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The literature on the identification of rules of customary international law is extensive. Commentators have focused on isolating the methodologies by which international courts and tribunals identify customary international law, with most of the debate revolving around the use of induction, or deduction and assertion as methods of custom identification. However, the existing literature has overlooked that the choice among custom identification methodologies takes place behind closed doors, during confidential deliberation processes. When all that scholars see may be deduction or assertion, international courts and tribunals may have ascertained the existence of customary rules by induction, but induction may not have made it into the final text of the decision. This article elaborates on the impact of judicial deliberations at the International Court of Justice on the choice among custom identification methodologies. It argues that individual-driven stages of deliberations favour custom identification by induction, while collegial stages promote custom identification by non-inductive methodologies.

In The Rome Statute as Evidence of Customary International Law, Yudan Tan offers a detailed analysis of topical issues concerning the Rome Statute of the International Criminal Court as evidence of customary international law.

Recent decades have witnessed an impressive process of normative development in international law. Numerous new treaties have been concluded, at global and regional levels, establishing far-reaching international legal and regulatory regimes in important areas such as human rights, international trade, environmental protection, criminal law, intellectual property, and more. New political and judicial institutions have been established to develop, apply and adjudicate these rules. This trend has been accompanied by the growing consolidation of treaty norms into international custom, and increased references to international law in domestic settings. As a result of these developments, international relations have now reached an unprecedented level of normative density and intensity, but they have also given rise to the phenomenon of ‘fragmentation’. The debate over the fragmentation of international law has largely focused on conflicts: conflicts of norms and conflicts of authority. However, the same developments that have given rise to greater conflict and contradiction in international law, have also produced a growing amount of normative equivalence between rules in different fields of international law. New treaty rules often echo existing international customary norms. Regional arrangements reinforce undertakings that already exist at the global level; and common concerns and solutions appear in many international legal fields. This book focuses on such instances of normative parallelism, developing the concept of ‘multisourced equivalent norms’ in international law, with contributions by leading international law experts exploring the legal and political implications of the concept in a variety of contexts that span the full spectrum of international legal norms and institutions. By concentrating on situations governed by a multitude of similar norms, the book emphasizes the importance of legal contexts and institutional settings to international law-interpretation and application.

This article discusses the relationship between particular and general customary international law, grappling with academic views affirming that, ordinarily, the emergence of the former is a stage in the consolidation of the latter. It is argued that the higher standard of State consent required for the configuration of bilateral or regional custom suggests otherwise. In addition, it is also contended that a distinctive kind of opinio juris must be present for particular custom to arise: a conviction from the States concerned that their conduct is governed by particular (as opposed to general) customary law.

International legal scholars have long recognized the importance of the rules and processes by which states adhere to international legal obligations and “translate” them into their domestic legal systems. Research by political scientists on specific issue areas likewise increasingly recognizes that domestic implementation is crucial to international law compliance and effectiveness. Yet the lack of systematic data makes it difficult to assemble an overall picture of the relationship between international law and domestic law around the world, let alone to document its evolution over time. Recent qualitative surveys of state practice have begun to fill that gap, but provide only a snapshot in time and are limited to relatively few countries. Some quantitative projects cover more countries, but address only a limited number of questions based solely on the text of national constitutions.

The International Committee of the Red Cross's study of Customary International Humanitarian Law by Jean-Marie Henckaerts and Louise Doswald-Beck (Cambridge University Press, 2005) contains a unique collection of evidence of the practice of States and non-State actors in the field of international humanitarian law, together with the authors' assessment of that practice and their compilation of rules of customary law based on that assessment. The study invites comment on its compilation of rules. Perspectives on the ICRC Study on Customary International Humanitarian Law was originally published in 2007, and results from a year-long examination of the study by a group of military lawyers, academics and practitioners, all with experience in international humanitarian law. The book discusses the study, its methodology and its rules and provides a critical analysis of them. It adds its own contribution to scholarship on the interpretation and application of international humanitarian law.

Can a few primarily Western States expand the right to self-defence against non-State actors, incorporating the unwilling or unable standard? Even on a traditional reading of customary law formation, the answer is no because proponents have failed to attract consistent and widespread support. What is more, using our interactional international law approach, we show that efforts to date have not been successful because they have failed to address fundamental rule of law concerns. The current state of world politics has perhaps caught proponents of the unwilling or unable standard in a difficult bind. We suggest how proponents might carefully develop the law on self-defence against non-State actors.

Practice shows the existence of complex legal situations in which customary international rules applicable to the Member States interfere, even indirectly, with the competences of the Union, and vice versa. On the one hand, the implementation of a rule of customary international law by Member States could affect rights and obligations under EU law. On the other hand, the exercise of EU competences could affect the rights and obligations conferred on Member States by customary law. In these situations, the Union must reconcile two “equal and opposite” needs. On the one hand, it must ensure that Member States’ exercise of rights and obligations under customary international law does not undermine the effectiveness of EU law. On the other hand, it must prevent EU competences from interfering with the rules of customary international law applicable to the Member States. This Article aims to explore how the Union reconciles the exercise of EU competences with the exercise of Member States’ competences under customary international law. After examining the most prominent models that could theoretically be used to coordinate the two spheres of competence (section II), the attention will turn to the approach adopted by the ECJ (section III) to determine whether this approach affects the prerogatives of the EU Member States as sovereign states under international law (section IV).

This article argues that the International Court of Justice’s (ICJ) account of the customary law of environmental impact assessment (EIA) is incomplete. While acknowledging the role of the harm prevention principle in formulating the customary obligation to conduct EIAs, the ICJ has ignored the duty to cooperate, notwithstanding the latter duty’s equally strong standing in international environmental law. Ignoring the duty to cooperate pushes the court towards a formal and sequential understanding of EIA, which undervalues the centrality of notice and consultation in EIA. In effect, viewed through the harm prevention lens alone, EIA is largely understood in instrumental and technical terms; whereas, if the duty to cooperate is brought back in, EIA’s deliberative and ‘other-regarding’ nature is more clearly seen. This, in turn, recognises the normative and political role of EIA in structuring State interactions respecting environmental disputes.

Much scholarship on customary international law has examined the merits of induction, deduction, and assertion as approaches to custom identification. Save for where international tribunals identify custom by assertion, writers have viewed custom identification that does not rely on evidence of State practice and opinio juris as an example of deductive reasoning. However, writers have stated that, at best, deduction is reasoning from the general to the particular. This article draws on legal philosophy to define the contours of deductive reasoning and argues that pure deduction, namely deduction not combined with other forms of reasoning, is an unsound approach to custom identification. This argument is tested by reference to cases of custom identification by the International Court of Justice, categorised according to three types of deduction: normative, functional, and analogical. This article also explores the authority and utility of custom identification by pure deduction and its impact on content determination.