《法学资料集》（英文版） A Theory of Customary International Law

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A Theory of Customary International Law Jackl. Goldsmithand eric A. Posner*e 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. "1 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. 2 It lacks a centrali lawmaker. a centralized executive enforcer. and a centralized authoritative decision-maker. The content of cil seems to track the nterests 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 Associate Professor of Law, University of Chicago 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 I Restatement(Third)of the Foreign Relations Law of the United States S 102(2) (1986) 2 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.

A Theory of Customary International Law Jack L. Goldsmith* and Eric A. Posner** 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.”1 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.2 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 * Associate Professor of Law, University of Chicago. ** 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. 1 Restatement (Third) of the Foreign Relations Law of the United States § 102(2) (1986). 2 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.”)

CHICAGO WORKING PAPER IN LAW AND ECONOMICS of CiL's standard definition raise perennial, and unanswered, uestions. 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 exist 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. 3 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 orivate 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 3 Our approach has many affinities with the rational choice school in international relations. See Cooperation Under Anarchy(ed. Kenneth Oye 1986): Jame Morrow, Modeling the Forms of International Cooperation: Distribution Versus Information, 48 Int'l Org. 387(1994); Duncan Nidal, Coordination Versus Prisoners Dilemma: Implications for International Cooperation, 79 Am. PoL. S 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 ontains 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 onstructivist school of international IL 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)

CHICAGO WORKING PAPER IN LAW AND ECONOMICS 2 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. 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.3 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 3 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)

CUSTOMARY INTERNATIONAL LAW 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 labelCiL "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 prisoners 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 regulariti 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 The theory suggests that most international behavioral regularities associated with CIL reflect coincidence of interest or coercion. These cases are trivial and have no normative content for states independently pursue their self-interest without generating gains from interaction. The theory also contemplates that some international behavioral regularities associated with CIL will reflect cooperation or coordination, but the theory suggests that these egularities 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

3 CUSTOMARY INTERNATIONAL LAW 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. The theory suggests that most international behavioral regularities associated with CIL reflect coincidence of interest or coercion. These cases are trivial and have no normative content, for states independently pursue their self-interest without generating gains from interaction. The theory also contemplates that some international behavioral regularities associated with CIL will 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

CHICAGO WORKING PAPER IN LAW AND ECONOMICS that controls the behavior of states; it is 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 cooperation or coordination arise only in bilateral contexts Section III tests the theory using case studies from four traditional areas of CIL: neutrality, diplomatic immunity, prize, and maritime jurisdiction. We chose to study these areas of the law because they represent a broad spectrum of CIL norms, and becaus these CIL norms are, according to conventional accounts, among the most robust that exist. The case studies teach several lessons The nain lesson is that cil as traditionally understood has little explanatory power. The international behaviors said to constitute CIL are actually disparate and changing practices that follow different logics depending on the interaction of state interests in rticular contexts. The case studies suggest that the behaviors associated with Cil do not reflect a unitary underlying logic, and hat Cil understood as a normative force does no independent work in guiding national behavior. The case studies als so reveal how commentators and courts commit errors of induction in moving from the observation of a behavioral regularity to the conclusion that a CiL rule exists. In addition, in analyzing CIL courts and commentators rely too heavily on what nations say at the expense of what they do and why they do it, and they tend to limit cil to behavioral regularities that are good"from their normative perspective to be CIL, denigrating regularities that are bad as comity"or a violation or an exception to the CIL rule. Finally, the case studies confirm that cil does not reflect multilateral, law-like behavioral regularities Section IV considers several extensions of the analysis. It unpacks the artificial assumption of a unitary state interest that lies

