Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Laws

Program

Law

Supervisor

Oosterveld, Valerie

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.

Summary for Lay Audience

The International Court of Justice (ICJ) – the United Nations court which considers state-to-state disputes – considers three types of international law when it judges cases: treaties, customary international law, and general principles of law. This thesis focuses on customary international law because it is the most critiqued source of the three. The ICJ defines customary international law as consisting of state practice and the acceptance of practice as law (known as opinio juris). This two-component principle has generated many questions: what amounts to state practice? How is opinio juris measured? Are state practice and opinio juris different? This thesis examines how the ICJ has defined and identified customary international law, particularly how it has decided that state practice and opinio juris exist when concluding that a rule has reached the level of custom.

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