What Is Customary international law?
Customary international law refers to legal obligations arising from the consistent practice of states that they follow out of a sense of legal obligation. It is a foundational component of International Legal Frameworks and operates alongside treaties and general principles of law as a primary source of international law. Unlike formal written conventions, customary international law is unwritten, emerging from the actual behavior of states coupled with their belief that such behavior is legally required. This dual requirement, often referred to as "state practice" and "opinio juris", is essential for a norm to achieve the status of customary international law.
The International Law Commission (ILC) emphasizes that identifying customary international law necessitates establishing both a general practice and the acceptance of that practice as law (opinio juris).9 This means that a mere habitual action is not enough; states must engage in the practice because they believe they are legally compelled to do so. Customary international law generally applies to all states, irrespective of whether they have explicitly consented to a specific treaty, provided they have not persistently objected to its formation.
History and Origin
The origins of international law, including customary norms, can be traced back thousands of years to cooperative agreements among ancient peoples, such as treaties between rulers in Mesopotamia and Egypt.8 Concepts that underpin the modern international legal order, like jus gentium (law of nations), were established during the Roman Empire to govern relations between Roman citizens and foreigners, often imbued with a sense of universal application based on natural law.7
The modern system of customary international law began to formalize during the European Renaissance. Influential jurists such as Hugo Grotius, often regarded as a significant figure in the development of international law, worked to organize it into a comprehensive system, notably in his work De Jure Belli ac Pacis (On the Law of War and Peace) in 1625.6 Throughout history, the development of customary international law has been influenced by global events, interactions among states, and the evolving understanding of state sovereignty and international relations.
Key Takeaways
- Customary international law arises from the consistent practice of states combined with their belief that such practice is legally obligatory (opinio juris).
- It is an unwritten source of international law, distinct from formal treaties.
- The International Court of Justice (ICJ) and other international bodies frequently rely on customary international law in their rulings.
- It applies generally to all states unless a state has been a persistent objector to its formation.
- Challenges include identifying clear state practice and opinio juris, as well as enforcement issues due to the absence of a centralized global authority.
Interpreting Customary international law
Interpreting customary international law involves a careful assessment of state practice and opinio juris. State practice refers to the consistent and widespread conduct of states, including their official statements, diplomatic correspondence, legislative acts, and judicial decisions. It is crucial to determine if these actions demonstrate a general pattern rather than isolated incidents.
Opinio juris, the "psychological element," requires discerning whether states engage in a practice because they believe it is legally required, rather than out of courtesy, political expediency, or habit. This often involves examining statements made by state representatives, voting records in international organizations, and the justifications states provide for their actions. The process can be complex due to the unwritten nature of customary international law, requiring extensive legal analysis of available evidence.5 Understanding these elements is vital for states engaging in international trade or managing potential geopolitical risk.
Hypothetical Example
Consider a hypothetical scenario where Country A and Country B share a maritime border. For decades, both countries have consistently allowed fishing vessels from the other nation to operate within a specific 12-nautical-mile zone from their respective coasts without requiring special permits, as long as the vessels adhere to basic safety and environmental regulations. Neither country has a formal trade agreement or investment treaty explicitly codifying this arrangement.
One day, Country A decides to impose strict permit requirements and fees on Country B's fishing vessels entering its 12-nautical-mile zone. Country B protests, arguing that a customary international law has developed between them, granting mutual access without such restrictions, and that Country A is now violating this established norm. For Country B to successfully argue the existence of customary international law, it would need to demonstrate two things: first, that the practice of free access has been consistent and widespread between the two countries for a significant period (state practice); and second, that both countries engaged in this practice because they believed it was a legal obligation, not merely a friendly gesture (opinio juris). Evidence might include diplomatic notes, navigational charts, or lack of previous enforcement actions for decades.
Practical Applications
Customary international law plays a significant role in various aspects of global governance and cross-border interactions, indirectly influencing financial markets and market stability. It underpins many fundamental principles of international relations, such as state sovereignty, non-intervention in internal affairs, and the prohibition on the use of force, all of which impact foreign direct investment and cross-border transactions.
