Throw Custom to the Wind: Examining the Life Cycle of Customary International Law in the Absence of a Custom-Making Moment

Doctrinal Indeterminacy and Customary International Law

Customary International Law (CIL) has been the victim of its own existential fluidity. From the duality paradox over its elements to dubious questions over its purpose as a source of international law, CIL has become a theoretic quicksilver. It has escaped clear-cut definition even in spite of the International Law Commission’s herculean attempt at charting out its draft conclusions on CIL identification, which themselves produced more questions than were answers, especially concerning the role of International Organizations’ practice (especially 18–20) and, hence, the potential uneasy relationship between draft conclusions 4.2 and 12.2. While some consensus has admittedly been reached, the discussion on CIL itself is a staple of much international legal scepticism and criticism.

One can easily place Jean d’Aspremont’s recent contribution to the debate on CIL within this critical context. During the recent TRICI-Law conference and elsewhere more broadly, d’Aspremont continued his previous criticism of modern CIL doctrine (generally expressed here and here) by claiming that there is no such thing as a ‘custom-making moment’ in the CIL life cycle. While this contestation is not altogether new, its implications for the way we view, interpret and apply CIL are material. In this post, d’Aspremont’s claim, as well as will be first examined by putting attention to two arguments he presents. Subsequently, the overall implications drawn from the revision of the CIL life cycle will be considered. Finally, the case will be made for maintaining the orthodox understanding of the life cycle and why it remains both true and useful despite its critics’ doubts.

CIL Doctrine and The Sceptic’s Apologia

The CIL life cycle, as presently understood following the ICJ’s laconic indications in The Gulf of Maine (para 111) and North Sea Continental Shelf (Germany/Netherlands) (para 37) cases, begins with the process of CIL formation entails the combination of objective ‘practice’ and subjective belief (or desire) that the rule in question is legally binding (‘opinio juris’) in light of article 38(1)(b) ICJ Statute. Consequently, this is identified on the principle but not necessarily exclusive basis of inductive reasoning, which Talmon defines as ‘inference of a general rule from a pattern of empirically observable individual instances of State practice and opinio juris’. Subsequently, rules must be interpreted by international tribunals through a deductive reasoning process; otherwise, as Merkouris makes clear, courts and tribunals risk needing to either re-identify CIL every time when applying it, or assert its content without analysing its respective development. Indeed, subsequent to its initial identification and interpretation, the content of CIL may be modified, which requires the task of re-interpreting these changes.

In arguing against the existence of a custom-making moment, d’Aspremont’s argument is generally premised around the proposed ‘social theory of sources’, which entails ‘that practice of law-applying authorities, and not the will of States, is what nourishes the sources of international law’. On this basis, d’Aspremont points out (p 15–16) that the ascertainment of a source ‘produces a static snapshot of rules existing at the moment of cognition’. While the result is ‘by definition static’, the process of ascertainment is itself subject to ‘evolution and changes’, particularly ‘the law-applier’s agendas’ and the process by which ‘observers’ elect law-appliers. Consequently, this ‘social practice’ has implications for the criteria used to identify the sources of international law.

Second, the ‘law-ascertainment’ process is itself ‘an interpretative process’ (p 16), which requires the rules for identification to themselves be subject to a hermeneutic process that is itself distinct from ‘content-determination interpretation’. Indeed, in arguing this claim, depending on the degree of flexibility and the points of attention, the very ‘rulelessness’ of the sources of international law entails that ascertaining CIL entirely depends upon the law-applier and not the evidence independent of it. Since this interpretative act is only restricted by social factors, this gives the interpreter power to define what is and what is not international law, subject to communitarian restrictions (p 20–28).

While the CIL doctrine maintains that its formation (the ‘custom-making moment’) is the result of the two-element criteria found in article being met and that international courts and tribunals have the role of identifying, interpreting and applying it, the social theory of sources promoted by d’Aspremont differs significantly. It suggests that law-appliers and observers are shaped by social factors, namely communitarian restrictions, when identifying CIL. This selectiveness ultimately makes us question whether the processes of CIL formation and identification are indeed separate. As such, the ‘custom-making moment’ is neither logically necessary, nor consistent with the recast theory of international law sources

Consequences and Criticisms

As a thought experiment, the suspension of belief in the CIL life cycle reinvigorates our criticism of what data constitutes practice and opinio juris, the power relations illustrated by the assumption of tacit consent to the formation of CIL, and even the role of international courts beyond mere passive agents. However, taking this reasoning to its logical extreme exceeds what is logically necessary. Courts and tribunals do serve an important role in deducing CIL and, subsequently, interpreting and applying it. By rejecting the classic CIL life cycle, however, we risk not only denying the very ontology of CIL as being based upon the strict belief in a norm’s legal binding-ness and overemphasising the roles of courts; rather, we begin viewing the very process of induction as being based on courts’ dubious selection of CIL-affirming practice and, resultantly, not as a judicial process. While there may be some grain of truth, d’Aspremont’s claim ultimately presents international courts as creative forces, which are themselves conditioned by their restrictions.

The life of CIL is rightly in a state of constant development, a symptom of the dynamism that characterises international relations. However, while the identification process can be challenged by some given the limited evidence used by courts and tribunals, it would be impossible to envisage identification without a point when ‘social reality [transforms] into a posited rule having the power of law’. Consequently, we are left with three options: to accept that international courts are CIL-makers and CIL identification is synonymous with creation; that an actual custom-making moment exists that is distinct from identification, or a combination of both. While the first scenario is evidently unrealistic because it envisages law-appliers as sole CIL-makers, which is questionable for its complete disregard of their constraints (e.g., Prosecutor v Rutaganda, para 86), the remaining two deserve consideration.

The chief ground for accepting the last scenario, that identification partially overlaps with creation, is that law-ascertaining actors, especially the ICJ, can ‘assert’ CIL and its content when identifying it. Omri Sender and Michael Wood dispute this and claim ‘assertion’ is not a ‘methodology for determining the existence of a rule of [CIL]’, but rather ‘a way of ‘stating a [familiar] conclusion’. A more potent argument here may be that an assertion does not in itself induce States’ compliance; consequently, it cannot be norm-creating since it does not transform practice into rules. While the two may indeed sometimes overlap temporally, they are not materially the same.

Finally, without the full CIL life cycle as previously envisaged, it would be impossible to properly apply a consistent interpretative process. By removing the presently envisaged custom-making moment from the framework, we are theoretically left without a stable and consistent general ontological understanding of CIL’s operation, their form and what impact they have. While rightly d’Aspremont maintains that a distinction between ‘law-ascertainment interpretation’ and the interpretation of CIL content exists, it is difficult to imagine the latter becoming a systematised process. If CIL formation is a construct of law-appliers’ agendas, albeit themselves restricted, then a clear methodology or subset of principles for CIL interpretation would be impossible to objectively maintain.


Ultimately, too much hinges on the custom-making moment, the transformation of the two components into positive law, be it a single instant or a stage that goes undetected until international courts and tribunals are required to identify it. While d’Aspremont provokes us to revisit the development of CIL and offers an alternative perspective, we should still accept that there is a moment when the accumulation of practice and belief in it gain normative standing within international law and thus undergo interpretation, codification or modification. While, like interpretation, identification is itself a rigorous art, it is burdened with two important caveats: that it is based on established on the basis of consistent, norm-creating past behaviour and restricted by induction. Even though the formalist doctrine of CIL is susceptible to error and the social reality is itself an important consideration, there is no logical necessity to throw away the baby with the bathwater.