionally thought to constitute a unitary CIL as variations of one of four different behavioral logics. First, some of what is called CIL is better thought of as behavior arising from coincidence of interest, where behavioral regularities result from the private advantage each state obtains from the same action regardless of the action of the other. Second, some of what is called CIL is better thought of as arising from coercion, where a powerful state (or coalition of states with convergent interests) forces or threatens to force other states to engage in acts that they would not do in the absence of such force. Although we take no position on how the label “CIL” ought to be used, scholars who use this label to refer to behavior arising from coincidence of interest or coercion usually are under the erroneous impression that the behavior reflects successful international cooperation. Third are cases of true cooperation. These cases are best modeled as a bilateral iterated prisoner’s dilemma in which two states receive relatively high payoffs over the long term as long as both states resist the temptation to cheat in the short term. If certain conditions are met, the resulting behavioral regularity can be one in which the higher payoffs are obtained. Fourth, some behavioral regularities associated with CIL can arise when states face and solve bilateral coordination problems. In these cases, states receive higher payoffs if they take identical or symmetrical actions than if they do not. Both cooperation and coordination can be robust in bilateral contexts, but will not likely occur in multilateral contexts. 5 Our approach has many affinities with the rational choice school in international relations. See Cooperation Under Anarchy (ed. Kenneth Oye 1986); James Morrow, Modeling the Forms of International Cooperation: Distribution Versus Information, 48 Int’l Org. 387 (1994); Duncan Snidal, Coordination Versus Prisoners’ Dilemma: Implications for International Cooperation, 79 Am. Pol. Sc. Rev. 923 (1985). In recent years international law scholarship has begun to borrow heavily from the international relations literature. See Anne-Marie Slaughter, et al., International Law and International Relations: A New Generation of Interdisciplinary Scholarship, 92 Am. J. Int’l L. 367 (1998) (survey); Jeffrey Dunhoff & Joel Trachtman, Economic Analysis of International Law: An Invitation and a Caveat (forthcoming) (different survey). However, this literature contains no theory of CIL, a huge gap considering the fundamental role of CIL in international law. There has been no comprehensive analysis of customary international law through the lens of rational choice, game theory, and related approaches. See Dunhoff & Trachtman, supra, at __ (appendix). Michael Byers draws on the constructivist school of international relations to gives an account of CIL that differs from ours in methodology and conclusion. See Michael Byers, Custom, Power, and the Power of Rules, 17 Mich. J. Int’l L. 109 (1995). Fernando Teson briefly sketches a game-theoretic account of CIL in order to criticize it on positive and especially normative grounds. See Fernando Teson, A Philosophy of International Law 74-77 (1998). 2 The theory suggests that many international behavioral regularities result from states independently pursuing their self-interest without generating gains from interaction. These cases are trivial and have no normative content. Some international behavioral regularities do reflect cooperation or coordination, but the theory suggests that these regularities will arise in bilateral, not multilateral, interactions. What appear to be multilateral CIL norms, then, are illusions, the product of some combination of (a) coincidence of interests among all, or almost all, states, (b) coercion by one or a few powerful states, or (c) a prisoner’s dilemma or a coordination game played out in discrete bilateral contexts. This theory differs from the standard conception of CIL in several fundamental respects. It rejects the usual explanations of CIL based on opinio juris, legality, morality, and related concepts. States do not comply with norms of CIL because of a sense of moral or legal obligation; rather, their compliance and the norms themselves emerge from the states’ pursuit of self-interested policies on the international stage. In other words, CIL is not an exogenous force that controls the behavior of states, the way domestic law controls the behavior of citizens; it is instead a label people attach to behavior that is generated endogenously from the interactions of states pursuing their self-interest. In addition, our theory rejects the traditional claim that the behaviors associated with CIL reflect a single, unitary logic. These behaviors instead reflect various and importantly different logical structures played out in discrete, historically contingent contexts. Finally, the theory is skeptical of the existence of law-like, multilateral behavioral regularities that are typically thought to constitute CIL. It holds that multinational regularities will invariably reflect coincidence of interest or coercion (and thus not be law-like), and that regularities that reflect