Summary

Article 38 of the Statute of the International Court of Justice lists “international custom, as evidence of a general practice accepted as law” as the second source of law to be used by the Court. In other words, customary international law (CIL) requires state practice and opinio juris, the belief that the practice is legally required. A basic principle of international law is that sovereign states must consent to be bound by international legal requirements. Therefore, for a norm to become CIL, a widespread group of states must consistently follow the norm and indicate, either explicitly or implicitly, that they consent to the norm. Consistent action is important in two ways: consistent state practice following the norm indicates state consent to be bound by the norm and consistent objection to the norm indicates that the state does not consent to the norm. To avoid being bound by a rule of CIL, a state must persistently object to the rule during and after its formation. Changing CIL requires new state practice and evidence that opinio juris supports the new, not the old, state practice. Debates surrounding state practice include the number of states required to demonstrate “widespread” action, whether the states must be representative of the community of states, and how long consistent practice must occur before CIL is formed. Opinio juris is debated because it is subjective unless there is a specific, official statement that there is a belief that the practice is legally required.

Once a state consents, implicitly or explicitly, to a CIL rule, it cannot withdraw that consent. States that gain independence after a CIL rule is established are bound by that rule if the former government was not a persistent objector. This is problematic, particularly for former colonies that were not able to object during the formation of existing CIL rules because they were not considered “sovereign states.” Scholars supporting this perspective argue that, prior to decolonization, CIL was used to control the colonies and, since their independence, it is used by the colonizers to maintain their power and perpetuate inequality.