Lecture 4 missed recording
Customary International Law: Core Elements
This lecture continues the discussion of the three primary sources of international obligation—treaties, custom, and general principles of law—with a detailed focus on customary international law. Custom is defined in the statute of the International Court of Justice (ICJ) as international custom as evidence of general practice, accepted as law. The two essential elements for custom are: (i) general state practice, and (ii) a sense of legal obligation or legal entitlement by states in adopting that practice, known in Latin as opinio juris (also given in full as opinio juris sive necessitatus). The lecturer notes the Latin term explicitly and also uses the shorthand pinio juris a few times, which should be read as opinio juris. The discussion also addresses how long custom can take to develop, how treaty practice interacts with customary law, proof challenges, and potential limits on custom via a persistent objector, as well as regional or local variants of custom.
Two Core Requirements of Customary International Law
Customary international law rests on two main requirements:
General state practice, interpreted as consistent and representative practice across relevant states.
A belief by states that such practice is legally obligatory or obligatory in terms of entitlement under customary international law (opinio juris or opinio juris sive necessitatus).
The key point is that consistency is important but not absolute rigidity. The ICJ in the Nicaragua case emphasized that a rule of customary international law does not require perfect conformity; some states may oppose the rule, yet custom can still develop. This allowance for imperfect conformity underlines that custom evolves through widespread, representative practice and the belief that that practice has legal force, not through universal agreement alone.
General Practice: What It Looks Like in Practice
General practice means that states follow a practice regularly and in a representative way. It does not require every state to conform absolutely. The practice must be representative of the relevant states and groups, especially when a rule affects specific groups of states more than others. For example, in continental shelf disputes, the practice of coastal states—those with a coastline—and maritime states with large fishing fleets would be considered representative for the rule governing continental shelves and related rights. The International Law Commission’s 2018 work on customary international law supports the view that practice must be representative and significant, not merely numerical; it is not a mechanical tally of states.
Representative practice often involves major groupings of states, including both developed and developing states. In some domains, there is a functional representation element—for example, in continental shelves you look at coastal states; in fishing rights you consider states with coastline and those with large fishing fleets; in investment law you consider capital importing and capital exporting states. The overarching idea is to assess whether there is consistent practice across those different, relevant groupings rather than merely counting states.
Opinion Juris (Opinio Juris) and the Belief in Legal Obligation
Opinio juris is the belief that a practice is required or is obligatory under law, not merely a habit or policy choice. It is the sense of legal obligation that accompanies the practice and distinguishes customary law from mere diplomatic usage or convenience. The dual requirement—general practice plus opinio juris—ensures that customary rules reflect what states feel legally bound to do, not just what they happen to do.
Proof, Difficulties, and the Persistent Objector
Proving a rule as customary involves showing both extensive and representative practice and evidence of opinio juris. There are challenges in proof, including controversies about what counts as sufficient practice and which states are “specially affected” by a rule. Judge Tanaka, in discussions related to the continental shelf, emphasized that you cannot determine the existence of a customary rule purely by counting states. Instead, you must assess whether the practice is representative and significantly has the effect of creating a sense of legal obligation among states affected by the rule. The concept of “specially affected” states is key: for continental shelves, coastal states are usually regarded as specially affected; landlocked states may be treated differently.
There is also the persistent objector rule. This rule provides an “escape hatch” for dissenting states: if a state persistently objects to a developing customary rule from the outset, that state cannot be bound by that rule once it crystallizes, provided the requirements of the persistent objector rule are met. This mechanism ensures that even if most states eventually recognize a rule as customary, resolute dissenters can avoid binding effect, though this is balanced against the general tendency for custom to develop despite some opposition.
Regional and Local Custom; Global Norms vs Local Variants
Most customary law is global in character, but regional and local custom also exist. In some areas, regional practice may crystallize around particular norms due to shared geography, history, or legal culture. The continental shelf example illustrates the idea that practice can be regionally focused (special considerations for coastal states) while remaining part of a broader global framework. The notes emphasize that while global norms are common, regional and local customs play a significant role and must be assessed in light of their particular contexts.
The Relationship Between Treaties and Custom: How Practice Can Crystallize into Custom
A central theme of the lecture is the relationship between treaty practice and customary international law, particularly how long it takes for custom to develop and how treaty arrangements can influence that development. The North Sea Continental Shelf cases provide a crucial example of this relationship and of evaluating whether a treaty provision has become customary practice.
The North Sea Continental Shelf Cases (1969): Background and Key Issues
The North Sea Continental Shelf cases involve a dispute between West Germany (Federal Republic of Germany), Denmark, and the Netherlands over the boundary line for the continental shelf between their adjacent coasts. Denmark and the Netherlands were parties to the 1958 Geneva Convention on the Continental Shelf, while West Germany was not a party. The central question was whether the equidistance rule (which draws a boundary based on a perpendicular line from the baselines) codified in Article 6 ext{(2)} of the Geneva Convention on the Continental Shelf reflected a preexisting customary rule, and if not, whether state practice between 1958 and 1969 could have crystallized such a rule as customary.
