An Examination of the International Court of Justice’s Approach to Customary International Law
Abstract
Article 38(1) of the Statute of the International Court of Justice (ICJ) is regarded as the pre-eminent authority on the sources of public international law. Of the sources in this Article, none has been questioned as much as international custom, also referred to as customary international law. The ICJ has ruled that customary international law crystallizes when there is a conjugation of state practice and opinio juris, the subjective feeling by states that they must undertake the state practice. However, that seemingly simple definition leads to several questions: what amounts to state practice? How is opinio juris measured? Are state practice and opinio juris qualitatively different? Through an examination of ICJ cases, this thesis examines how that Court has – and has not - answered these questions when defining and identifying customary international law.