Customary International Law
Article 38(1)(b) of the ICJ Statute: International custom, as evidence of a general practice
accepted as law
• To determine the existence of a rule of custom it is necessary to ascertain whether there
is general practice (Usus) that is accepted as law (Opinio juris):
• General Practice (Usus) – Conduct element
• Accepted as law (Opinio juris) - Mental element
Both requirements have been confirmed in Continental Shelf Case ICJ, 1985
General Information:
Customary international law consists of rules that derive from the consistent practice of
States followed out of a sense of legal obligation or the belief that those rules are legally
binding (usus and opinio juris)
• Unwritten
• Universally applicable – Binding on all States*
• Evolves over time – Progressive development of international law
• Customary international law, International custom, rules of custom, custom
State Practice (Usus) + Accepted as law (Opinio juris) = CIL
Example 1: Unwritten rules about driving on Example 2 : Piracy
the Left Side of the Road
• Usus - Everyone drives on left side of • Usus – States refrain from acts of Piracy
the road = common practice (consistent • Opinio Juris – States refrain from acts of
and uniform) Piracy BECAUSE they believe that it is
• Opinio juris – We believe that we have illegal under international law
to drive on the left side of the road.
Usus
• Usus is the uniform and consistent State practice.
• Usus can be physical practices, verbal acts, inaction
Usus can take many forms:
1. Acts taken by states in their diplomatic relations with one another;
2. Acts taken internally by states through their legislatures or courts;
3. Acts taken by states before international organizations; and
4. Deliberate inaction by states
Duration and consistency of practice How long should state practice continue to qualify as
usus?
• No rigid time element – depends upon circumstances of each case.
• Duration not the most important component of state practice.
Anglo-Norwegian Fisheries Case, ICJ: There must be ‘constant and sufficiently long practice’,
but also that ‘too much importance need not be attached to the few uncertainties or
contradictions, real or apparent’ that may exist in practice.
Nicaragua case: ‘absolutely rigorous conformity with the rule’ not required.
Opinio Juris
Opinio juris is the belief that states are following a practice because they consider it to be a
legal obligation.
In the North Sea Continental Shelf Cases (ICJ) stated : that the States concerned must feel
that they are conforming to what amounts to a legal obligation.
• How can it be proven that a State is considered to be legally bound?
Evidence of Opinio Juris
Treaty Participation:
Nicaragua Case: The ICJ looked at the behaviour of States in treaties to assess whether they
acted out of a sense of legal duty.
• The willingness of states to enter into and comply with treaties indicates that they
believe in the legal obligations set out in those agreements.
General Assembly Resolutions:
Use of Nuclear Weapons Case (1996): The ICJ examined how states voted on UN resolutions
to determine if they believed their actions were legally required.
• Consistent support for resolutions can show that States view the practice as mandatory
under international law.
Official Publications:
States documents and reports that outline a State's adherence to certain international
practices can indicate opinio juris.
• For example, a national policy document or even a legislation emphasizing compliance
with norms of international humanitarian law shows that the State believes these norms
are legally binding
Customary International Law
To determine if a practice and its associated opinio juris have become customary
international law, it is not about a specific number of states. Instead, international law
follows certain considerations:
• Widespread and representative participation
• There is no rigid numerical threshold
• Majority participation
The persistent Objector
When is a State Not bound by a rule of custom?
• Persistent Objector Rule: if a state refuses to consent to the new norm/rule of CIL, then
the State will not be bound by it.
• If a state can prove that it tacitly or expressly opposed a rule of customary law during its
formation, then it is not bound by that rule.
Key Elements:
- Consistent Objection: The state must consistently object to the rule while it is
emerging.
- Clear Communication: The objection must be clearly communicated to other states
and international bodies.
- Timing: The objection must be maintained from the time the practice begins to
develop into a norm.
Customary International Law
What is the Relationship between treaty law and customary International law?
• Sometimes treaties codify existing rules of custom
• A treaty may clarify a rule of custom that emerged prior to conclusion of the treaty
• Eg … The Vienna Convention on Diplomatic Relations (1961) codified many diplomatic
practices that were already widely accepted as customary international law
