Customary International Law: How Do Courts Do it?

33 Pages Posted: 9 Feb 2015

See all articles by Stephen J. Choi

Stephen J. Choi

New York University School of Law

G. Mitu Gulati

Duke University School of Law

Date Written: February 7, 2015


The textbook rule for when a custom among nations can be said to have evolved into international law has two parts. First, there has to be widespread acceptance of this practice among nations. And second, nations have to believe that the practice is legally compelled. This rule — which on its face requires a rigorous empirical inquiry into the practices and mental states of nations — is, to put it mildly, difficult (maybe impossible) to apply. The reasons for that range from a lack of logical consistency in the textbook rule to the lack of empirical, historical and anthropological expertise on the part of the courts who are supposed to apply it. The question we are interested in is what do international courts do in the face of this type of legal rule, particularly given that there is often a pressing global need for new types of customary international law rules to be found. The answer: They ignore it. In theory, they could follow the textbook rule and never find new customary international law. That is not the case. Courts do frequently find new rules of customary international law and appear to follow a particular formula in doing so. The formula is that they look primarily to international treaties as support.

Keywords: custom, international law, international courts

JEL Classification: K33

Suggested Citation

Choi, Stephen J. and Gulati, Gaurang Mitu, Customary International Law: How Do Courts Do it? (February 7, 2015). Available at SSRN: or

Stephen J. Choi

New York University School of Law ( email )

40 Washington Square South
New York, NY 10012-1099
United States

Gaurang Mitu Gulati (Contact Author)

Duke University School of Law ( email )

210 Science Drive
Box 90362
Durham, NC 27708
United States

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