What are Customary International Laws?

By Martha Inaingo

Published on: October 5, 2023 at 10:07 IST

Municipal laws has so many sources: primary or original sources and secondary or adopted sources. Customs are a major source of law not just in the municipal level but on the international level as well. Customs are compelling rules that are obtained from usages or practices that have been accepted as law and thus have the force of law to command obedience.

This article delves deep into the dynamic realm of customary international law by discussing the general overview of what customary international law are, and whether customary international laws are recognized, binding and enforceable as laws.

Statutory Provisions for Customary International Law

Article 38 of the Statute of the International Court of Justice, provides that the ICJ would determine disputes by applying international law in accordance with any of the following:

1. International conventions, whether general or particular, establishing rules expressly recognized by the contesting states ;
2. International custom, as evidence of a general practice accepted as law;
3. The general principles of law recognized by civilized nations ;
4. Subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.

Aforementioned provision recognizes international customs as a source of international law that is applicable by the court in the determination of disputes. A locus classicus of such application is the Lotus Case (France Vs Turkey),^[1] where a collision occurred in the high seas between a French vessel- Lotus and a Turkish vessel- Boz-Kourt. The victims of the collision were Turkish nationals and the Turkish government tried and sentenced the alleged French offenders in their court. The dispute was referred to the Permanent Court of International Justice, which held inter alia that a State cannot exercise its jurisdiction outside its territory unless an international treaty or customary law permits it to do so.

What are Customary International Laws?

Customary international law refers to international obligations arising from established international practices, as opposed to obligations arising from formal written conventions and treaties. Customary international law results from a general and consistent practice of states that they follow from a sense of legal obligation. Two examples of customary international laws are the doctrine of non-refoulement e.g, asylum and refugee rights, and the granting of immunity for visiting heads of state. ^[2]

Article 38 of the Statute of the International Court of Justice, explicitly states international custom as the second source of law to be used by the Court. Customary international law therefore consists of rules that come from a general practice accepted as law and exist independent of treaty law.^[3]

Customary international law reflects certain practices that States follow in a repeated and consistent manner and that they accept as law ( opinio juris ). Defined by the International Court of Justice (ICJ) as “evidence of a general practice accepted as law” (Art. 38.1 of ICJ Statute), customary law is one of the oldest sources of international law, alongside the law codified in international conventions. In the international arena, States create legal norms not only by expressly stating their will through international conventions, which is referred to as conventional law, but also through their conduct.^[4]

Elements of Customary International Law

Customary international law refers to the body of rules and principles that are derived from the consistent practice of states and other international actors, accompanied by a belief that such practices are legally obligatory (opinio juris). Unlike treaties, which are formal agreements between states, customary international law evolves over time as states engage in consistent and general practices that are accepted as law.

The elements of customary International law include:

• Unwritten rules: Customary international law is not based on written laws, they are not written down or codified in a particular source. They are based on continuous state practice along with states’ belief that they are acting in accordance with a binding norm. Customary law often precedes written law, it’s large unwritten but may be codified later, for instance, in the form of a convention or a formal resolution adopted by the General Assembly of the UN or by the International Law Commission.^[5]
• State Practice: Customary international law consists of rules of law derived from the consistent conduct of States acting out of the belief that the law required them to act that way; that is the widespread repetition by States of similar international acts over time.^[6] Customary international law originates from standards of behavior recognized and accepted as legitimate and beneficial. This conduct of States build up the “precedents” that can be invoked as proof of such law. Conversely, repeated acts violating the law may result in the progressive erosion of the custom, if they are not condemned openly.

State practice is generally determined by two factors: (1) the number of states that behave in accordance with the practice; and (2) the length of time over which those states have behaved in accordance with the practice.^[7]

The actual practice of states covers the duration, consistency, repetition, and generality of a particular kind of behaviour by states. All these factors are relevant in determining whether a practice may form the basis of a binding international custom. The ICJ has required that practices amount to a “constant and uniform usage” or be “extensive and virtually uniform” to be considered binding.^[8]

Once a practice becomes a custom, all states in the international community are bound by it whether or not individual states have expressly consented, except in cases where a state has objected from the start of the custom. A particular practice may be restricted to a specified group of states e.g., the Latin American states or even to two states, in which cases the standard for acceptance as a custom is generally high.

Customs can develop from a generalizable treaty provision, and a binding customary rule and a multilateral treaty provision on the same subject matter may exist at the same time, e.g., the right to self-defense.