Prior to the 1958 convention, Article 6 ext{(1)} addressed opposite states and used a median line, while Article 6 ext{(2)} dealt with adjacent states and provided that, in the absence of agreement, the boundary would be determined by the application of the equidistance principle from the nearest baselines. Denmark and the Netherlands argued a trilogy of positions: (1) the 1958 treaty codified a preexisting customary rule; (2) the negotiations at the 1958 conference crystallized a customary rule from an preexisting practice; or (3) an evolving practice between 1958 and 1969 had coalesced into a rule of customary international law. West Germany argued against all three propositions.
The Court’s analysis concluded that there was no preexisting customary rule codified in 1958 that compelled West Germany to accept equidistance. The Court also rejected the argument that the 1958 conference alone crystallized a customary rule by itself at that time. However, the Court acknowledged that it is possible for customary rules to develop over a relatively short period. The Court stated that within such a period, state practice (including that of specially affected states) would need to be extensive and virtually uniform and show general recognition of a legal obligation. The Court recognized the theoretical possibility of quick development, but in the particular case found that the requisite extensive and virtually uniform practice sufficiency had not been shown.
In the course of explaining the timing, the Court used a vivid metaphor: the 1958 conference could be seen as a midwife in the birth of a customary rule, but the Court found no evidence that equidistance had crystallized as a customary rule by 1969. The Court thus did not find a binding customary rule on West Germany in relation to equidistance for the continental shelf in that period, though it left open the possibility that custom could develop in a relatively short timeframe if the requisite practice existed.
The Court’s judgment set out a clear standard: even if a rule could potentially develop within a short timeframe (roughly a decade or so), the evidence had to show extensive and virtually uniform practice, including the practice of states whose interests are specially affected, and that this practice demonstrated a general recognition of a legal obligation. In the North Sea case, the Court found that the conditions were not met, but it also affirmed that custom could in principle develop quickly under the right circumstances.
Practical Implications: Treaty Practice, Custom, and the Role of Dissent
The North Sea Continental Shelf decision illustrates how treaties can influence the formation of customary rules and how practice over time contributes to the emergence of customary norms. It shows that a treaty’s text may reflect existing customary practice, but a treaty can also catalyze the crystallization of custom through the subsequent state practice surrounding negotiations, signing, and adherence. The decision also highlights the role of dissent and the persistent objector rule, reinforcing that a small group of states may oppose a developing norm while the majority moves toward customary status.
Connections to Previous Lectures and Real-World Relevance
The discussion ties back to earlier lectures on the nature of peremptory norms (jus cogens) and the interplay between treaties and custom. It also relates to practical governance questions, such as how states negotiate boundaries, resource rights, and investment rules, and how those negotiated rules may become customary over time. The ICJ’s approach to customary law—focusing on representative practice, opinio juris, and the caveat that practice need not be perfectly conforming—offers a framework for evaluating evolving norms in contemporary international law and for assessing what counts as customary when states negotiate new treaties or revise old ones.
Summary of Key Points and Takeaways
Customary international law rests on two elements: (i) general state practice (consistent and representative), and (ii) opinio juris (belief that practice is legally obligatory).
Absolute conformity is not required for a rule to become customary; some opposition from states does not automatically prevent the emergence of customary norms.
The persistent objector rule provides an opt-out mechanism for dissenting states, subject to meeting its requirements.
“Specially affected” vs. “representative” practice guides the assessment of when a practice becomes customary, with particular emphasis on states directly affected by a rule (e.g., coastal states in continental shelf cases).
Regional and local customs exist but must be analyzed within their specific contexts; global norms are common but not exclusive.
The relationship between treaties and custom is dynamic: treaty practice can crystallize into custom, and custom can influence treaty interpretation and subsequent negotiations.
The North Sea Continental Shelf cases illustrate how a rule can potentially develop quickly into custom, but require extensive, virtually uniform practice and recognition of legal obligation; the 1958 Geneva Convention did not alone crystallize equidistance as customary by 1969 in that case, though the possibility of quicker development remains valid in principle.
The “midwife” metaphor underscores the role of treaty negotiations as catalysts in the birth of customary rules, rather than as the sole source of those rules.
Numerical and textual references used in this note include: the Geneva Convention on the Continental Shelf (1958) with Articles 6(1) and 6(2), the period from 1958 to 1969 (just over 11 years), the existence of approximately 40 landlocked states, and the general principle that state practice needs to be extensive and virtually uniform to give rise to a binding customary rule.