Choi & Gulati: Customary International Law: How Do Courts Do it?

Stephen J. Choi (New York Univ. - Law) & G. Mitu Gulati (Duke Univ. - Law) have posted Customary International Law: How Do Courts Do it? Here's the abstract:

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.