Customary Law - International Law

For a rule to be seen as a rule of customary IL which binds all states, there are two requirements: consistent and general state practice and opinio juris (North Sea Continental Shelf Cases).

• There may be regional customs in particular areas (Asylum case)

• New custom will override old customary law

‘State practice’ is practice showing that states are acting in compliance with a particular rule. There must be consistent and general (not erratic) state practice by a significant majority of states.

1. State practice need not be absolutely uniform and universal , as long as it is extensive and ‘virtually uniform’(North Sea Continental Shelf Cases);

2. State practice must be, in general, consistent with the rule and instances of inconsistent conduct must have been treated as breaches of the rule rather than recognition of a new rule (Nicaragua Case)

3. State practice need not have been occurring over a period of extensive duration, as long as these requirements are shown (North Sea Continental Shelf)

   • Custom can arise almost instantaneously between two states if there are no other claims (eg rules applicable to the use of outer space between the USA and USSR)

4. The states ‘most affected’ - those with relevant interests at risk - must have acceded in the purported rule (North Sea Continental Shelf Cases/Legality of Nuclear Weapons Case)

Evidence of state practice is ‘any act or statement by a state from which views about customary law can be inferred’ (Akehurst, quoted in Trigg). Evidence includes:

‘Opinio juris’ is a belief that the practice the state is undertaking is a legal obligation and that it is bound to follow the rule. This subjective element must be proved by evidence that the rule ‘is of such a nature, and has been so widely and generally accepted, that it can hardly be supposed that any… state would repudiate it’ (West Rand Central Gold Ming Co).

• Given the difficulties of evidencing this subjective requirement, judicial inquiry has sometimes return to examination of state practice (see, eg, Congo v Belgium, where the majority of the ICJ confined its analysis to state practice); however, opinio juris is often nonetheless held to be important (Legality of Nuclear Weapons)

Evidence of opinio juris

• GA (see, eg, Israeli Wall case, Nicaragua, Kosovo Advisory Opinion) and SC resolutions (see, eg, Israeli Wall case): these are often considered by tribunals without pointing to whether they reflect state practice or opinio juris, showing they are evidence of both

• Treaties are not evidence of state practice, since a state obeying a rule because it is in a treaty is not evidence that it believes it is customary law

  • However, treaties may crystallise into customary law [see below]

• Breach of a rule may be evidence of opinio juris of a particular state if it tries to justify its inconsistent conduct: this shows it considers itself bound rather than that another norm is developing (Nicaragua case)

• Other evidence as for state practice

Custom may crystallise after a treaty expressing that norm

1. treaties may crystallise into customary law if the treaty is of a potentially norm-creating character (North Sea Continental Shelf case)

   1. ability to make reservations to an article may mean it is not of a norm-creating character (North Sea Continental Shelf)

2. this may occur over even a short period if there is widespread and representative participation in a convention, provided the participants include those states whose interests are particularly affected (North Sea Continental Shelf cases)

3. the treaty and custom will coexist and even states not party to such treaty will be bound after it becomes custom (Nicaragua case)

   1. where a party has not accepted an international tribunal’s jurisdiction on multilateral treaty adjudication in particular circumstances, it may still be brought to court where the rule is also custom (Nicaragua case)

The persistent objector rule allows a state to object to the formation of a norm of customary international law, and thus to ‘opt out’ of its operation (Anglo-Norwegian Fisheries case).

1. This is only applicable where the customary rule is de lege ferenda (an emerging rule); states may not object to established rules.

2. Objection can be demonstrated by making a statement or by consistent practice contrary to the rule

3. Even if there is evidence of opinio juris by an overwhelming...