Customary international law

Customary international law is an aspect of international law involving the principle of custom. Along with general principles of law and treaties, custom is considered by the International Court of Justice, jurists, the United Nations, and its member states to be among the primary sources of international law.

Many governments accept in principle the existence of customary international law, although there are differing opinions as to what rules are contained in it.

In 1950, the International Law Commission listed the following sources as forms of evidence to customary international law: treaties, decisions of national and international courts, national legislation, opinions of national legal advisors, diplomatic correspondence, and practice of international organizations.[1] In 2018, the Commission adopted Conclusions on Identification of Customary International Law with commentaries.[2] The United Nations General Assembly welcomed the Conclusions and encouraged their widest possible dissemination.[3]

Recognition of customary international law

The International Court of Justice Statute defines customary international law in Article 38(1)(b) as "a general practice accepted as law".[4] This is generally determined through two factors: the general practice of states and what states have accepted as law (opinio juris sive necessitatis).[5]

There are several kinds of customary international laws recognized by states. Some customary international laws rise to the level of jus cogens through acceptance by the international community as non-derogable rights, while other customary international law may simply be followed by a small group of states. States are typically bound by customary international law regardless of whether the states have codified these laws domestically or through treaties.

Jus cogens

A peremptory norm (also called jus cogens, Latin for "compelling law") is a fundamental principle of international law which is accepted by the international community of states as a norm from which no derogation is ever permitted (non-derogable). These norms are rooted from Natural Law principles,[6] and any laws conflicting with it should be considered null and void.[7] Examples include various international crimes; a state violates customary international law if it permits or engages in slavery, torture, genocide, war of aggression, or crimes against humanity.[8]

Jus cogens and customary international law are not interchangeable. All jus cogens are customary international law through their adoption by states, but not all customary international laws rise to the level of peremptory norms. States can deviate from customary international law by enacting treaties and conflicting laws, but jus cogens are non-derogable.