How Does Customary International Law Evolve, and How Can Its Existence and Content Be Reliably Proven?

Customary international law (CIL) is a foundational pillar of the international legal system, comprising rules that states accept as binding legal obligations, even in the absence of a formal treaty. Unlike treaties, which generally bind only the states that have consented to them, CIL rules are typically binding on all states. This makes understanding its formation, evolution, and methods of proof essential for anyone navigating the complexities of international relations, trade, and dispute resolution. This article delves into the intricate process by which CIL evolves and the rigorous, yet often challenging, methods used to ascertain its existence and content.

The Twin Pillars of Customary International Law: State Practice and Opinio Juris

The traditional and most widely accepted theory for the formation of customary international law, consistently applied by the International Court of Justice (ICJ) and other international tribunals, rests on two core elements:

1. State Practice (usus or diuturnitas): This is the objective, material element, reflecting the actual conduct of states.
2. Opinio Juris sive Necessitatis (acceptance as law): This is the subjective, psychological element, signifying that states engage in a particular practice out of a sense of legal obligation or right, rather than mere habit, courtesy (comity), or political expediency.

The ICJ Statute, in Article 38(1)(b), refers to "international custom, as evidence of a general practice accepted as law," encapsulating this two-element approach. Both elements must be present and identifiable for a rule of CIL to be established.

1. Unpacking State Practice (Usus)

State practice encompasses a wide spectrum of state conduct in their international relations. This includes not only physical actions but also verbal acts, and even, in certain contexts, inaction or silence.

Forms of State Practice:
Evidence of state practice can be drawn from a diverse range of sources:

• Official governmental acts and statements: This includes diplomatic correspondence, policy statements, press releases, official manuals on legal questions (e.g., military manuals), and comments made by governments on draft treaties or legal proposals.
• National legislation: Domestic laws enacted by states can indicate their position on a particular international legal issue. For instance, national laws on the extent of the territorial sea or sovereign immunity have historically contributed to the formation of CIL.
• Decisions of national courts: Judgments of domestic courts, especially higher courts, on matters of international law can serve as evidence of how a state understands and applies international legal rules. The ICJ has referred to national judicial decisions as state practice, for example, in the Arrest Warrant of 11 April 2000 (Democratic Republic of the Congo v. Belgium) case (Judgment of February 14, 2002).
• Treaty practice: The conclusion of bilateral or multilateral treaties, particularly those that are widely ratified and declare or codify certain rules, can be strong evidence of state practice supporting those rules.
• Conduct in connection with resolutions of international organizations: How states vote on, and subsequently adhere to, resolutions of international bodies like the UN General Assembly can indicate their practice and legal views, especially for resolutions dealing with legal principles.
• Omissions or inaction: In certain circumstances, a state's consistent failure to act or protest against the practice of other states can be interpreted as acquiescence and thus contribute to the formation of a customary rule, provided it is accompanied by opinio juris. The ICJ considered a state's inaction in the Fisheries Case (United Kingdom v. Norway) (Judgment of December 18, 1951).

Characteristics of Relevant State Practice:
For state practice to contribute to the formation of CIL, it generally needs to exhibit certain characteristics:

• Generality: The practice should be sufficiently widespread, meaning it should involve a significant number of states. It does not require universal participation, but it should be representative, including states whose interests are "specially affected" by the rule in question. The ICJ affirmed this in the North Sea Continental Shelf Cases (Federal Republic of Germany/Denmark; Federal Republic of Germany/Netherlands) (Judgment of February 20, 1969). The influence of developed nations, due to their resources for information dissemination and active participation in international fora, can be disproportionately significant in practice, although formally all states are equal in the CIL formation process.
• Consistency and Uniformity: The practice should be reasonably consistent. While complete uniformity is not required, and some contrary practice does not necessarily negate the existence of a rule, substantial inconsistency can prevent a rule from crystallizing. If a state acts contrary to an established rule but defends its conduct by appealing to exceptions or justifications within the rule itself, such action may paradoxically confirm rather than weaken the rule, as seen in the Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America) case (Merits, Judgment of June 27, 1986).
• Duration: While traditionally CIL was seen as developing over a long period, the ICJ in the North Sea Continental Shelf Cases acknowledged that "the passage of only a short period of time is not necessarily, or of itself, a bar to the formation of a new rule of customary international law," provided the practice is extensive and virtually uniform, and accompanied by opinio juris. This opens the door for the relatively rapid formation of custom in response to new developments, sometimes referred to as "instant custom," particularly in areas like space law historically, or potentially cyber law today.