CHICAGO WORKING PAPER IN LAW AND ECONOMICS 4 that controls the behavior of states; it is 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 cooperation or coordination arise only in bilateral contexts. Section III tests the theory using case studies from four traditional areas of CIL: neutrality, diplomatic immunity, prize, and maritime jurisdiction. We chose to study these areas of the law because they represent a broad spectrum of CIL norms, and because these CIL norms are, according to conventional accounts, among the most robust that exist. The case studies teach several lessons. The main lesson is that CIL as traditionally understood has little explanatory power. The international behaviors said to constitute CIL are actually disparate and changing practices that follow different logics depending on the interaction of state interests in particular contexts. The case studies suggest that the behaviors associated with CIL do not reflect a unitary underlying logic, and that CIL understood as a normative force does no independent work in guiding national behavior. The case studies also reveal how commentators and courts commit errors of induction in moving from the observation of a behavioral regularity to the conclusion that a CIL rule exists. In addition, in analyzing CIL courts and commentators rely too heavily on what nations say at the expense of what they do and why they do it, and they tend to limit CIL to behavioral regularities that are “good” from their normative perspective to be CIL, denigrating regularities that are bad as “comity” or a violation or an exception to the CIL rule. Finally, the case studies confirm that CIL does not reflect multilateral, law-like behavioral regularities. Section IV considers several extensions of the analysis. It unpacks the artificial assumption of a unitary state interest that lies

CUSTOMARY INTERNATIONAL LAW at the heart of our theory This leads us to examine domestic constitutional arrangements that identify and enforce the national interest implicated by CIL. Section IV also considers what our theory might teach about the other main form of international law, the treaty. In addition, it speculates about how our theory fits with contemporary discussions about the role of international organizations. Finally, it examines the implications of our analysis for modern international human rights lay I Standard views of cil The treaty and cil are the two primary forms of international law. Because they lack a centralized judicial and enforcement regime and because violations often go unpunished, both treaties and CIL have long been plagued by doubts about whether they establish genuine legal obligations. 4 CIL suffers additional doubts about its legitimacy that do not burden treaties. Treaties are express promises that are almost always embodied in written form; they often have built-in dispute resolution mechanisms such as international arbitration; and they only bind signatories. By contrast, CIL is unwritten;it is said to arise spontaneously from the decentralized practices of nations; the criteria for its identification are(as we shall more fully below) unclear; it is said to bind all nations in the world; and it does not contain within itself a mechanism for resolving disputes and enforcing its norms. Nonetheless, conventional wisdom holds that the obligations created by Cil bind nations with the same force as treaties. 5 CiL is typically defined as the collection of international behavioral regularities that nations over time come to view as binding on them as a matter of law. 6 This standard definition contain two elements. There must be a widespread and uniform 4 For overviews, see D.J. Harris. Cases and Materials on International Law 1-15 (1998): Louis Henkin, et al, International Law 10-41(3d ed. 1994) 5 See Restatement(Third), supra note 1, S 102 cmt. j 6 See id, s 102(2)(defining CIL as"general and consistent practice that states follow from a sense of legal obligation"); Statute of the International Court of Justice, art. 38(1)(b)(including within sources of international law " international custom, as evidence of a general practice accepted as law")

5 CUSTOMARY INTERNATIONAL LAW at the heart of our theory. This leads us to examine domestic constitutional arrangements that identify and enforce the national interest implicated by CIL. Section IV also considers what our theory might teach about the other main form of international law, the treaty. In addition, it speculates about how our theory fits with contemporary discussions about the role of international organizations. Finally, it examines the implications of our analysis for modern international human rights law. I. Standard Views of CIL The treaty and CIL are the two primary forms of international law. Because they lack a centralized judicial and enforcement regime, and because violations often go unpunished, both treaties and CIL have long been plagued by doubts about whether they establish genuine legal obligations.4 CIL suffers additional doubts about its legitimacy that do not burden treaties. Treaties are express promises that are almost always embodied in written form; they often have built-in dispute resolution mechanisms such as international arbitration; and they only bind signatories. By contrast, CIL is unwritten; it is said to arise spontaneously from the decentralized practices of nations; the criteria for its identification are (as we shall more fully below) unclear; it is said to bind all nations in the world; and it does not contain within itself a mechanism for resolving disputes and enforcing its norms. Nonetheless, conventional wisdom holds that the obligations created by CIL bind nations with the same force as treaties.5 CIL is typically defined as the collection of international behavioral regularities that nations over time come to view as binding on them as a matter of law.6 This standard definition contain two elements. There must be a widespread and uniform 4 For overviews, see D.J. Harris, Cases and Materials on International Law 1-15 (1998); Louis Henkin, et al, International Law 10-41 (3d ed. 1994). 5 See Restatement (Third), supra note 1, § 102 cmt. j. 6 See id., § 102(2) (defining CIL as “general and consistent practice that states follow from a sense of legal obligation”); Statute of the International Court of Justice, art. 38(1)(b) (including within sources of international law “international custom, as evidence of a general practice accepted as law”)