• Opinio Juris: This is the general believe that the observed state practice is legally obligatory. Customary international law arises when a significant number of states consistently engage in a pattern of behavior and the conviction has developed among states that this behavior is required by international law.^[9]

Customary International Law can therefore be established by the evidence of state practice and opinio juris, the belief that the practice is legally required.

Although all states may contribute to the development of a new or modified custom, the major states generally play a greater role and possess a greater significance in the establishment of customs. For example, during the 1960s the United States and the Soviet Union played a far more crucial role in the development of customs relating to space law than did the states that had little or no practice in this area.

After a practice has been established, a second element converts a mere usage into a binding custom, the practice must be accepted as opinio juris sive necessitatis (Latin: “opinion that an act is necessary by rule of law”). In the North Sea Continental Shelf cases, the ICJ stated that the practice in question must have “occurred in such a way as to show a general recognition that a rule of law or legal obligation is involved.”^[10]

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 a customary international law, 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 customary international law, a state must persistently object to the rule during and after its formation. For an International custom to be changed a new state practice and evidence that opinio juris supports the new, not the old, state practice must be established.^[11]

Are Customary International Laws Binding and enforceable?

Treaty-based international law and customary international law can exist in parallel and may be in conflict with one another; treaty-based international law will not necessarily override customary international law. International customary law is as binding on States as the international conventions to which they are parties (as evidenced by Art. 38 of the International Court of Justice Statute). The fact that a State has not signed an international convention has no bearing on its obligations under customary law.^[12]

Currently, the four 1949 Geneva Conventions, as well as most provisions of the 1977 Additional Protocols, have gained the status of customary international law. This means that even States that have not ratified them must abide by their rules.

In its judgment in the Nicaragua Vs United States of America case, the International Court of Justice noted that the violations of a rule of customary law do not remove the existence of its customary character. “The Court does not consider that, for a rule to be established as customary, the corresponding practice must be in absolutely rigorous conformity with the rule. In order to deduce the existence of customary rules, the Court deems it sufficient that the conduct of States should, in general, be consistent with such rules, and that instances of State conduct inconsistent with a given rule should generally have been treated as breaches of that rule, not as indications of the recognition of a new rule. If a State acts in a way prima facie incompatible with a recognized rule, but defends its conduct by appealing to exceptions or justifications contained within the rule itself, then whether or not the State’s conduct is in fact justifiable on that basis, the significance of that attitude is to confirm rather than to weaken the rule”^[13]

The features of customary international law gives it the unique elements stated and it is very significant in the international arena in so many ways:

• International Customs reflect the decentralized nature of the international system.
• Customary law addresses situations and cases not specifically provided for in treaty law and deals with problems of contradictory interpretation. Customary law plays a very important role in the law of armed conflict and humanitarian action because it codifies interactions and confrontations between States on the one hand and non-state actors on the other.
• Customary law also plays an important role in filling the voids left by written law, whether because it does not exist or because it cannot be applied—for instance, owing to the complex procedure of signing, ratifying, and possibly issuing reservations to an international convention. In fact, it is the second source of law to which the International Court of Justice refers in framing its decisions (Art. 38.1 of ICJ Statute).^[14]

Conclusion

Customary International Laws are sources of International law that are evidenced from generally accepted state practices that are recognized as having the force of law and therefore binding. The ICJ Statute provides that the court recognizes as a law that is applicable in the resolution of disputes. Thus, a number of cases have been determined by the court reiterating that a State may do or may not do only what a treaty or customary international law prescribes or proscribes.

Customary International Laws are binding on their own and does not need a supplementary authority to make them enforceable.

1. lotus-case-summary/↑

2. customary_international_law ↑

3. war-and-law/treaties-customary-law/customary-law↑

4. humanitarian-law.org/content/article/3/customary-international-law/> ↑

5. https://law.duke.edu/ilrt/cust_law_2.htm> ↑

6. Ibid ↑
7. <https://guides.law.sc.edu/c.php?g=315476&p=2108171> ↑
8. North Sea Continental Shelf cases
9. University of South Carolina Library (n vii) ↑

10. <https://www.britannica.com/topic/international-law/Custom> ↑

11. <internationalstudies/display/10.1093/acrefor
12. https://guide-humanitarian-law.org/content/article/3/customary-international-law/ ↑
13. guide-humanitarian-law.org/content/article/3/customary-international-law/> ↑
14. <https://guide-humanitarian-law.org/content/article/3/customary-international-law/> ↑