International custom is one of the primary sources of international law, recognized under Article 38(1)(b) of the Statute of the International Court of Justice (ICJ). It refers to long-standing practices followed by states out of a sense of legal obligation, known as opinio juris.
It is not written in treaties but evolves organically through consistent and general practice accepted as law by the international community.
Essential Elements of International Custom
To qualify as a binding international custom, two key elements must be present:
- State Practice (usus):
There must be widespread and consistent behavior by states over time. This practice includes official acts, diplomatic notes, legislation, and court decisions. - Opinio Juris:
States must follow the practice because they believe they are legally obligated to do so—not merely out of courtesy or habit.
Only when both elements are present does a custom attain legal force.
Legal Recognition and Importance
The International Court of Justice (ICJ) has repeatedly recognized international as a binding source of law, even in the absence of treaties. Examples include:
- Non-use of force in international relations
- Immunity of foreign diplomats
- Freedom of the high seas
In many cases, international customary law binds even those states that haven’t signed a specific treaty, making it universal in scope.
Examples of International Custom
- Prohibition of genocide
- Non-refoulement in refugee law
- Immunity of state officials
- Laws of warfare, as recognized in the Hague and Geneva Conventions
Such practices are accepted and observed universally, making them part of customary international law.
Role in Modern International Law
In today’s global system, international custom continues to play a major role when:
- No treaty exists between disputing parties
- Filling legal gaps in existing international agreements
- Interpreting ambiguous treaty provisions
Custom evolves with changing norms and behaviors, ensuring that international law remains dynamic and adaptable.
