Customary norms have historically governed societies both nationally and internationally long before the emergence of legislators and solid legal institutions. From tribal organisation to mercantile practices, custom has operated as a binding source of order, dispute resolution, and social regulation.
Customary norms and Law – John Austin and afterwards
In the 19th century, John Austin, whose legal positivism has long shaped common law jurisprudence, claimed that only explicit or tacit command of a sovereign or a court to follow a customary rule makes such a rule part of the legal system (Austin, 1832: 28). His ‘high positivism’ believed that custom as a ‘bottom-up’ law was an oxymoron (Bederman, 2010: 169-70). Oliver Wendell Holmes also claimed that courts turn part of custom into law by refining coercive commands out of it. Custom, then, is merely the basis for law, but no part of custom can stand as law on its own (Hadjigeorgiou, 2020: 53).
A bit similar position was later expressed by Raz, who claimed that unless a custom is formally endorsed by the legislator or a court, it is not law, but a moral standard (Lefkowitz, 2005: 409).
Some scholars disagreed with these statements. For instance, Jeremy Bentham, even though being an inspiration for Austin, admitted the prescriptive role of custom where it raises legitimate expectations of compliance (Murphy, 2014: 84; Bentham, 1828: 231-51).
Although H. L. A. Hart’s position on customary law still has not been clearly understood, while disagreeing with Austin’s theory observed that not every norm needs the sovereign’s express imprimatur to be law. Rather, many laws confer powers on private individuals to make their own rules, with the advance expectation of official enforcement” (Bederman, 2010: 123; Hart, 1976: 26).
Customary norms and International Law
Does the picture change when we consider the question within the context of international law?
If we look at the application of Austin’s claim to customary international law, it is possible to present several arguments which aim to show that Austin’s theory cannot be applied on customary international law without considering peculiarities of international law.
We can follow this line of argumentation:
1) To claim that the intervention of a court is needed creates a certain paradox when confronted with the normative reality of contemporary legal systems.
If the intervention of a court is what constitutes a norm belonging to the system, then a norm that the court applies is not part of the system. This means that, in light of contemporary normative systems, around like one-half or even more of the norms in contemporary systems would not be part of them.
If we look at international law, the situation becomes even more concerning. International humanitarian law, international law of the sea, international environmental law and even international human rights law were originally created by customs. At the same time, not all these areas of international law are purely positive nowadays. Some of them continue to exist with a significant prevalence of general customary norms regulating States’ behaviour.
2) This paradox leads to the conclusion that normative effectiveness implies a loss of validity.
Normative effectiveness means that the norm is followed by the agents. If the norm is followed by the agents, there are no disputes. If there are no disputes, the courts do not intervene. And therefore, since the courts do not intervene, and their intervention was constitutive, normative effectiveness implies invalidity. It seems redundant and is exactly the opposite of what everyone has backed for centuries.
Besides, international law was originally created and developed through customs, as I have already mentioned above. Historically, jus gentium was even equalised with consuetudo by some authors (e.g., Francisco Suárez). International law has gone through at least 500 years of customs. Empirically speaking, it would even be impossible for courts to confirm the existence of tens of thousands of customs in international law. Doing approximate math, this basically means that the international community needed at least several thousand litigations. Poor PCIJ and later ICJ, together with some international arbitrations that would have to deal with that.
3) The norms do not exist in isolation, that is, for you to recognise the norm as a norm, you need to integrate it into a system.
The conviction of bindingness (opinio juris) is a proof that the norm belongs to the system. Additionally, the practice of states demonstrates which sources are acknowledged as giving rise to rules having the force of law.
4) Building up on what has been said in my previous three arguments, we can look at two types of customs suggested by Bentham: in pays and in foro.
Bentham claimed that the former is shared and practiced by a group of private individuals, while the latter is shared and practiced by public officials. However, such understanding in international law is impossible since there are neither ‘private’ individuals nor ‘public officials’ per se, but there are states that cover both fronts. Thus, the understanding of these two customs in international law should be adjusted to better accommodate the differences between international law and the national one. In this way, customs in pays could be considered as the practice of subjects of international law, namely states, and customs in foro are applied in the practice of the courts.
For some reason, the large majority of existing theories of CIL on the sources of law are largely modelled after a form of custom in foro. As such, those theories seek to establish the existence, scope and content of rules on sources of law without due consideration of any relevant general practice of states and the conviction of bindingness but based on the decisions of PCIJ or ICJ (Mejía-Lemos, 2022: 157-8).
By contrast to the national system, in international law it should be custom in pays that by accumulating state practice and opinio juris creates a norm that should be considered the source of law that regulates States’ behaviour. Based on this norm, they engage in law-making or in law-identification which eventually can also be used in connection with dispute settlement proceedings.
A final remark
Summarising the abovementioned, I should stress once again that customary international law is not merely a shadow of judicial authority – it is a living, autonomous source of international law. While courts do help clarify some of the norms of customary international law, their intervention is not decisive for the pertaining of a custom to international legal system. Practice, opinio juris and recognition are the true engines of its normativity.
This work was realized under a PhD Scholarship granted by the Portuguese national funds through the FCT – Fundação para a Ciência e a Tecnologia de Portugal.
The work on this blogpost is the result of the presentation done at the ALF young scholar conference organised within the Horizon Twinning project “Advancing Cooperation on The Foundations of Law – ALF” (project no. 101079177) financed by the European Union.
SUGGESTED CITATION: Kaplina, Vladyslava: “Who Needs a Judge? The Case for Authority of Customary International Law”, FOLBlog, 2025/12/08, https://fol.ius.bg.ac.rs/2025/12/08/who-needs-a-judge-the-case-for-authority-of-customary-international-law/