Customary International Law in the Reasoning of International Courts and Tribunals

Abstract

The methodology for identifying customary international law has long been established, most prominently in the jurisprudence of the International Court of Justice. This methodology has also recently been affirmed by the International Law Commission. International courts and tribunals have nevertheless adopted various shortcuts in their legal reasoning when ascertaining the existence and content of customary international law. The use of such shortcuts does not always withstand scrutiny, particularly where evidence of State practice and opinio juris is divided or scant. This is also the case where conclusions are drawn exclusively on the basis of certain written materials, prior decisions of other courts and tribunals, or the work of the International Law Commission which, more often than not, combines elements of codification and progressive development. While recognizing that international courts and tribunals face various institutional and practical constraints, this chapter argues that they should nonetheless show greater care in their legal reasoning and demonstration of customary international law, absent which they risk being perceived not only as the idle scribes of custom but as law-making agents proper, which may in a long run damage both the authority of their decisions and that of custom itself as a source of international law.