It is often observed in the literature on customary international law that the identification practice of the International Court of Justice for customary norms deviates from the traditional definition of customary law in Art. 38 (1) lit. b of the ICJ Statute. However, while there are many normative and descriptive accounts on customary law and the Courts practice, few studies try to explain the jurisprudence of the ICJ. This study aims at closing this gap. I argue that the ICJs argumentation pattern is due to the institutional constraints that the Court faces. In order for its decisions to be accepted, it has to signal impartiality through its reasoning. However, the analysis of state practice necessarily entails the selection of particular instances of practice, which could tarnish the image of an impartial court. In contrast, if the Court resorts to the consent of the parties or widely accepted international documents, it signals impartiality.