CHICAGO WORKING PAPER IN LAW AND ECONOMICS practice of nations. And nations must engage in the practice out of a sense of legal obligation. This second requirement, often referred to as opinio juris is the central concept of CIL. Because opinio juris fers to the reason why a nation acts in accordance with a behavioral regularity, it is often described as the "psychological"element of CIL.It is what distinguishes a national act done voluntarily or out of comity from one that a nation follows because required to do so by law. Courts and scholars say that a longstanding practice among nations"ripens "or "hardens" into a rule of cil when it becomes legally binding This standard account of cil suffers from well-known difficulties. g No one agrees about which types of national actions count as state practice. 0 Policy statements, national legislation, and diplomatic correspondence are the least controversial sources T reaties-especially multilateral treaties, but also bilateral often used as evidence of but in an inconsistent and under theorized way. I1 The writings of jurists are a common but highly tendentious source of CIL I2 Even more controversially, United Nations General Assembly Resolutions and other non-binding statements and resolutions by multilateral bodies are often viewed as evidence of CIL. 3 Those who study and use CIL-courts 7 See Ian Brownlie, Principles of Public International Law 7-9(4th ed. 1990) Anthony D'Amato, The Concept of Custom in International Law 47-55, 66-73 (1971) 8 See, e.g., The Paquete Habana, 175 U.S. 677(1900)(By an ancient usage among civilized nations, beginning centuries ago, and gradually ripening into rule of international law, cost fishing vessels .. have been recognized as exempt from capture as prize of war. " 9 See D'Amato, supra note Fidler, supra note 10 See Fidler, supra note__, at 201-04: Brownlie, supra note_,at 5 1I See H W.A. Thirlway, International Customary Law and Codification 80-94 (1972); Wolfke, supra note_, at 68-72 12 See, for example, Kadic v. Karadzic, 70 F3d 232, 238(2d Cir. 1995); see generally Brownlie, supra note_,at 24-25(noting reasons for"caution" in using publicists as a source of law); G.J.H. van Hoof, Rethinking the Sources of International Law 176-178( 1983)(explaining and criticizing role of publicists) 13 For analyses of the significance for CIL of General Assembly Resolutions Oscar Schacter, International Law in Theory and Practice, 178 Res. des Cours

CHICAGO WORKING PAPER IN LAW AND ECONOMICS 6 practice of nations. And nations must engage in the practice out of a sense of legal obligation. This second requirement, often referred to as opinio juris, is the central concept of CIL. Because opinio juris refers to the reason why a nation acts in accordance with a behavioral regularity, it is often described as the “psychological” element of CIL.7 It is what distinguishes a national act done voluntarily or out of comity from one that a nation follows because required to do so by law. Courts and scholars say that a longstanding practice among nations “ripens” or “hardens” into a rule of CIL when it becomes accepted by nations as legally binding.8 This standard account of CIL suffers from well-known difficulties.9 No one agrees about which types of national actions count as state practice.10 Policy statements, national legislation, and diplomatic correspondence are the least controversial sources. Treaties—especially multilateral treaties, but also bilateral ones—are often used as evidence of CIL, but in an inconsistent and undertheorized way.11 The writings of jurists are a common but highly tendentious source of CIL.12 Even more controversially, United Nations General Assembly Resolutions and other non-binding statements and resolutions by multilateral bodies are often viewed as evidence of CIL.13 Those who study and use CIL—courts, 7 See Ian Brownlie, Principles of Public International Law 7-9 (4th ed. 1990); Anthony D’Amato, The Concept of Custom in International Law 47-55, 66-73 (1971). 8 See, e.g., The Paquete Habana, 175 U.S. 677 (1900) (“By an ancient usage among civilized nations, beginning centuries ago, and gradually ripening into a rule of international law, cost fishing vessels . . . have been recognized as exempt . . . from capture as prize of war.”) 9 See D’Amato, supra note __; Fidler, supra note __. 10 See Fidler, supra note __, at 201-04; Brownlie, supra note __, at 5. 11 See H.W.A. Thirlway, International Customary Law and Codification 80-94 (1972); Wolfke, supra note __, at 68-72. 12 See, for example, Kadic v. Karadzic, 70 F.3d 232, 238 (2d Cir. 1995); see generally Brownlie, supra note __, at 24-25 (noting reasons for “caution” in using publicists as a source of law); G.J.H. van Hoof, Rethinking the Sources of International Law 176-178 (1983) (explaining and criticizing role of publicists). 13 For analyses of the significance for CIL of General Assembly Resolutions, see Oscar Schacter, International Law in Theory and Practice, 178 Res. des Cours

