During its sixty-fourth session, in 2012, the International Law Commission (ILC) decided to place the topic “Formation and evidence of customary international law” on its current programme of work, and appointed Michael Wood as Special Rapporteur. In his first report, Michael Wood outlined his proposed contribution. He discussed earlier work of the ILC on the topic and identified the aim of the report, inter-alia, as a source of “…guidance to those called upon to apply rules of customary international law on how to identify such rules in concrete cases” and to describe the current state of international law on the formation and evidence of rules of customary international law, without prejudice to developments that might occur in the future. He discussed succinctly customary international law’s relationship to other sources listed in Article 38 of the Statute of the International Court of Justice, including treaties and general principles of international law. An interesting aspect is that he disputes arguments that treaties may be “a more important source of international law” than custom. He argues that “such generalizations are neither particularly illuminating nor necessarily accurate.” He argues that,
“Even in fields where there are widely accepted “codification” conventions, the rules of customary international law continue to govern questions not regulated by the conventions and continue to apply in relations with and between non-parties. Rules of customary international law may also fill possible lacunae in treaties, and assist in their interpretation. An international court may also decide that it may apply customary international law where a particular treaty cannot be applied because of limits on its jurisdiction (for example in the Nicaragua case).”
While the above mentioned aspects, including various terminology used to describe customary international law, its constituent elements and attitude of States would be expanded in successive reports, Wood gives an overview of various jurisprudence of the ICJ and other international, regional and national courts and tribunals on the formation of customary international law. Since we look at some of the ICJ’s jurisprudence in our class, I extracted the section(paras 54 – 65) on the ICJ’s practice in deducing customary law below (minus the footnotes, which are also informative).
The full report can be accessed here. Students are strongly encouraged to read pp 21 -41.
54. The case law of the International Court of Justice and its predecessor, the Permanent Court of International Justice, will be of great significance for the Commission’s work on the present topic. The Court’s primary function in relation to customary international law is to identify and apply customary rules as necessary for deciding the cases before it.Its judgments (including separate and dissenting opinions) shed much light on the general approach to the formation and evidence of customary international law (when “[w]hat ‘is’ becomes what ‘must be’”), including on specific aspects of these processes.
55. Examining the Court’s frequent application of Article 38.1 (b) of its Statute, by which it “perform[s] its perfectly normal function of assessing the various elements of State practice and legal opinion adduced … as indicating the development of a rule of customary law”, affords an overview of the Court’s approach to the matter. As the judgments referred to below indicate, the Court has clearly and consistently held — as did its predecessor — that customary international law is formed through State practice accompanied by opinio juris.
56. In the Lotus case, the Permanent Court of International Justice stated that international law emanates from the free will of States as expressed in conventions or “by usages generally accepted as expressing principles of law”. It emphasized the distinction between the two constitutive elements of customary international law, stressing the need for both to be present in order to ground a finding of such law:
“Even if the rarity of the judicial decisions to be found among the reported cases were sufficient to prove in point of fact the circumstances … it would merely show that States had often, in practice, abstained from instituting criminal proceedings, and not that they recognised themselves as being obliged to do so; for only if such abstentions were based on their being conscious of having a duty to abstain, would it be possible to speak of an international custom.”
57. The classic statement of the International Court of Justice on the processes of formation and evidence of rules of customary international law is to be found in the North Sea Continental Shelf cases:
“Although 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 on the basis of what was originally a purely conventional rule, an indispensable requirement would be that within the period in question, short though it might be, State practice, including that of States whose interests are specially affected, should have been both extensive and virtually uniform in the sense of the provision invoked; and should moreover have occurred in such a way as to show a general recognition that a rule of law or legal obligation is involved.
… The essential point in this connection — and it seems necessary to stress it — is that even if these instances of action by non-parties to the Convention were much more numerous than they in fact are, they would not, even in the aggregate, suffice in themselves to constitute the opinio juris; — for in order to achieve this result, two conditions must be fulfilled. Not only must the acts concerned amount to a settled practice [une pratique constante, in the French text], but they must also be such or be carried out in such a way as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it. The need for such a belief i.e. the existence of a subjective element, is implicit in the very notion of the opinio juris sive necessitatis. The States concerned must therefore feel that they are conforming to what amounts to a legal obligation. The frequency, or even habitual character of the acts is not in itself enough. There are many international acts, e.g. in the field of ceremonial and protocol, which are performed almost invariably, but which are motivated only by considerations of courtesy, convenience or tradition, and not by any legal sense of duty.”
