The Impact of Customary International Law and General Principles of Law on the Horizontal System of Enforcement

Abstract

This chapter considers the influence which sources of international law, with the exception of treaty law, have on the horizontal system of enforcement. There exists relatively widespread consensus to the effect that important treaty provisions, such as the aut dedere aut judicare rule, have passed into customary international law. Such consensus, although broad, cannot be considered as overwhelming particularly because, in the first place, even conventionally, the aut dedere aut judicare rule presents itself in many forms. Such different formulae and their subsequent differentiated adherence could have a prejudicial impact on the customary status of the aut dedere aut judicare rule. This is because, since the 1985 Continental Shelf case, the ICJ asserted that the substance of customary international law must be looked for primarily at the actual practice and opinio juris of States. A traceable and influential nexus also exists between customary international law, on the one hand, and general principles of law, on the other hand. Domestic courts can play a role in forming customary international law, in that their decisions constitute State practice. Such dicta can constitute both State practice and opinio juris. Municipal judgments also have the potential to form and identify general principles of law. This is particularly so in relation to substantive international criminal law (especially the constitutive elements of core crimes), but it is not so self-evident when State practice on extradition is examined with the sources of international law (for such purpose, customary international law and general principles of law) in mind.