CUSTOMARY INTERNATIONAL LAW arbitrators, diplomats, politicians, scholars-invoke these sources selectively and usually tendentiously No one, moreover, agrees about how widespread and uniform state practice must be. In theory the practice is supposed to be general in the sense that all or almost all of the nations of the world engage in it. 14 But it is practically impossible to determine whether 190 or so nations of the world engage in a particular practice. CiL is thus usually based on a highly selective survey of state practice that includes major powers and interested nations I5 Increasingly, courts and scholars sometimes ignore the state practice requirement altogether. 16 For example, they refer to a CL prohibition on torture at the same time that they acknowledge that many nations of the world torture their citizens. 7 It is thus unclear when, and to what degree, the state practice requirement must be satisfied The opinio juris requirement raises more problems. 18 To what does the psychological state refer? How does one identify it? There are no settled answers. Courts and scholars sometimes infer it from the existence of a widespread behavioral regularity. I9 But if opinio Juris can be inferred from behavioral regularities, it is redundant with the requirement of a widespread and uniform state practice, which 111-121(1982-V); Wolkfke, supra note_, at 84; Stephen M. Schwebel, The Effect of Resolutions of the U N. General Assembly on Customary Internationa Law. 1979 Proc. Am. Soc. Intl L. 301 14 See Brownlie, supra note_,at 5-6 15 See Wolfke, supra note_, at 78-79; Jonathan Charney, Universal Inter- national Law, 87 Am. J. Int'l L. 529, 537(1993) 16 See Curtis A. Bradley and Jack L. Goldsmith, Customary International Law as Federal Common Law: A Critique of the Modern Position, 110 Harv. L. Rev 815,839-40(1997) 17 See Filartiga v. Penal-Irala 2d 876. 882(2d Cir. 1980): Bruno Simma Philip Alston, The Sources of Human Rights Custom, Jus Cogens, and General Principles, 12 Austl. Y B Int'l L. 82, 90(1992) 18 ee Thirlway, supra note at 47( The precise definition of the opinio juris, the psychological element in the formation of custom, the philosopher's stone which transmutes the inert mass of accumulated usage into the gold of binding leg rules, has probably caused more academic controversy than all the actual contested claims made by states on the basis of alleged custom, put together. " 19 See Brownlie, supra note_, at 7(citing examples)

7 CUSTOMARY INTERNATIONAL LAW arbitrators, diplomats, politicians, scholars—invoke these sources selectively and usually tendentiously. No one, moreover, agrees about how widespread and uniform state practice must be. In theory the practice is supposed to be “general” in the sense that all or almost all of the nations of the world engage in it.14 But it is practically impossible to determine whether 190 or so nations of the world engage in a particular practice. CIL is thus usually based on a highly selective survey of state practice that includes major powers and interested nations.15 Increasingly, courts and scholars sometimes ignore the state practice requirement altogether.16 For example, they refer to a CIL prohibition on torture at the same time that they acknowledge that many nations of the world torture their citizens.17 It is thus unclear when, and to what degree, the state practice requirement must be satisfied. The opinio juris requirement raises more problems.18 To what does the psychological state refer? How does one identify it? There are no settled answers. Courts and scholars sometimes infer it from the existence of a widespread behavioral regularity.19 But if opinio juris can be inferred from behavioral regularities, it is redundant with the requirement of a widespread and uniform state practice, which, 111-121 (1982-V); Wolkfke, supra note __, at 84; Stephen M. Schwebel, The Effect of Resolutions of the U.N. General Assembly on Customary International Law, 1979 Proc. Am. Soc. Int’l L. 301. 14 See Brownlie, supra note __, at 5-6. 15 See Wolfke, supra note __, at 78-79; Jonathan Charney, Universal International Law, 87 Am. J. Int’l L. 529, 537 (1993). 16 See Curtis A. Bradley and Jack L. Goldsmith, Customary International Law as Federal Common Law: A Critique of the Modern Position, 110 Harv. L. Rev. 815, 839-40 (1997). 17 See Filartiga v. Penal-Irala, 630 F.2d 876, 882 (2d Cir. 1980); Bruno Simma & Philip Alston, The Sources of Human Rights Law: Custom, Jus Cogens, and General Principles, 12 Austl. Y.B. Int’l L. 82, 90 (1992). 18 See Thirlway, supra note, at 47 (“The precise definition of the opinio juris, the psychological element in the formation of custom, the philosopher’s stone which transmutes the inert mass of accumulated usage into the gold of binding legal rules, has probably caused more academic controversy than all the actual contested claims made by states on the basis of alleged custom, put together.”). 19 See Brownlie, supra note __, at 7 (citing examples)

