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International Law in National Legal Systems: An Empirical Investigation

  • Pierre-Hugues Verdier (a1) and Mila Versteeg (a2)


International legal scholars have long recognized the importance of the rules and processes by which states adhere to international legal obligations and “translate” them into their domestic legal systems. Research by political scientists on specific issue areas likewise increasingly recognizes that domestic implementation is crucial to international law compliance and effectiveness. Yet the lack of systematic data makes it difficult to assemble an overall picture of the relationship between international law and domestic law around the world, let alone to document its evolution over time. Recent qualitative surveys of state practice have begun to fill that gap, but provide only a snapshot in time and are limited to relatively few countries. Some quantitative projects cover more countries, but address only a limited number of questions based solely on the text of national constitutions.



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1 See, e.g., Knop, Karen, Here and There: International Law in Domestic Courts, 32 N.Y.U. J. Int’l L. & Pol. 501 (2000); Koh, Harold Hongju, Why Do Nations Obey International Law?, 106 Yale L.J. 2599 (1997).

2 See, e.g., Beth A. Simmons, Mobilizing for Human Rights: International Law in Domestic Politics (2009); Lupu, Yonatan, Best Evidence: The Role of Information in Domestic Judicial Enforcement of International Human Rights Agreements, 67 Int’l Org. 469 (2013); Moravcsik, Andrew, Liberal Theories of International Law, in Interdisciplinary Perspectives on International Law and International Relations: The State of the Art 83, 96–100 (Dunoff, Jeffrey L. & Pollack, Mark A. eds., 2013).

3 See, e.g., International Law and Domestic Legal Systems: Incorporation, Transformation, and Persuasion (Dinah Shelton ed., 2011); The Role of Domestic Courts in Treaty Enforcement: A Comparative Study (David Sloss ed., 2009); National Treaty Law and Practice (Duncan Hollis et al. eds., 2005).

4 See, e.g., Ginsburg, Tom, et al., Commitment and Diffusion: How and Why National Constitutions Incorporate International Law, 2008 U. Ill. L. Rev. 201 ; Hathaway, Oona A., Treaties’ End: The Past, Present, and Future of International Law making in the United States, 117 Yale L.J. 1236 (2008).

5 Lori Fisler Damrosch & Sean D. Murphy, International Law: Cases and Materials 621 (6th ed. 2014).

6 Id.

7 Id.

8 James Crawford, Brownlie’s Principles of Public International Law 50 (8th ed. 2012).

9 See, e.g., Pellet, Alain, Vous avez dit “monisme”? Quelques banalités de bon sens sur l’impossibilité du prétendu monisme constitutionnel à L•lrançaise, in L’Architecturedudroit: Melangesenl’Honneur De Michel Troper 827 (de Béchillon, Denys et al. eds., 2006). In this article, while we generally avoid using the terms “monist” and “dualist,” it is sometimes necessary to do so to avoid repetition or describe how legal systems are conventionally classified. In such cases, we use the term “monist” to designate countries where treaties have direct effect upon ratification without further action by the legislature (even if legislative approval is required prior to ratification or if formal steps need to be taken by other branches to bring the treaty into effect) and “dualist” to designate countries where legislative action is required to incorporate a ratified treaty into domestic law. The monist-dualist distinction is much less salient for CIL, and we avoid its use in that context.

10 International Law and Domestic Legal Systems, supra note 3; National Treaty Law and Practice, supra note 3; Treaty Making—Expression of Consent to be Bound by a Treaty (Council of Europe ed., 2001).

11 Hathaway, supra note 4.

12 Ginsburg, et al., supra note 4.

13 We thank the Comparative Constitutions Project for providing us access to their historical repository of constitutions.

14 Of the 101 countries in our sample, 30 percent are not currently “fully democratic.” We define “fully democratic” as a score of 6 or higher on the Polity Iv democracy scale that is commonly used in the political science literature.

15 Indeed, the correlation between democracy and legislative involvement in treaty making is close to zero. See infra note 22 and accompanying text.

16 See Law, David S. & Versteeg, Mila, Constitutional Variation Among Strains of Authoritarianism, in Constitutions in Authoritarian Regimes 165 (Ginsburg, Tom & Simpser, Alberto eds., 2013).

17 See Levitsky, Steven & Way, Lucan A., Competitive Authoritarianism: Hybrid Regimes after the Cold War (2010); Rule by Law: The Politics of Courts in Authoritarian Regimes (Tom Ginsburg & Tamir Moustafa eds., 2008); Howard, Marc Morjé & Roessler, Philip G., Liberalizing Electoral Outcomes in Competitive Authoritarian Regimes, 50 Am. J. Pol. Sci. 365 (2006); Levitsky, Steven & Way, Lucan A., The Rise of Competitive Authoritarianism, 13 J. Democracy 51 (2002).

18 See Kumm, Mattias, The Legitimacy of International Law: A Constitutionalist Framework of Analysis, 15 Eur. J. Int’l L. 907 (2004).

