Prof. Kenneth S. Gallant – No Ex Post Facto Criminal Laws: Legality and its Meaning for Comparative and International Law

The non-retroactivity of crimes and punishments has become a rule of customary international law. How it did so is an interesting and complex story about the use of comparative law in the making of international law.

This rule of international human rights law can be demonstrated as rigorously from practice and opinio juris as any other rule of customary international law. It is sometimes said that less evidence of state practice is necessary to treat an international human right as customary international law. Rules of human rights law, however, are far safer and more secure if grounded in practice as well as opinio juris. Legality is an excellent tool for making such a demonstration of technique in human rights law.

Because legality, as with other human rights, concerns the relationship between states and their own nationals, not just states and others’ nationals, a wider variety of practice sources needs to be considered than is often used in deal with evidence of customary international law that concerns only state interactions with each other. In addition, the practice and opinio juris of international organizations, including international tribunals, has become primary evidence for (not just a subsidiary means of determining) customary international law.

Comparative law technique is necessary to determine the specific content of this rule. Without comparative analysis, it would be difficult to determine which of the many versions of legality in the laws of national and international tribunals constitutes the version required by international law.