Comparative law method is essential to determining the customary international law status of rules of human rights law. Doing the hard, detailed work of comparative law is necessary if we are to give up on the unfortunate tendency to make overly broad, unsupported claims that wide varieties of human rights have passed into customary international law.
The traditional use of only interstate practice in determining rules of customary international law is insufficient where the rules concern relationships between states and individuals, especially their own nationals. This, however, is the essence of human rights law.
Comparative law techniques allow, and are necessary to, determining the scope and content of human rights guarantees across different states and legal systems. Only through use of these techniques can we determine state practice on human rights, and the reasons that the rights are protected (opinion juris) — i.e., which rights are protected by customary international law. This piece uses one example, the determination that non-retroactivity of crimes and punishments has passed into customary international law, and shows which version of that principle has become the rule of customary international law.
This sort of research may sometimes lead to unwelcome conclusions that, in fact, some rights have not yet become customary international law, or have become customary only in weak or incomplete forms.
Kenneth S. Gallant, Use of Comparative Law in Determining the Customary International Law of Human Rights, 24 Fla. J. Int'l L. 429 (2012).