19 Haftel, Yoram Z. & Thompson, Alexander, Delayed Ratification: The Domestic Fate of Bilateral Investment Treaties, 67 Int’l Org. 355, 361 (2013).

20 Jackson, John H., The Status of Treaties in Domestic Legal Systems: A Policy Analysis, 86 AJIL 310, 324 (1992).

21 Id. at 325.

22 As noted above, this requirement is found both in democracies and in non-democratic regimes. The correlation between democracy and whether a country requires prior legislative approval is 0.06, which means that the two features are almost entirely unrelated. Just as autocracies commonly hold elections, they require the legislature to approve treaty ratification, upholding a formal separation of power between the executive and legislative branches.

23 Specifically, in many civil law countries, the constitution distinguishes between legislative and regulatory domains. The legislative domain comprises laws approved by parliament as a whole, while the regulatory domain consists of regulations adopted without parliamentary approval. In many cases, the constitution explicitly sets out which substantive issues fall within the legislative domain. Where treaties deal with issues that fall within the legislative domain, parliamentary approval is required.

24 In the words of Gladstone in an 1891 speech, “if the House of Commons can by any possibility lose the power of the control of the grants of public money, depend upon it your very liberty will be worth very little in comparison.” 2 The Speeches of the Right Hon. W. E. Gladstone 343 (A. W. Hutton & H. J. Cohen eds., 1902).

25 One might object that such formal treaty-making procedures are meaningless if the executive can bypass them by entering into international agreements through other means, such as executive agreements. However, in countries that require prior legislative approval of specific categories of treaties, the virtually uniform rule is that executive agreements may not constitutionally be used in these areas.

26 The correlation between democracy and legislative approval requirements for treaties that alter domestic law is -0.03, while the correlation between democracy and legislative approval requirements for treaties that require domestic spending is -0.11.

27 Cope, Kevin & Movassagh, Hooman, Comparative International Law and National Legislatures, in Compar Ative International Law (Roberts, Anthea et al. eds., forthcoming 2016).

28 See generally Mayhew, David R., Congress: The Electoral Connection (1974).

29 See Jackson, supra note 20, at 325.

30 See generally Anne-Marie Slaughter, A New World Order 65–103 (2004).

31 Here again, it is not necessarily the countries with the greatest democratic pedigree that have adopted this requirement: the correlation between democracy and ex ante democratic approval is close to zero in this subsample (0.12).

32 Constitutional Reform and Governance Act, 2010, c. 25, § 20 (U.K.).

33 The government may return the treaty to Parliament with a statement explaining why the treaty should nevertheless be ratified, triggering a new twenty-one-day period during which only the House of Commons (not the House of Lords) may block ratification. Id. § 20(4)–(5). The Act goes further than the previously applicable Ponson by Rule, under which Parliament could review treaties but not defeat ratification by the executive. See Arabella Thorp, Parliamentary Scrutiny of Treaties: Up to 2010, at 9 (2009), available at

34 Lucinda Maer et al., House of Commons Library, Constitutional Reform and Governance Bill 21–22 (2009), available at

35 See Posner, Eric A. & Sykes, Alan O., Economic Foundations of International Law 141 (2013). Increasing domestic hurdles to treaty making may also strengthen a state’s bargaining position ex ante, as its counterparts will anticipate that an insufficiently favorable treaty may not gain domestic approval. See Putnam, Robert D., Diplomacy and Domestic Politics: The Logic of Two-Level Games, 42 Int’l Org. 427, 452–53 (1988). However, it is unclear whether ex ante approval or ex post implementation requirements should systematically be more effective in this respect— except of course for treaties that do not require domestic implementation for their effectiveness.

36 Importantly, these are not the only ways in which international law can enter national law. For example, in some well-known cases, national courts have considered a state’s international legal obligations—albeit formally unincorporated—in circumscribing permissible administrative action. See, e.g., Minister of State for Immigration and Ethnic Affairs v Teoh (1995) 183 CLR 273 (Austl.). The degree of influence that international law can exert within a national legal system is ultimately a matter of degree. See Kingsbury, Benedict, The Concept of “Law” in Global Administrative Law, 20 Eur. J. Int’l L. 23 (2009). In this contribution, we focus on those aspects of the relationship—including the formal applicability of ratified treaties by national courts and the explicit articulation of an interpretive presumption of conformity—that are well-documented across many countries and therefore lend themselves to consistent coding.

37 See, e.g., Buergenthal, Thomas, Self-Executing and Non-Self-Executing Treaties in National and International Law, 235 Recueil des Cours 303, 317 (1992).

38 In 2011, Belarus, Iran, Morocco, Tajikistan, Turkmenistan, and Ukraine were all rated as “not independent” by the CIRI human rights dataset (which rates judicial independence on a three-point scale “not independent,” “partially independent,” and “generally independent”). See David L. Cingranelli et al., The CIRI Human Rights Dataset, CIRI Human Rights Data Project (Apr. 4, 2014), at Egypt, Latvia, and Turkey were all rated “partially independent.” Id. Only Estonia was rated as “independent.” Id.

