The Nature and Context of Rules and the Identification of Customary International Law

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Abstract The International Court of Justice (ICJ) regularly invokes a two-element test for the identification of customary international law: state practice accompanied by opinio juris. Yet the Court’s application of this test has appeared inconsistent. In some cases, an absence of evidence of opinio juris leads to a finding that an alleged rule of customary international law does not exist; yet in other cases, there is no clear evidence of opinio juris on the part of states participating in the practice, but the Court nevertheless concludes that the alleged customary rule exists. In other cases, the Court concludes that a customary rule exists apparently based on evidence of opinio juris alone. This article argues that these judgments do not undermine the ICJ’s claim to be applying a two-element analysis but reveal something fundamental about how customary international law is identified – namely, that how the existence of state practice and opinio juris is evaluated may vary depending on the nature of the customary rule under investigation (for example, whether it is a permissive or prohibitive rule) and the context of underlying international law rules in which that alleged new customary international law rule is located.