How It Works
Customary International Law Formation happens gradually and organically through the consistent and general behavior of states. This behavior, known as 'state practice,' must be widespread and representative, showing that states actually follow a particular rule in their international relations. But practice alone isn't enough. There has to be a belief that this behavior is required by law, called 'opinio juris.' Together, these two elements form a binding international norm.
Why It Matters
Customary international law is crucial because it fills gaps where treaties don't exist or don't cover specific situations. It binds all states, not just those that have signed agreements, creating a shared legal Framework that guides international conduct. This universality helps maintain order and predictability in global affairs.
Customary International Law vs Treaty Law
While treaties are written agreements explicitly consented to by states, customary international law emerges from practice and belief over time without written consent. Treaties bind only the parties involved, but customary law generally applies to all states. However, treaties can codify or modify customary rules, and sometimes customary law can override treaties if the latter conflict with fundamental principles.
Real-World Examples
The prohibition of torture is a customary international law norm, upheld by consistent state practice and the belief that torture is illegal, regardless of treaty ratification. Another example is the rule against piracy on the high seas, which has been universally recognized through customary law.
Common Misconceptions
One common misconception is that only formal agreements create international law. In reality, customary international law is equally binding. Another misunderstanding is that any repeated state action becomes law; however, without opinio juris—the belief that the practice is legally obligatory—it remains mere habit or policy, not law.
ILC Conclusions on Identification
The International Law Commission's 2018 Conclusions on Identification of Customary International Law provide the most authoritative contemporary framework for analyzing customary international law formation. The Conclusions affirm the two-element approach (state practice + opinio juris), specify how each element should be assessed, and address particular issues like persistent objection, regional custom, and the role of inaction.
The ILC Conclusions have been widely cited in international jurisprudence and provide the standard framework for assessing whether particular norms have achieved customary status. The Conclusions are themselves not binding but represent the considered analysis of the principal international law-codification body.
Example
The widespread state practice of diplomatic immunity, combined with the belief that it is legally obligatory, has established it as customary international law binding on all states.