CHICAGO WORKING PAPER IN LAW AND ECONOMICS by concession, is insufficient by itself to establish CIL. To avoid this problem, courts and scholars sometimes(but only sometimes independent evidence that a nation acted from a sense obligation, such as a statement by an important government official ratification of a treaty that contains a norm similar to the cil norm in question, or an attitude of approval toward a General Assembly Resolution. 20 The appropriate conditions on the use of such evidence remains unsettled, and indeed the evidence is never considered in a systematic fashion These definitional problems with opinio juris flow in part from more serious conceptual difficulties. There is no convincing explanation of the process by which a voluntary behavioral regularity transforms itself into a binding legal obligation. 21 Opinio juris is described as the psychological component of cil because it refers to an attitude that nations supposedly have toward a behavioral regularity. The idea is mysterious because the legal obligation is created by a nation's belief in the existence of the legal obligation. As D'Amato notes, this is circular reasoning. 22 Opinio juris is really a conclusion about a practice's status as international law; it does not explain how a widespread and uniform practice becomes law We have described some of the many uncertainties that bedevil the standard conception of CIL. These problems are well known They are the subject of an enormous literature that endlessly(and in our opinion unproductively) debates definitional issues, the relative significance of practice and opinio juris and other conceptual matters internal to the traditional account. 23 Although our theory has implications for many of these issues, such issues are not the main focus of our analysis. Instead, we focus on two sets of issues that are 20 Id at 7-9(citing examples) 21 For a catalogue of failed attempts, see D'Amato, supra note_,at,66- 22 D'amato captures this circularity with a question: "How can custom create law if its psychological component requires action in conscious accordance with law preexisting the action"? D'Amato, supra note 66. He analyzes the many futile attempts to avoid this paradox, id at 47-56, 66-68 23 The canonical treatments of CIL include D'Amato, supra note supra note Thirlway note: and Michael Akehurst. Custom Source of International Law, 47 Brit. Y B. Int'l L. 1(1974-1975

CHICAGO WORKING PAPER IN LAW AND ECONOMICS 8 by concession, is insufficient by itself to establish CIL. To avoid this problem, courts and scholars sometimes (but only sometimes) require independent evidence that a nation acted from a sense of obligation, such as a statement by an important government official, ratification of a treaty that contains a norm similar to the CIL norm in question, or an attitude of approval toward a General Assembly Resolution.20 The appropriate conditions on the use of such evidence remains unsettled, and indeed the evidence is never considered in a systematic fashion. These definitional problems with opinio juris flow in part from more serious conceptual difficulties. There is no convincing explanation of the process by which a voluntary behavioral regularity transforms itself into a binding legal obligation.21 Opinio juris is described as the psychological component of CIL because it refers to an attitude that nations supposedly have toward a behavioral regularity. The idea is mysterious because the legal obligation is created by a nation’s belief in the existence of the legal obligation. As D’Amato notes, this is circular reasoning.22 Opinio juris is really a conclusion about a practice’s status as international law; it does not explain how a widespread and uniform practice becomes law. We have described some of the many uncertainties that bedevil the standard conception of CIL. These problems are well known. They are the subject of an enormous literature that endlessly (and in our opinion unproductively) debates definitional issues, the relative significance of practice and opinio juris, and other conceptual matters internal to the traditional account.23 Although our theory has implications for many of these issues, such issues are not the main focus of our analysis. Instead, we focus on two sets of issues that are 20 Id. at 7-9 (citing examples). 21 For a catalogue of failed attempts, see D’Amato, supra note __, at 47-56, 66- 72. 22 D’amato captures this circularity with a question: “How can custom create law if its psychological component requires action in conscious accordance with law preexisting the action”? D’Amato, supra note __, at 66. He analyzes the many futile attempts to avoid this paradox, id. at 47-56, 66-68. 23 The canonical treatments of CIL include D’Amato, supra note __; Wolfke, supra note __; Thirlway, supra note __; and Michael Akehurst, Custom as a Source of International Law, 47 Brit. Y.B. Int’l L. 1 (1974-1975)

