Ascertaining Customary International Law: An Inquiry into the Methods Used by Domestic Courts

Based on analysis of a large number of recent domestic court cases on matters of customary international law (2000–2014), this article demonstrates that, rather similar to the International Court of Justice, domestic courts do not normally identify customary norms of customary international law on the basis of the textbook method of ascertaining a general practice accepted as law. Rather, they tend to outsource the determination of custom to treaties, non-binding documents, doctrine or international judicial practice. Sometimes, it appears that domestic courts simply assert, without citing persuasive practice authority, the existence of a customary norm. In rare cases, however, domestic courts do engage in extensive analysis.