Customary international law often seems like a riddle wrapped in a mystery inside an enigma. According to Manley O. Hudson, even the drafters of the International Court of Justice Statute “had no very clear idea as to what constituted international custom.” The situation has not changed much since then.

I got my first taste of the difficulties in identifying custom when I was a junior attorney at the U.S. Department of State and was assigned the task of preparing the U.S. submission in a juvenile death penalty case before the Inter-American Commission on Human Rights. The juvenile death penalty is prohibited by the International Covenant on Civil and Political Rights (ICCPR) and the American Convention on Human Rights, but the question in the Inter-American Commission case was whether it is also prohibited as a matter of customary international law.