The purpose of this chapter is to discuss the role of judicial dialogue between international courts in the interpretation of customary international human rights law. Judicial dialogue refers to international courts’ spontaneous practice of referencing other international courts’ decisions or international instruments that are outside the international court’s own judicial system. International courts engage in this practice in order to both identify rules of customary international human rights law and reach common interpretations on the meaning and scope of norms protecting human rights. Through the analysis of international courts’ case law, this chapter discusses the impact of judicial dialogue consisting in cross-references to legal norms and judicialdecisions on the interpretation of rules protecting human rights, especially when judges use case law from other courts in support of their interpretation.

Effectiveness and efficiency in judicial decision-making are the most important objectives of any court. While this concerns primarily the final decisions that are rendered, it is also relevant to the judicial process and the legal reasoning that a court or tribunal carries out to reach its decision, in order to ensure continuity and coherence. Traditional understandings of the international judiciary have seen the judges’ role as one where they discover and declare the law by applying it at face value to the legal issues that have arisen within the case, thereby achieving effectiveness through what is said to be direct and clear application of the law. This sits rather uneasily with the identification of customary international law (CIL), which is by its very nature unwritten and established by identifying evidence of state practice and opinio juris. The aim of this chapter is to examine instances of judicial activism in the decision-making of international courts and tribunals during the determination and application of CIL and how that allows for either judicial effectiveness or ambiguity.

The uncertain character of customary international law (CIL) has been discussed time and time again among academics and practitioners. To most of them, the uncertain character of CIL entails a twofold defect: first, uncertainty is perceived with respect to the identification of the rule, since we may not know whether there is a valid legal rule; and second, uncertainty is perceived with respect to the content of the rule, since we may not know the precise meaning of the rule. Yet, what seems to be missing from the entire discussion is the mechanism by which CIL functions. Although a number of theories have been formulated, there are no persuasive answers that would explain when and how changes in CIL occur. In other words, the dynamics of CIL, as the driving force behind its evolution remain essentially unexplored. Providing answers to these questions requires an in-depth understanding not only of the structure of CIL but also of the processes that occur in and out of CIL during its operation as a social system. This chapter uses complexity science to describe the functioning of CIL and explore CIL’s construction as a social system.

The interpretation of unwritten norms is fraught with difficulty, as the boundaries between the existence of a norm and the determination of its content can become blurred. Interpreters may return to the evidence of the norm’s existence in order to determine its content or it may be that interpretation itself is part of the constitutive process of unwritten norms. This confusion is exacerbated by a lack of established methods and procedures for the interpretation of unwritten international law, which includes not only custom but also general principles of law. While it is commonplace to speak of custom and general principles under the umbrella of ‘general international law’, it is unclear whether questions of interpretation are to be approached in the same manner for both categories of norms or whether custom and general principles may assist in the interpretation of one another. The central objective of this chapter is to examine the interactions between these two categories of norms in the context of interpretation. More specifically, it considers whether general principles of law may play a role in the interpretation of customary rules.

This chapter explores how the interpretation of customary international law (CIL) can be shaped by the underlying premises and political values of a system. The argument it develops focuses on how investor–state arbitration has interpreted the CIL law rule establishing that the actions of state-owned enterprises will be attributed to the controlling state, as expressed in Article 8 of the Articles on the Responsibility of States for Internationally Wrongful Acts.

In its case law the International Court of Justice has repeatedly suggested the idea that rules of customary international law (CIL) do not operate in a vacuum but, instead, are to be understood against the background of other rules of the international legal system. This observation, although somewhat unsurprising, shows that the sources of international law exist in close interconnection – something that is also visible if one looks at the rules of interpretation contained in Article 31 of the Vienna Convention on the Law of Treaties. Accordingly, ‘any relevant rules of international law applicable in the relations between the parties’ must be taken into account, together with the context, when interpreting treaty provisions. The question addressed in this chapter is whether or not the same can be said of the interpretation of customary rules. In other words, if we look at the practice of international courts and tribunals, is it possible to reach the conclusion that CIL rules, too, must be interpreted with the cognizance of any relevant rules of international law applicable between the parties?

