Determining Customary International Law: The ICJ’s Methodology between Induction, Deduction and Assertion

Methodology is probably not the strong point of the International Court of Justice (ICJ) or, indeed, of international law in general. Unlike its approach to methods of treaty interpretation, the ICJ has hardly ever stated its methodology for determining the existence, content and scope of the rules of customary international law that it applies. There are only isolated references in the ICJ’s jurisprudence to the inductive and deductive method of law determination. In the Gulf of Maine case, a Chamber of the Court stated that ‘customary international law […] comprises a set of customary rules whose presence in the opinio juris of States can be tested by induction based on the analysis of a sufficiently extensive and convincing practice and not by deduction from preconceived ideas’ ([1984] ICJ Rep 246 [111]). The use of the words ‘can be’, rather than ‘is’, implies that customary international law rules can also be discovered deductively. That deduction is part of the ICJ’s methodological arsenal is demonstrated by the fact that in the North Sea Continental Shelf cases five judges used the deductive method in their separate or dissenting opinions. For example, Judge Tanaka stated that ‘[i]n the event that the customary law character of the principle of equidistance cannot be proved, there exists another reason which seems more cogent for recognizing this character. That is the deduction of the necessity of this principle from the fundamental concept of the continental shelf’ ([1969] ICJ Rep 179). In the ICJ’s more recent jurisprudence, the Arrest Warrant case is widely seen an example of deductive reasoning, while the Jurisdictional Immunities of the State case is regarded as a prime example of the Court using the inductive method.

It is not only the ICJ itself that has largely remained silent on its methodology for the determination of customary international law. The legal literature has also had little to say on this subject. The great debate in the 1960s between Georg Schwarzenberger and Wilfred Jenks over the right method in international adjudication remains an isolated incident. [See Jenks, The Prospects of International Adjudication (1964), at 617–662 (‘Inductive and Deductive Reasoning in International Adjudication’) & Georg Schwarzenberger, The Inductive Approach to International Law (1965), at 115–164 (‘The Inductive Approach Refuted?’)]  Jenks saw in Schwarzenberger’s inductive approach to international law ‘a challenge to creative jurisprudence’, while, for Schwarzenberger, the deductive method was nothing more than ‘judicial legislation’ in disguise. In view of the fact that determining the law always also means developing and, ultimately, creating the law, it is surprising that the question of the ICJ’s methodology has attracted such little interest.

The article aims to refocus attention on the methodology used by the ICJ when determining the rules of customary international law that it applies, and to highlight the role played by methodology in the development of customary international law. It starts by defining the terms ‘induction’ and ‘deduction’ and examining their use by the ICJ. It then explores the situations in which the ICJ uses inductive and deductive reasoning, the different forms and functions of deduction and the relationship between the two methods. The article challenges the various theories distinguishing between inductive and deductive custom and demonstrates that the main method employed by the ICJ is neither induction nor deduction but, rather, assertion.

Over the years, the Court has pulled a number of customary international law ‘rabbits’ out of its hat. The article shows that in the large majority of cases the Court does not offer any (inductive or deductive) reasoning but simply asserts the law as it sees fit. Of course, one would not expect the Court to examine State practice and opinio juris, or employ a process of deductive reasoning, in order to establish long and well-recognized rules of customary international law, such as the inviolability of diplomatic agents. After all, the Court’s judgments are not papers in a student law review where every statement must be supported by a reference. However, the Court’s assertions have not been limited to what may be regarded as notorious custom. The Court has not only asserted positive rules, but it has also denied the existence of (alleged) rules of customary international law simply by stating that there is no ‘uniform and widespread State practice’ without providing any support for this assertion.

The article’s main findings can be summarized in the following 15 propositions:

1. The ICJ rarely explicitly states its methodology for determining the rules of international law.
2. Induction and deduction are epistemological methods used by the ICJ to reach certain conclusions in the process of identifying existing rules of customary international law.
3. The ICJ’s methodology for identifying rules of customary international law must be distinguished from the elements and evidence of those rules.
4. Customary international law is, as a rule, ascertained by inductive reasoning.
5. The inductive method finds its limits in the requirements of customary international law.
6. The ICJ resorts to deductive reasoning in order to avoid a non liquet.
7. The ICJ does not employ one single deductive method but resorts to normative, functional and analogical deduction.
8. Deductive reasoning is not limited to identifying rules of customary international law but may also be used to confirm the results reached by induction, replace or lower the standard of inductive evidence or establish a burden of proof necessary for the inductive method to reach a result.
9. Induction and deduction are not two competing or opposing monolithic analytical methods but, in practice, are intermixed.
10. The ICJ does not distinguish between ‘traditional custom’ identified through an inductive process and ‘modern custom’ derived by a deductive process.
11. The use of the two methods is neither traditional nor modern in a chronological sense but has been consistent throughout the ICJ’s jurisprudence.
12. The ICJ’s jurisprudence does not support a special category of ‘deductive customary law’ that is deduced from the constitutional foundations of the international community.
13. The choice of methodology may result in differing outcomes, but there is no evidence that the ICJ deliberately chooses a certain method to produce a predetermined result.
14. The inductive method is as subjective, unpredictable and prone to law creation by the ICJ as the deductive method.
15. The main method employed by the ICJ is not induction or deduction but assertion.The gateway for judicial legislation is thus neither induction nor deduction but, rather, the ICJ’s use of assertion as a method to determine rules of customary international law. No matter what it may say on the subject, in a majority of cases the ICJ has not examined the practice and opinio juris of States but, instead, has simply asserted the rules that it applies. Even its own members have criticized this methodological approach. However, assertion is not always merely a convenient methodological shortcut. There are situations where the inductive and deductive methods will not allow the Court to fulfil its normal judicial function of determining the applicable rules of customary international law because induction is virtually impossible or because there are no relevant general rules or principles from which to deduce the applicable law. Judicial assertion is the price States have to pay for the ICJ not to declare an epistemological non liquet. It must be careful, however, not to overstep the limits of the method of assertion. If the ICJ’s assertions do not convince its clients, States may simply stay away from the Court.