2. Ascertaining Opinio Juris Sive Necessitatis

The requirement of opinio juris is what distinguishes legally binding custom from mere usage, habit, or comity (acts performed out of courtesy or convenience rather than a sense of legal duty). A state must follow a certain practice because it believes it is legally required to do so. The ICJ in the Lotus Case (France v. Turkey) (Judgment of September 7, 1927) famously stated that only if abstention from exercising criminal jurisdiction was based on states "being conscious of having a duty to abstain would it be possible to speak of an international custom." Similarly, in the North Sea Continental Shelf Cases, the Court found that frequent state action was not, in itself, sufficient; states "must also feel that they are conforming to what amounts to a legal obligation."

Evidencing Opinio Juris:
Proving this subjective element can be notoriously difficult. Opinio juris is often inferred from the context of the practice itself, or from explicit statements made by states. Evidence may include:

• Official statements by governments (e.g., in diplomatic notes, parliamentary debates, or before international bodies).
• Voting patterns and explanations of votes on resolutions of international organizations, particularly the UN General Assembly, when these resolutions purport to declare or develop legal rules. The ICJ has considered UNGA resolutions as potential evidence of opinio juris in cases like Nicaragua v. USA and the Legality of the Threat or Use of Nuclear Weapons (Advisory Opinion of July 8, 1996).
• Treaty provisions, especially in preambles or in widely ratified codification treaties that declare existing law.
• Decisions of national and international courts that refer to rules as being legally binding.
• The International Law Commission's (ILC) conclusions on the state of customary law, which are often based on extensive surveys of state practice and opinio juris.

The theoretical debate about opinio juris often centers on whether it represents a state's tacit consent to be bound (a subjective theory close to contract) or its recognition of an existing or emerging objective legal norm (an objective or legal conviction theory). While the "tacit consent" view faces difficulties in explaining how CIL binds new states or becomes general law, the "legal conviction" approach raises the logical conundrum of how a state can believe a rule is law before it has actually become law (the "opinio juris paradox"). In practice, international tribunals, including the ICJ, appear to lean towards the "legal conviction" model, looking for evidence that states generally consider a practice to be required by law, without necessarily requiring individual consent from every state.

The Dynamic and Intertwined Nature of Practice and Opinio Juris

While analytically distinct, state practice and opinio juris are often deeply intertwined and not always easy to separate in the real world. The same piece of evidence can sometimes attest to both practice and opinio juris. For example, a state's enactment of domestic legislation criminalizing piracy reflects its practice of combating piracy and also its opinio juris that piracy is an international crime that it has a right (or duty) to suppress.

Furthermore, the characterization of an act as "state practice" relevant to a particular rule often depends on the accompanying opinio juris. An act of aggression is practice, but it's practice contrary to the prohibition on the use of force. If a state uses force but claims it is acting in self-defense, this claim (an expression of opinio juris) frames the practice as potentially consistent with, and even reinforcing, the law on self-defense, rather than undermining the prohibition on the use of force.

In some instances, particularly with UN General Assembly resolutions or declarations from diplomatic conferences, the verbal acts (statements, votes) themselves can be seen as a form of state practice and direct evidence of opinio juris simultaneously, blurring the lines between the two elements.

Proving Customary International Law: A Multifaceted Endeavor

Demonstrating the existence of a CIL rule is a rigorous process that requires careful examination of all available evidence. There is no single, definitive checklist.

The Role of International Courts and Tribunals:
International courts, particularly the ICJ, play a crucial role in identifying and articulating rules of CIL. While their decisions are technically binding only on the parties to a specific case (ICJ Statute, Article 59), their pronouncements on customary law are highly authoritative and often treated as strong evidence of the law by states, other tribunals, and scholars. The ICJ carefully sifts through state practice and evidence of opinio juris in its judgments and advisory opinions.
Other international tribunals, such as the International Criminal Tribunal for the former Yugoslavia (ICTY), have also made significant contributions to clarifying CIL, especially in fields like international humanitarian law and international criminal law. The ICTY Appeals Chamber in the Tadić case (Decision on the Defence Motion for Interlocutory Appeal on Jurisdiction, October 2, 1995), for example, extensively reviewed state practice to determine the customary status of rules applicable in non-international armed conflicts.