39 The correlation is 0.17. of the twenty-two countries in our data that were rated as “not independent” by the CIRI dataset in 2011, sixteen recognized a distinction between self-executing and non-self-executing treaties, while only six did not recognize such a distinction.

40 Buergenthal, supra note 37, at 317.

41 Medellín v. Texas, 552 U.S. 491, 506–20 (2008); see Bradley, Curtis A., Intents, Presumptions and Non-Self Executing Treaties, 102 AJIL 540 (2008); Vásquez, Carlos Manuel, Treaties as Law of the Land: The Supremacy Clause and the Judicial Enforcement of Treaties, 122 Harv. L. Rev. 599 (2008).

42 Cámara Federal de Apelaciones [CFed.] [Federal Court of Appeals], 2/6/1998, “Viceconte, Mariela c. Estado Nacional”/Acción de Amparo,” La Ley [L.L.] (1998-F-305) (Arg.) (holding that the government’s failure to manufacture a vaccine against Argentine hemorrhagic fever constituted a violation of Article 12 of the Icescr); Corte Suprema de Justicia de la Nación [Csjn] [National Supreme Court of Justice], 24/10/2000, “Campodónico de Beviacqua, Ana Carina c. Ministerio de Salud y Acción Social/recurso de hecho,” La Ley [L.L.] (2001-D-23) (Arg.) (finding that article 12 of the Icescr required the government to continue providing a drug to a child who had an immunological condition). See Byrnes, Andrew, Second-Class Rights Yet Again? Economic, Social, and Cultural Rights in the Report of the National Human Rights Consultation, 33 Unsw L.J. 193, 206–07 (2010); Byrne, Iain, Enforcing the Right to Health: Innovative Lessons from Domestic Courts, in Realizing the Right to Health 525, 527 n.15 (Clapham, Andrew & Robinson, Mary eds., 2009).

43 See Patrick Daillier, Mathias Forteau & Alain Pellet, Droit International Public 254–55 (8th ed. 2009).

44 See, e.g., Ginsburg et al., supra note 4.

45 The Federalist No. 64, at 362 (John Jay) (Clinton Rossiter ed., 1961).

46 See Viljoen, Frans, International Human Rights Law in Africa 533–37 (2nd ed. 2012).

47 As Anthea Roberts points out, courts as well as legislatures can be important fora for the “hybridization” of international and domestic norms. Roberts, Anthea, Comparative International Law? The Role of National Courts in Creating and Enforcing International Law, 60 Int’l & Comp. L.Q. 57 (2011).

48 Posner & Sykes, supra note 35, at 143.

49 Abdelmadjid Djebbar, La Politique Conventionnelle de l’Algérie (2000) (authors’ translation).

50 R v. Jones, [2006] UKHL 16, [20]–[23] (appeal taken from Eng.).

51 18 U.S.C. § 1651 (2012).

52 28 U.S.C. § 1350 (2012).

53 See, e.g., Constitución Política de Colombia [C.P.] art. 214.

54 See Requa, Marny A., A Human Rights Triumph? Dictatorship-era Crimes and the Chilean Supreme Court, 12 Hum. Rts. L. Rev. 79, 89–90 (2012).

55 Roberts, Anthea, Traditional and Modern Approaches to Customary International Law: A Reconciliation, 95 AJIL 757, 784–85 (2001); Verdier, Pierre-Hugues & Voeten, Erik, Precedent, Compliance, and Change in Customary International Law: An Explanatory Theory, 108 AJIL 389 (2014).

56 Foreign Sovereign Immunities Act of 1976, 28 U.S.C. §§ 1330, 1332, 1391(f), 1441(d), 1602–1611 (1994); State Immunity Act, 1978, 26 Eliz. 2, c. 33 (U.K.).

57 See, for example, Kiobel v. Royal Dutch Petroleum Co., 133 S.Ct. 1659 (2013); Sosa v. Alvarez-Machain, 542 U.S. 692 (2004); Filártiga v. Peña-Irala, 630 F.2d 876 (2d Cir. 1980), among dozens of appellate cases.

58 Verdier, Pierre-Hugues & Voeten, Erik, How Does Customary International Law Change? The Case of State Immunity, 59 Int’l Stud. Q. 209 (2015). of course, this is a concern only when the state values the rule generally, but wishes to justify its defection in a particular instance. If the state dislikes the rule and actively wants to undermine it, then it should make the breach as “noisy” as possible.

59 Knop, supra note 1, at 506.

60 Putnam, supra note 35, at 452–53.

* The authors would like to thank Benedict Kingsbury, Anthea Roberts, Paul Stephan, one anonymous reviewer, and participants in the 2014 Sokol Colloquium on Private International Law for helpful comments and suggestions on an earlier version.

International Law in National Legal Systems: An Empirical Investigation

  • Pierre-Hugues Verdier (a1) and Mila Versteeg (a2)


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