The process of identifying and interpreting norms of customary international law, while appearing to be primarily based on an inductive analysis of state practice and opinio juris, is sometimes a deductive exercise based on logic and reason. Logic permeates every decision in international law. Logic manifests itself inherently throughout the process and can be identified in all steps of reasoning in identifying, interpreting and applying customary international law. Logic, however, can constitute the application of either an inductive or deductive inference. This chapter focuses on situations in which the International Court of Justice (ICJ) and the Permanent Court of International Justice (PCIJ) applied a deductive approach, identifying or interpreting norms of customary international law without seeming to consult state practice and opinio juris. Specifically, it considers whether norms that can be reasonably inferred or deduced from existing rules, or that are simply logical for the operation of the international legal system, can be identified as norms of customary international law under a complementary, supplementary or distinctive interpretive approach.

This chapter addresses the chronological paradox of customary international law (CIL). The paradox is that for a new customary rule to be created states must believe that the law already obligates the behaviour specified in that rule (opinio juris). However, the behaviour in question can only be legally required once that rule has been created. As a result, creating a new customary rule would be impossible, or at the very least an incoherent process. This chapter challenges this conclusion. In addressing the chronological paradox, it provides a coherent interpretation of the creation of new customary rules. It argues that the sense of legal obligation (opinio juris) emerges from the general principle of good faith. Good faith leads to legal obligations, which compel a subgroup of states to engage in specific behaviour. Then, as a result of this subgroup’s repeated behaviour, a new customary rule emerges, obligating the entire community of states to act accordingly. To explain the shift from good faith to legal obligations and from legal obligations to customary rules, the chapter draws on interpretivism, social ontology and contemporary research on constitutive rules.

When faced with the inevitable task of interpreting customary international law (CIL), what should a court do and what should it consider? Should it engage in an ‘inductive’ process of sifting through available evidence of state practice and opinio juris or a deductive process designed to reason logically from principles embedded in the rule? Should the court invoke something like the rules of treaty interpretation with their focus on good faith, ordinary meaning, context, and object and purpose? International law doctrine falls short here. Figuring out how to interpret and apply custom requires a theory of custom, a focus on the normative stories we tell. This reveals that there is not just one story explaining why custom should be a source of law, but multiple stories. What we call ‘custom’ may represent or draw from at least three different sources of law: Negotiated Law, Legislated Law and Adjudicated Law. This chapter aims to show that the non-treaty rules derived from each draw on different sources of legitimacy, operate according to different logics, dictate different methods of interpretation, and favour different methods for resolving disputes.

The risk of possibly conflicting norms of customary international law (CIL) has received increased attention in recent international legal scholarship and practice. In the absence of commonly accepted or authoritative rules of conflict that may mitigate the ramifications which stem from a possible clash of opposing obligations under CIL, legal scholars and practitioners alike almost instinctively turn towards competent (judicial) authorities, thereby seeking advice on how to strike a balance between conflicting norms in conformity with applicable legal frameworks and regimes. Thus far, international courts and tribunals, including the International Court of Justice, have regularly refrained from outlining, conceiving and imposing coherent analytical and prescriptive means of establishing an equilibrium between (partially) opposing norms of CIL in situ. This chapter argues that international adjudicative bodies could contemplate resorting to the German constitutional law principle of practical concordance (praktische Konkordanz) and thereby draw on a legal methodology that has become well established and is regularly applied by the German Constitutional Court when ruling on fundamental rights.

Customary international law (CIL) is particularly vulnerable to the accusation that it is no more than ‘mere assertion’, a creation of the courts, if not downright fantasy. Yet it is in CIL that one finds the strongest claim to objectivity in international law. It is expressed in the doctrine that one of the elements of CIL is state practice, which represents the ‘objective’ element of CIL. It is thought to supplement the ‘subjective’ or ‘psychological’ element of CIL: opinio juris. This chapter argues that the notion of state practice as a set of ‘material facts’ that should be ‘identified’ and from which customary norms can be ‘induced’ is grounded in obsolete epistemology. The identification of state practice is more adequately described as a selection of what deserves to be counted as state practice. It is argued that the starting point for this selective process is opinio juris. Opinio juris does not come after the fact, as a subjective feeling of obligation that is superadded to a set of otherwise objective facts. Opinio juris is the indispensable conceptual framework without which habits and usages cannot even be ‘seen’ as state practice.