For instance, the principle of diplomatic immunity, which allows diplomats to perform their duties without fear of arrest or prosecution in host countries, largely derives from customary international law. Another prominent application is in international humanitarian law, where many rules governing armed conflict are considered customary, binding all states even if they haven't ratified specific treaties. The International Court of Justice (ICJ) frequently refers to customary international law in its judgments. A notable example is the Nicaragua v. United States case in 1986, where the ICJ found the United States to be in breach of its obligations under customary international law concerning the non-use of force and non-intervention. This highlights how customary international law provides a framework for dispute resolution and accountability among nations.
Limitations and Criticisms
Despite its importance, customary international law faces several limitations and criticisms. A primary challenge is its unwritten nature, which can lead to uncertainty and ambiguity regarding its precise content and scope.4 Unlike treaties, there isn't a single document to consult, making it difficult to definitively determine when a customary rule comes into existence or what its exact boundaries are. This lack of clear codification can also make it challenging to identify both state practice and opinio juris, as evidence may be diffuse and open to interpretation.3
Furthermore, the enforcement of customary international law can be problematic due to the absence of a centralized global authority with the power to ensure compliance.2 While international bodies like the ICJ exist, their jurisdiction often depends on the consent of the states involved, and there is no global police force to enforce rulings. This reliance on state consent and political will means that compliance is often voluntary, making it difficult to hold states accountable for non-compliance. Critics also point to the potential for selective enforcement and politicization, as powerful states may have more influence over the development and interpretation of customary norms.1 This can create challenges, particularly for issues concerning sovereign risk or the implementation of sanctions.
Customary international law vs. Treaty law
Customary international law and treaty law are both primary sources of international law, yet they differ significantly in their formation, nature, and application.
| Feature | Customary International Law | Treaty Law |
|---|---|---|
| Formation | Arises from consistent state practice (usus) and the belief that such practice is legally obligatory (opinio juris). | Created through formal, written agreements between two or more states (bilateral or multilateral treaties). |
| Nature | Unwritten, evolving through the conduct and legal convictions of states. | Written documents (conventions, covenants, protocols, agreements) with clear articles and provisions. |
| Binding Effect | Generally binds all states, except for persistent objectors, as long as the custom is established. | Binds only those states that have signed and ratified (or otherwise consented to be bound by) the treaty. |
| Flexibility | More adaptable to changing international circumstances, as it evolves organically with state practice. | Less flexible; amendments or termination typically require formal processes agreed upon by the parties. |
| Evidence | Inferred from state conduct, diplomatic correspondence, national legislation, judicial decisions, and statements of legal opinion. | Clearly documented in the text of the treaty itself, along with any reservations or declarations made by parties. |
The main point of confusion often lies in their relationship: a treaty can codify existing customary international law, or a widely ratified treaty can contribute to the formation of new customary international law by demonstrating widespread state practice and opinio juris. However, a state that is not a party to a treaty is not bound by its provisions unless those provisions have also attained the status of customary international law.
FAQs
What are the two elements of customary international law?
The two essential elements are "state practice" and "opinio juris". State practice refers to the general and consistent conduct of states, while opinio juris is the belief that such conduct is legally required.
Is customary international law written or unwritten?
Customary international law is primarily unwritten. Its rules emerge from the actual behavior of states over time, coupled with their conviction that these behaviors are legally binding, rather than from a formal written document.
How does customary international law affect multinational corporations?
While customary international law primarily governs states, its principles can indirectly affect multinational corporations by shaping the legal frameworks and regulatory environments within which they operate. For example, norms related to human rights, environmental protection, or non-discrimination, if recognized as customary international law, may influence corporate social responsibility standards or legal expectations in the jurisdiction where they operate.
Can customary international law be changed?
Yes, customary international law can evolve or change over time as state practice and opinio juris change. If states consistently begin to act differently and demonstrate a new legal conviction regarding a particular matter, an existing customary rule can be modified or a new one can emerge. This process is generally slow and requires widespread acceptance by the international community.
What is the difference between custom and customary international law?
"Custom" in a general sense refers to habitual practices or traditions. "Customary international law" is a specific legal concept that requires not only a consistent practice (custom) but also the accompanying belief that the practice is legally binding (opinio juris). Without opinio juris, a widespread practice remains merely a custom or usage, not a source of international law.