CUSTOMARY INTERNATIONAL LAW rarely discussed in the international law literature but that are fundamental to understanding CIL. The first set of issues concerns the unarticulated and undefended assumptions that underlie the traditional conception of CIL. Despite the many disagreements within the traditional paradigm, the parties to this debate assume that Cil is unitary, universal and exogenous CIL is unitary in the sense that all the behaviors it describes have an identical logical form that is described in the standard definition cil is universal in the sense that its obligations bind all nations except those that "persistently object during the development of the CIL norm. 24 And CIL is an exogenous influence on national behavior in the sense that it guides, shapes, and influences national actions. When nations are law abiding they conform their behavior to CIL. When they violate Cil they act in defiance of it. Our theory of cil challenges each of these optic The second set of issues on which we focus concerns the traditional paradigm's inability to explain international behavior. For example, the traditional paradigm has no account for how CIL originates. 25 It does not explain how international behavioral regularities emerge from disorder. As we saw above, it also fails to explain how nations move from a"mere"behavioral regularity to a behavioral regularity that nations follow from a sense of legal obligation The traditional account cannot explain how CiL rules change over time. 26 To take one of scores of examples the ostensible CIL ale governing a nations jurisdiction over its coasts changed from a cannon-shot rule to a three-mile rule to a twelve mile rule with many qualifications. 27 On the traditional account, the process of change is necessarily illegal, since some states must initiate a departure from 24 On the persistent objector rule, see Restatement(Third), supra note_, at s 102, comment d; Ted L Stein, The Approach of the Different Drummer: The Principle of the Persistent Objector in International Law, 26 Harv. Int'l L J. 457 (1985) 25 ee D'Amato, supra note, at 4. 26 f, supra note, at 97-105 27 This is a simplification. We explore this rule more fully infra

9 CUSTOMARY INTERNATIONAL LAW rarely discussed in the international law literature, but that are fundamental to understanding CIL. The first set of issues concerns the unarticulated and undefended assumptions that underlie the traditional conception of CIL. Despite the many disagreements within the traditional paradigm, the parties to this debate assume that CIL is unitary, universal, and exogenous. CIL is unitary in the sense that all the behaviors it describes have an identical logical form that is described in the standard definition. CIL is universal in the sense that its obligations bind all nations except those that “persistently object” during the development of the CIL norm.24 And CIL is an exogenous influence on national behavior in the sense that it guides, shapes, and influences national actions. When nations are lawabiding they conform their behavior to CIL. When they violate CIL they act in defiance of it. Our theory of CIL challenges each of these assumptions. The second set of issues on which we focus concerns the traditional paradigm’s inability to explain international behavior. For example, the traditional paradigm has no account for how CIL originates.25 It does not explain how international behavioral regularities emerge from disorder. As we saw above, it also fails to explain how nations move from a “mere” behavioral regularity to a behavioral regularity that nations follow from a sense of legal obligation. The traditional account cannot explain how CIL rules change over time.26 To take one of scores of examples: the ostensible CIL rule governing a nation’s jurisdiction over its coasts changed from a cannon-shot rule to a three-mile rule to a twelve mile rule with many qualifications.27 On the traditional account, the process of change is necessarily illegal, since some states must initiate a departure from 24 On the persistent objector rule, see Restatement (Third), supra note __, at § 102, comment d; Ted L. Stein, The Approach of the Different Drummer: The Principle of the Persistent Objector in International Law, 26 Harv. Int’l L. J. 457 (1985). 25 See D’Amato, supra note __, at 4. 26 See id.; Hoof, supra note __, at 97-105. 27 This is a simplification. We explore this rule more fully infra

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