Some Reflections on Identifying Custom in Contemporary International Law

Custom has long been considered as the main, if not the exclusive, source of general international law. Since the earliest doctrinal reflections, custom has been conceived as an unconscious and involuntary way of law making that derives its effectiveness from the force of tradition. This is a basic definition common to all periods and all areas of law, regardless of the legal system involved in the phenomenon in question. In the course of the 20^th century, international law scholars have mostly discussed the monistic or dualistic nature of customary norms, the latter becoming the dominant vision. Indeed, it is possible to say that the almost totally prevailing view is that these facts are to be grouped in two elements, i.e. an objective one, the repeated behaviour of States (diuturnitas), and a subjective one, the belief that such behaviour depends on a legal obligation (opinio juris sive necessitatis). Indeed, widespread repetition of behaviours within a community is the necessary condition to distinguish an unwritten normative fact based on spontaneous practice (such as a custom) from a voluntary normative act, a unilateral decision, or a practice which could later be quickly abandoned. Moreover, without regular repetition and wide dissemination of the practice we could no longer discern opinio juris ac necessitatis as evidence for the conviction by the community as a whole that the usage in question is respected as a legal rule. In addition to the above, it should be noted that the identification of an international customary norm has never been a serious issue in the past. As pre-modern commentators made clear, custom is manifest in concrete actions by the members of a society. The claims made and resistance opposing them, in the context of a specific dispute would therefore be the most convincing evidence for the existence or non-existence of a customary norm. This is also consistent with the nature of custom as a normative fact, gradually asserting itself in the reality of social behaviour as a rule of conduct. According to this approach, statements or abstract normative acts that do not correspond to what happens in the reality of intersubjective relations cannot therefore be considered sufficient evidence. With particular regard to international law, courts have traditionally looked to such practice as the official positions of the executive, protests and reactions to the alleged breach of a customary obligation, and acquiescence to the claims of others. This concept of customary law never posed any particular problem when international law was only a homogeneous group of unwritten rules, regulating the power of States to exclusively govern a certain territory. Indeed, such rules are rooted in ancient practice dating back to the late Middle Ages and giving rise to the first principles of public law in Europe, namely the rules of territorial sovereignty, diplomatic immunity, and limitations to reprisals. Lastly, it should be noted that the majority of disputes concerning the existence and/or interpretation of these norms were resolved, until recently, through direct negotiations between States, with arbitration often limited to the fair calculation of damages... (segue)