Interpretation is ubiquitous in everyday life. We constantly interpret a variety of objects. Interpretation is central to the practice of international law, too. Arguing about international law’s content is the everyday business of international lawyers, and this often includes arguing about the existence and content of norms of customary international law (CIL). Although a number of scholars recognise that CIL can be interpreted, disagreements remain as to the precise methods and extent of CIL interpretation. Such disagreements are born of a common concern to secure competently made, coherent and accurate interpretations of CIL, given the latter’s non-textual nature. This chapter aims to explore in a preliminary manner two related questions regarding CIL interpretation: (1) Is it necessary, or even possible, to strive towards coherence in the interpretation of CIL? (2) Are there any possible indicators of (in-)coherence in that respect? Providing answers to these questions depends on how one understands coherence in the first place, including its relation to legal reasoning. A substantial part of the chapter will therefore deal with that as well.

This chapter provides a detailed account of the impact that the Commission’s work has had in shaping the Court’s case-law. In addition to surveying and classifying all those instances in which the Court has to date been ready to refer expressly to the Commission’s output, the chapter demonstrates that reliance on the Commission’s work has often been more implicit. The question is then posed as to the basis for such recourse and the advantage afforded by it.

The final chapter looks at the relationship between international and EU law. Also for this relationship it is decisive to, first, clearly identify how the EU can establish a consensus at the larger international level. This implies, on the one hand, that the EU is a subject of international law (Article 47 TEU and Article 335 TFEU) and, on the other hand, that the EU is competent to act concerning the subject matter (Article 3–6 TFEU). As both conditions are clearly satisfied by now, it is necessary to find out which organ is authorized to conclude an international norm (Article 216–219 TFEU). Second, it is important to determine the content of the consensus. This interpretation process is primarily the task of the level at which the consensus has been agreed. This is the international level. Third, this chapter holds that the effect of this consensus on the level of the smaller circle depends on whether the norm of the international circle is solely applicable, directly applicable or individualizing. This is demonstrated in this chapter by looking more closely at how the effect of an international treaty and customary international law unfolds within the EU legal order.

As state ownership of private firms grows, morphs, and globalizes, states increasingly channel their influence through the financialized markets. The ensuing merger of the state’s commercial and sovereign roles suggests that state ownership is, again, becoming a vector of sovereign authority. This chapter analyzes the international legal system that has developed around surging state ownership. It suggests that the legal construction of distinctive “shareholder identities” in international economic law plays a key role in this complex regulatory matrix. Specifically, the chapter focuses on how arbitral tribunals adjudicating claims arising from international investment treaties use attribution, a doctrine of customary international law, in creating, maintaining, and disciplining state shareholders. Arbitral tribunals use the analytical category of the state shareholder in order to delineate and construct state and company identities and to understand the economic, political, and legal implications of those identities in the the global economy. Accordingly, the interactions between substantive international economic law and the law of state responsibility form important, but underappreciated, elements of this constitutive process, which comes to affect the institutional design of state shareholding and disincentivize hands-on control over state-owned entities.

This chapter examines two ways the customary norm could have emerged post-1945: the two-element approach or that article 2(4) gave rise to a new customary rule of its own impact, following the approach of the ICJ in the North Sea Continental Shelf Cases. It demonstrates the challenges of applying the two-element approach to the customary prohibition of the use of force due to the presence of the parallel and near-universal treaty obligation in article 2(4) of the UN Charter, which makes it difficult to identify sufficient relevant State practice and opinio juris outside the treaty. Establishing evidence of the customary rule and its content in this way depends on a number of theoretical issues that remain unsettled or over which significant controversy exists. This chapter then applies the criteria set out by the ICJ in the North Sea Continental Shelf Cases to article 2(4) and argues that article 2(4) meets this test. This chapter concludes that, in contrast to the right to self-defence in article 51 of the UN Charter which explicitly has its origins in customary international law, article 2(4) is itself the origin of the customary international law prohibition of the use of force.

This chapter examines the relationship between the prohibition of the use of force in article 2(4) of the UN Charter and customary international law: if they are identical, the role the customary rule plays in the interpretation of article 2(4) and which one to interpret or apply to determine the meaning of a prohibited ‘use of force’. In doing so it examines the following concepts: the use of pre-existing or subsequently developing custom to fill gaps in the treaty, the use of subsequently developing custom to informally modify the interpretation of the treaty, an evolutive interpretation of the UN Charter and informal treaty modification through subsequent practice. It argues that since article 2(4) is the origin of the customary prohibition, it is not appropriate to use pre-existing or subsequently developing customary international law to fill gaps in interpretation of article 2(4) nor to use subsequently developing custom to modify article 2(4). It concludes that due to the present relationship between the customary and Charter prohibitions, the preferable approach to determine the meaning of prohibited force under international law is to focus on interpreting the UN Charter.