Custom, and How a Rule Becomes One
Chapter Four
Syllabus topic 1, "Environmental Federalism and International Order"
Pages 11 to 13 of 663
In one line
Custom is a rule that becomes law because States have behaved that way for long enough and have done so believing they were legally obliged to, and it is the only source that binds a State which has never signed anything.
In exam wording: international custom, under Article 38, paragraph 1(b), of the Statute of the International Court of Justice, is a general practice accepted as law, and it requires two elements: a settled and general practice of States, and opinio juris sive necessitatis, the conviction that the practice is obligatory.
Why this matters more here than anywhere else
Because the great environmental principles are not in any treaty India has joined. There is no convention that says "the precautionary principle shall apply in India". Yet the Supreme Court applied it in Vellore Citizens' Welfare Forum v. Union of India, and the route it used was custom. If the precautionary principle is customary international law, and if customary international law not contrary to Indian statute is deemed part of Indian law, then the principle is Indian law without Parliament doing anything at all. Every step of that reasoning depends on what custom is.
The two elements
One: general practice. What States actually do, and also what they say and how they vote. The practice must be:
- General, not universal. Not every State need have done it. The practice of the States particularly affected counts for more.
- Consistent, though not perfectly uniform. Occasional breaches do not destroy a rule; they are treated as breaches of it.
- Of some duration. There is no fixed period. A rule can form quickly where practice is dense and States are agreed.
Two: opinio juris. The belief that the practice is required by law. This is what separates law from habit. States exchange ambassadors and States hold state dinners; only the first is done out of a sense of legal obligation.
Opinio juris is the harder element to prove, and for environmental principles it is usually proved from what States say: how they vote on General Assembly resolutions, what they write into treaty preambles, what they argue before tribunals.
Broken down: three ways custom is proved in this subject
From a declaration repeated into a treaty. Principle 21 of the Stockholm Declaration 1972 was repeated as Principle 2 of the Rio Declaration 1992, written into the preamble of the Convention on Biological Diversity 1992 and of the Framework Convention on Climate Change 1992, and stated by the International Court of Justice in the Legality of the Threat or Use of Nuclear Weapons Advisory Opinion to be part of the corpus of international law relating to the environment. That is as good a proof of custom as this subject offers.
Custom, and How a Rule Becomes One
From arbitral practice. The rule in Trail Smelter was applied by the Court in Corfu Channel and again in Pulp Mills on the River Uruguay, where the Court called the principle of prevention a customary rule with its origins in the due diligence required of a State in its territory.
From a national court's own finding. In Vellore Citizens' Welfare Forum the Supreme Court of India held that sustainable development had been accepted as part of customary international law, and that the precautionary principle and the polluter pays principle were essential features of it. That is a national finding, not an international one, but it is what made the principles operative in India.
The persistent objector
A State that objects clearly and consistently from the moment a rule begins to form, and keeps objecting, is not bound by it when it crystallises. The doctrine is narrow: the objection must be from the start, it must be public, and it must be maintained. It cannot be raised after the rule exists.
Distinctions that carry marks
| Treaty | Custom | |
|---|---|---|
| How made | Written, negotiated, signed, ratified | Grown out of practice |
| Who is bound | Parties only | All States, except a persistent objector |
| Proof | Produce the text and the list of parties | Prove practice and opinio juris |
| Change | By amendment, following the treaty's own rules | By new practice displacing the old |
| Reservations | Possible unless excluded | Impossible |
Worked example
Anjali is arguing before an Indian High Court that a State pollution control board must apply the precautionary principle even though no Indian statute mentions it by name.
Her argument in four steps, and each step is a step this chapter has just supplied:
- Principle 15 of the Rio Declaration states the precautionary approach, and it is repeated in a long series of treaties and resolutions to which India is a party.
- That repetition, with the practice of national regulators, establishes the practice and the opinio juris that custom requires.
- Customary international law that is not contrary to Indian statute is deemed incorporated into Indian law and must be followed by Indian courts. That is the holding in Vellore Citizens' Welfare Forum v. Union of India, AIR 1996 SC 2715.
- No Indian statute contradicts the principle, so the board must apply it.
What it does NOT mean
Custom is not what most States want. It is what they do plus what they believe they must do. A resolution passed by a hundred and eighty votes is evidence of opinio juris; it is not itself custom.
A single arbitral award does not make custom. Trail Smelter was one tribunal deciding one dispute between two States in 1941. What made its rule customary was the eighty years of repetition that followed.
Custom, and How a Rule Becomes One
Custom is not frozen. The obligation to conduct an environmental impact assessment was not custom in 1972 and was held to be a requirement of general international law in Pulp Mills in 2010.
Quick revision
- Article 38(1)(b): custom is a general practice accepted as law.
- Two elements: practice, and opinio juris.
- Custom binds States that have signed nothing; the only escape is persistent objection from the start.
- Principle 21 of Stockholm is the standard example of a soft-law statement that became custom.
- Vellore is the Indian route: custom not contrary to statute is part of Indian law.
Test yourself
1. State the two elements of custom and give an example of each in this subject. Practice: States routinely notify neighbours of activities that may cause transboundary harm. Opinio juris: they say in treaties and before tribunals that they are obliged to.
2. Why can India be bound by a rule it never signed? Because custom binds all States. Only a persistent objector, objecting from the moment the rule began to form, escapes.
3. What is the difference between a habit and a custom in international law? Opinio juris. Both involve consistent practice; only custom is done out of a sense of legal obligation.
The rest of this subject
These notes are cut from the University's printed syllabus. Open the syllabus itself, or the past papers, for the same subject.