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 CoursCHICAGO 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 under￾theorized 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