Making Sense of Customary International Law

Monica Hakimi*

This Article addresses a longstanding puzzle about customary international law (CIL): How can it be, at once, so central to the practice of international law—routinely invoked and applied in a broad range of settings—and the source of such persistent confusion and derision? The centrality of CIL suggests that, for the many people who use it, it is not only comprehensible but worthwhile. They presumably use it for a reason. But then, what accounts for all the muddle and disdain?

The Article argues that the problem lies less in the everyday operation of CIL than in the conceptual baggage that is brought to bear on it. Most contemporary accounts of CIL reflect what can be called a “rulebook conception.” They presuppose that, in order for a given proposition to be CIL, it must apply more or less in the same way in all cases of a given type, rather than fluctuate without established criteria from one situation to the next. This rulebook conception is wrong. It does not accurately describe the range of normative material that global actors, in the ordinary course, use and treat as CIL. And because it is wrong, it systematically sows confusion and leads analysts to devalue CIL as a kind of international law. We should stop imagining that CIL operates like a rulebook and should recognize that it is an inherently contingent and variable kind of law.

*James V. Campbell Professor of Law, University of Michigan Law School. I benefited greatly from comments on earlier drafts from José Alvarez, Curtis Bradley, Kristina Daugirdas, Melissa Durkee, Ryan Goodman, Larry Helfer, Don Herzog, Julian Mortenson, Sean Murphy, Georg Nolte, Steven Ratner, David Stewart, Brian Willen, Ingrid Wuerth, and the participants in the Cyber-Colloquium on International Law and in workshops hosted by the American Society of International Law Interest Group on International Law in Domestic Courts (at Lewis & Clark Law School), the Max Planck Institute for Comparative Public Law and International Law, New York University Law School, and the University of Georgia Law School.

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