The International Court of Justice as a “Shortcut” to Identifying Custom

A frequently used shortcut to identifying rules of customary international law is to rely on statements by the International Court of Justice instead of conducting a more cumbersome investigation of state practice and opinio iuris. The purpose of this article is to consider when the Court’s statements align or may come to align with customary rules and, consequently, to what extent this shortcut is justified. Its value is in systematically exploring ideas that international lawyers may already have internalised; it may also help students of the subject to understand why reliance is placed on judicial decisions. Often, the Court simply elucidates pre-existing customary rules. But examples such as Factory at Chorzów, Fisheries, and Reservations to the Genocide Convention suggest that an additional or alternative justification for the shortcut may be stronger. This is the tendency of states to endorse or “ratify” statements by the Court through subsequent practice and opinio iuris.