The International Law Commission (ILC):
The ILC, a UN body of legal experts, plays a vital role in the codification and progressive development of international law. Its draft articles and commentaries often involve exhaustive research into state practice and opinio juris and are frequently relied upon by courts and states as evidence of CIL, even if the draft articles themselves have not yet been incorporated into a treaty.

Challenges in Ascertaining "General" Practice:
A significant challenge in proving CIL is the practical difficulty of surveying the practice of all, or even a representative majority, of the world's nearly 200 states. Historically, the practice of powerful Western states often dominated the formation of CIL. While this is less acceptable today, the reality remains that the practice of states with greater resources, more active diplomatic engagement, and better-documented legal positions tends to be more visible and influential. Ensuring a truly "general" assessment requires overcoming these potential biases and actively seeking out the practice and opinio juris of states from all regions and legal traditions.

Evolution, Change, and the Persistent Objector

Customary international law is not static; it can evolve over time to reflect changes in the international community, technological advancements, and shifting values.

Emergence and Desuetude:
New rules of CIL emerge when sufficient state practice and opinio juris coalesce around a new norm. Conversely, existing customary rules can fall into disuse (desuetude) if state practice overwhelmingly and consistently deviates from them, and this new practice is accompanied by an opinio juris supporting the change or abandonment of the old rule.

The "Instant Custom" Debate:
As mentioned, in certain fields characterized by rapid development and urgent global concerns (e.g., early space law, aspects of international environmental law or human rights law), some argue that CIL can form relatively quickly, even with limited physical practice, if there is widespread and strong opinio juris expressed, for example, through UN General Assembly resolutions adopted by overwhelming majorities. This idea of "instant custom" is controversial, as it challenges the traditional emphasis on extensive material practice over time. However, it reflects a trend where the opinio juris element, particularly as evidenced by collective pronouncements in international fora, may sometimes play a more prominent role in signaling rapid normative shifts.

The Persistent Objector Rule:
A state that has clearly, consistently, and from the earliest stages objected to the formation of a particular customary rule will, in principle, not be bound by that rule once it crystallizes. This "persistent objector" doctrine is an exception to the general applicability of CIL. However, its practical application is rare, and its theoretical underpinnings (often linked to theories of state consent) are debated. The burden of proving persistent objector status is high. Furthermore, it is widely accepted that the persistent objector rule cannot be invoked against rules of jus cogens.

Influence of Normative Content:
There's an emerging, though not fully settled, view that the substantive content of a potential rule can influence the stringency of the requirements for its formation as CIL. For rules protecting fundamental humanitarian values or essential community interests, some tribunals and scholars suggest that the threshold for demonstrating widespread and consistent practice might be somewhat lower, or that opinio juris can be more readily inferred. The ICJ in the Corfu Channel Case (United Kingdom v. Albania) (Merits, Judgment of April 9, 1949) referred to "elementary considerations of humanity" as a basis for certain obligations, even without extensive discussion of state practice in the traditional sense. The Martens Clause in humanitarian law treaties is also sometimes seen as reflecting a principle where "laws of humanity" and "dictates of public conscience" can inform legal obligations beyond codified rules. This suggests a nuanced interplay where compelling moral or community interests might accelerate or facilitate the recognition of certain practices as legally binding custom.

Conclusion

Customary international law remains a vital and dynamic source of international legal obligation. Its formation through the intricate interplay of state practice and opinio juris reflects the decentralized nature of the international legal system. Proving the existence and content of a CIL rule is a complex undertaking, requiring a thorough examination of diverse forms of evidence and a keen understanding of the motivations and beliefs underlying state conduct. While subject to evolution and occasional controversy regarding its modes of creation and identification, CIL continues to provide a fundamental framework of rules that shape the conduct of states and other international actors, adapting, albeit sometimes slowly, to the changing realities of the global landscape. For legal and business professionals operating internationally, an appreciation of how these unwritten rules are formed, proven, and evolve is indispensable.