Customary international law has been a contested concept for a long time. In 1939, a fateful year, Hans Kelsen remarked that the theory of customary international law “has no other function than to conceal [dissimuler] the important, not to say dominant, role that is played in the formation of customary law by the arbitrariness of the organ competent to apply the law.” Hans Kelsen, Théorie du Droit International Coutumier, 1 Revue Internationale de la Théorie du Droit [new series] 253, 266 (1939). (I note by the way that Kelsen, an Austrian, wrote this in French in a Franco-Austrian journal, the title of which was displayed in both French and German. This brings out how essential multilingualism is in the study and practice of international law and international legal research, a point that has been made before on this blog.) Critical attention to customary international law has increased recently. The literature is extensive, but emblematic of the trend is the recent article, Why I Stopped Believing in Customary International Law by Daniel Joyner, 9 Asian Journal of International Law 31 (2019). A key source is the collection of essays edited by Curtis A. Bradley, Custom’s Future: International Law in a Changing World (Cambridge University Press 2016).
The critique of customary international law has several aspects, but a central component is the observation that the standard model, the two-part definition of state practice and opinio juris, is not applied in practice. This criticism is aimed in particular at courts, both international and domestic. In Custom’s Future there is an essay by Stephen J. Choi and Mitu Gulati titled “Customary International Law: How Do Courts Do It?” This is a rigorous empirical study based on a data set of 175 determinations of customary international law by the International Court of Justice and some other international tribunals. If you can get through the mind-numbing statistical discussion, there is a lot to learn. Early in the essay there is a sub-section titled “Superhuman Research Skills.” The context is a discussion of the question whether there is a customary norm permitting a state to reject odious debts. The authors, who worked on this question, have this to say: “Not only was the type of evidence being requested unlikely to exist … but it was impossible to collect, as a practical matter, unless one somehow assembled an extraordinary team of anthropologists, economists, historians, political scientists and lawyers who would then be able to spend decades excavating the historical record.” The authors should have added international law librarians to the list of needed professions.
A key finding of the study is that the piece of evidence most frequently cited for the existence of a rule of customary international law is the treaty. But referring to treaties for this purpose is famously problematic. After all, the raison d’être for an international agreement might well be the sense of a gap in the law. The authors conclude that “[t]he data suggest that international courts do not come anywhere close to engaging in the type of analysis the officially stated two-part rule for the evolution of CIL sets up.” A similar result was reached by Stefan Talmon in his article, Determining Customary International Law: The ICJ’s Methodology between Induction, Deduction and Assertion, 26 European Journal of International Law 417 (2015). Concerning similar results for the domestic courts of the United States, see Ryan M. Scoville, Finding Customary International Law, 101 Iowa Law Review 1893 (2016).
It has to be conceded that the critique of customary international law is found in the academic international law literature. This is not a criticism of the critique, but an observation. In 2018 the International Law Commission of the United Nations adopted its Draft Conclusions on Identification of Customary International Law, with Commentaries. The Draft Conclusions can be found in the Commission’s report, A/73/10, and as an offprint on the Commission’s website. It has to be said that the Draft Conclusions stick resolutely to the standard model, with little recognition even in the commentaries of the concerns that have been raised.
Where does this leave international law librarians who are called on to advise on researching customary international law, and to teach the method? Clearly, there is a dilemma. I think it is still possible to start with the standard, two-element model, as classically stated in Article 38(1) of the Statute of the International Court of Justice. But more is clearly necessary. I think we have to raise the critique and its main components. Researchers and students need to be aware that researching customary international is beset with issues that a simple invocation of the standard model does not capture. The research guide International Legal Research in a Global Community by Heidi Frostestad Kuehl and Megan A. O’Brien (Carolina Academic Press 2018) integrates several elements of the critique into the discussion of researching customary international law without mentioning the critique explicitly.
What promise does the digital dispensation hold here? Is there, or will there soon be, a giant database of state practice of the 195 states of the world to be mined by an AI-driven robot for evidence of a customary norm of international law? I am comforted that the answer to the question is no.
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