58. The Court reaffirmed this in Military and Paramilitary Activities in and against Nicaragua, where it said that in order to consider what rules of customary international law were applicable it “has to direct its attention to the practice and opinio juris of States”, and that:
“… as was observed in the North Sea Continental Shelf cases, for a new customary rule to be formed, not only must the acts concerned ‘amount to a settled practice’ but they must be accompanied by the opinio juris sive necessitatis. Either the States taking such action or other States in a position to react to it, must have behaved so that their conduct is ‘evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it. The need for such a belief, i.e. the existence of a subjective element, is implicit in the very notion of opinio juris sive necessitatis’.”
59. In its judgment in Continental Shelf (Tunisia/Libyan Arab Jamahiriya) the Court referred to “the actual practice of States” as “expressive, or creative, of customary rules”. In the Gulf of Maine case, a Chamber of the Court observed that customary international law “comprises a limited set of norms for ensuring the co-existence and vital co-operation of the members of the international community, together with 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”.
60. When turning to an examination of customary international law in its Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons, the Court said at the outset that “[a]s the Court has stated, the substance of that law must be ‘looked for in the actual practice and opinio juris of States’ (Continental Shelf (Libyan Arab Jamahiriya/Malta), Judgment, I.C.J. Reports 1985, pp. 29-30, para. 27)”. Later in the Opinion it noted the existence of customary rules that “have been developed by the practice of States”.
61. The most recent extended pronouncement of the Court on its basic approach is to be found in Germany v. Italy, in which it said:
“It follows that the Court must determine, in accordance with Article 38 (1) (b) of its Statute, the existence of ‘international custom, as evidence of a general practice accepted as law’ … To do so, it must apply the criteria which it has repeatedly laid down for identifying a rule of customary international law. In particular, as the Court made clear in the North Sea Continental Shelf cases, the existence of a rule of customary international law requires that there be ‘a settled practice’ together with opinio juris …
Moreover, as the Court has also observed,
“It is of course axiomatic that the material of customary international law is to be looked for primarily in the actual practice and opinio juris of States, even though multilateral conventions may have an important role to play in recording and defining rules deriving from custom, or indeed in developing them. (Continental Shelf (Libyan Arab Jamahiriya/Malta), Judgment, I.C.J. Reports 1985, pp. 29-30, para. 27).”
62. At the risk of oversimplification, it may be said that there are two main approaches to the identification of particular rules of customary international law in the case law of the Court. In some cases the Court finds that a rule of customary international law exists (or does not exist) without detailed analysis.This may be because the matter is considered obvious (for example, because it is based on a previous finding of the Court or on what the Court views as unquestioned law). A number of examples may be found in the Court’s judgment of 19 November 2012 in Nicaragua v. Colombia (see footnote). In other cases the Court engages in a more detailed analysis of State practice and opinio juris in order to determine the existence or otherwise of a rule of customary international law. The Court’s judgment of 3 February 2012 in Germany v. Italy illustrates this approach. It is particularly these latter cases that are helpful in illustrating the Court’s approach to the formation and evidence of customary international law.
63. There is a considerable number of cases in which the Court has addressed specific aspects of the process of formation and identification of rules of customary international law, covering many of the issues that arise under the present topic, chief among them the nature of the State practice and opinio juris elements, and the relationship between treaties and customary international law. While such cases do not provide complete answers, they offer valuable guidance…
64. It is widely recognized in the literature that the International Court, through its jurisprudence, has enhanced the role of customary international law and clarified some of its aspects. At the same time, commentators have suggested that the Court has thus far provided only limited guidance on how a rule of customary international law is formed and is to be ascertained, having “a marked tendency to assert the existence of a customary rule more than to prove it”, and ultimately following a rather flexible approach. It has moreover been observed that the Court has not always been consistent in its use of terminology relating to customary international law, or in distinguishing the latter from general principles of law.
65. The President of the International Court of Justice, addressing the issue of the Court’s approach to customary international law, has recently explained:
“… authors are correct in drawing attention to the prevalent use of general statements of rules in the Court’s modern practice, although they take the point too far by insisting on theorizing this development. In fact, the Court has never abandoned its view, firmly rooted in the wording of the Statute, that customary international law is ‘general practice accepted as law’ — that is, in the words of a recent case, that ‘the existence of a rule of customary international law requires that there be a ‘settled practice’ together with opinio juris’. However, in practice, the Court has never found it necessary to undertake such an inquiry for every rule claimed to be customary in a particular case and instead has made use of the best and most expedient evidence available to determine whether a customary rule of this sort exists. Sometimes this entails a direct review of the material elements of custom on their own, while more often it will be sufficient to look to the considered views expressed by States and bodies like the International Law Commission as to whether a rule of customary law exists and what its content is, or at least to use rules that are clearly formulated in a written expression as a focal point to frame and guide an inquiry into the material elements of custom.”
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