International Custom: State Practice and Opinio Juris

Custom is the oldest source of international law and, unlike a treaty, binds almost all states. Article 38(1)(b) of the ICJ Statute refers to international custom, as evidence of a general practice accepted as law. This note explains the two elements of custom, how they are proved, the leading cases, including one involving India, the persistent objector rule, and the relationship between custom and treaties.

1. The Two Elements

Figure 1: Practice and acceptance as law

• Wording. Article 38(1)(b) is said to be inverted: custom is not evidence of a practice; rather, a general practice accepted as law is evidence of a custom.
• State practice. What states do and say: the practice must be general, though not universal, and consistent, though not perfectly uniform; no minimum duration is required.
• Opinio juris sive necessitatis. The belief that the practice is obligatory, or permitted, as a matter of law; without it, a practice is only usage, comity or courtesy, such as ceremonial salutes at sea.
• ILC conclusions, 2018. The International Law Commission's conclusions on identification of customary international law confirm the two element approach: each element must be ascertained separately, and the same material may evidence both.

2. Evidence of Practice

• Forms. Diplomatic correspondence, policy statements, official manuals, legislation, executive acts, decisions of national courts, treaties, and conduct in international organisations, including votes on resolutions.
• Inaction. Abstaining from action counts only if it is shown to rest on a sense of legal duty to abstain (Lotus)
• General Assembly resolutions. May evidence opinio juris, depending on their content and the conditions of adoption (Nuclear Weapons, 1996); in Chagos (2019), resolution 1514 (1960) was treated as reflecting customary law on self-determination.

3. The Leading Cases

Figure 2: Landmark decisions on custom

• Lotus (France v Turkey), PCIJ, 1927. Turkey's prosecution of a French officer after a collision on the high seas did not violate international law; France had not shown that states abstained from such prosecutions out of a sense of legal duty.
• Asylum (Colombia v Peru), 1950. A party relying on a regional custom must prove that it is established so as to bind the other party, in accordance with a constant and uniform usage; the practice on diplomatic asylum showed too much uncertainty and contradiction to support a right of unilateral qualification of the offence.
• Anglo-Norwegian Fisheries (UK v Norway), 1951. The ten mile rule for bays was not a general custom and, in any event, did not bind Norway, which had always opposed its application to its coast.
• Right of Passage over Indian Territory (Portugal v India), 1960. A local custom can exist between only two states; Portugal had a right of passage for private persons, civil officials and goods to its enclaves of Dadra and Nagar Haveli, but not for armed forces, armed police, arms and ammunition, so India had not acted contrary to its obligations by refusing passage.
• North Sea Continental Shelf (Germany v Denmark and Netherlands), 1969. The equidistance rule in Article 6 of the 1958 Convention was not customary and did not bind Germany, a non-party; a new custom needs practice, including that of specially affected states, that is extensive and virtually uniform and shows a general recognition that a legal obligation is involved, though a short time may suffice.
• Military and Paramilitary Activities (Nicaragua v USA), 1986. Practice need not be in absolutely rigorous conformity; conduct inconsistent with a rule should be treated as a breach, not as a new rule; and a customary rule continues to exist alongside an identical treaty rule.

4. Kinds of Custom and the Persistent Objector

• General custom. Binds all states, including new states, whether or not they took part in its formation.
• Regional custom. Among states of a region; must be strictly proved against the state concerned (Asylum)
• Local or bilateral custom. Between two states, established by long practice between them (Right of Passage)
• Persistent objector. A state that objects clearly and consistently while a rule is forming is not bound by it (Fisheries); a state that objects only after the rule is established is bound, and the doctrine does not apply to jus cogens.
• Instant custom. Some writers, such as Bin Cheng, argued that custom on outer space could arise almost instantly from opinio juris; most require some practice.

5. Custom and Treaties

Figure 3: Three ways a treaty interacts with custom (North Sea, 1969)

A treaty rule and a customary rule with the same content exist independently, so a state not party to a treaty, or one that has a reservation, may still be bound by the custom, and the ICJ may apply the custom where the treaty is outside its jurisdiction, as in Nicaragua (1986). A treaty provision can generate custom only if it is of a fundamentally norm creating character.

6. Frequently Asked Questions

Can a custom exist between only two states?

Yes. In Right of Passage over Indian Territory (1960), the ICJ saw no reason why a local custom based on long practice must involve more than two states, and recognised one between Portugal and India.

What is the difference between usage and custom?

Usage is a habitual practice followed without any sense of legal obligation, such as ceremonial courtesies. It becomes custom only when states follow it because they believe the law requires or permits it, that is, with opinio juris.

7. Related Topics

• Topic 7: Sources of International Law. Article 38 of the ICJ Statute.
• Topic 10: Jus Cogens and Obligations Erga Omnes. Custom that admits no derogation.
• Topic 33: State Jurisdiction. The Lotus principle.