This chapter expands on the concept of reciprocity, looking at how it can be defined, its key characteristics, and the three key functions it plays: in norm creation, as a condition, and in execution of the law. It examines the defining elements of reciprocity: proportionality, relativity, equality, and interdependence. The chapter goes on to examine the nature of reciprocity, and its relationship to custom and general principles, two sources of international law. It illustrates how it is reciprocity’s relationship with the structural characteristic of sovereign equality in international law that explains many of its roles and defining characteristics.

The conclusion highlights some conclusions on the nature and role of reciprocity in public international law, on the basis of the analysis in foregoing chapters. It highlights the link between reciprocity and the principle of sovereign equality, before briefly outlining the functions, limits, and some of the specific meanings of reciprocity in public international law.

The negative connotations of reciprocity and its link to conduct-based responses in a lawless context have led to a juxtaposition in legal doctrine of reciprocity with community interests, institutionalisation, and the existence of objective legality. This chapter provides an overview of existing approaches to the subject and introduces the argument of this book, that reciprocity in public international law is not antithetical to community interests and obligations, but that its importance is explained by a structural factor: the sovereign equality of states. This explains and predicts where reciprocity will be relevant, and where it will find its limits.

This chapter provides an in-depth analysis of the role reciprocity plays in treaties. First looking at treaty law and how reciprocity functions in the rules on reservations to treaties, the chapter goes on to examine reciprocity’s role in bilateral treaties, drawing on the examples of air transport agreements and energy agreements in particular, before looking at reciprocity’s role in multilateral treaties of the bilateralizable, interdependent, and integral types. It then analyzes treaties creating international organizations, particularly the EU and ILO, and differentiated obligations in environmental and trade law, illustrating how reciprocity operates in these types of instrument. Finally, the chapter addresses "objective regimes" and the effects of treaties on third parties. While some limitations exist on reciprocity, these do not depend on the substance of obligations but rather on the legal equality of the subjects involved.

This chapter addresess the operation of reciprocity in the enforcement of international law. It looks at the roles reciprocity may play in the consequences of a breach of international law, analyzing the exception of nonperformance and State responsibility. In particular, it analyzes the consequences that attach to the breach of erga omnes obligations or peremptory norms, and the rules and limitations on countermeasures. The structure and mechanisms of international responsibility demonstrate the persisting importance of reciprocity in international law and the structural limitations that international law’s horizontal nature poses to establishing specific consequences for breaches of community obligations.

This chapter looks at reciprocity in the jurisdiction of international courts and tribunals, and the differences in the role reciprocity plays in inter-State dispute settlement, and in dispute settlement procedures that involve non-State entities such as individuals. It looks at the well-developed role that reciprocity plays in the jurisdiction of the International Court of Justice, before turning to the system of compulsory dispute settlement in the United Nations Convention on the Law of the Sea (UNCLOS). The chapter then analyzes three types of instance which concern individuals: the International Criminal Court; human rights courts and complaint mechanisms; and investment tribunals, specifically the issue of the extension of jurisdiction on the basis of an MFN clause.

This chapter analyzes reciprocity in the social sciences, historical forms of law, and domestic contexts, including the law of contracts and in federal States, to shed light upon some of reciprocity’s fundamental characteristics. Reciprocity is not incompatible with the existence of a community, but necessarily requires a social relation, and one of its defining characteristics is its relationship with equality. Rather than being a negative concept, based on occasional and discontinued instances of interaction on the basis of reactions to conduct, reciprocity is a concept fundamental to the existence of social relations, and inherent to ideas of justice and fairness.

Following the finding in Chapter 3 that reciprocity encounters limitations when the subjects of a legal relationship are not equal, this chapter analyses the role of reciprocity in rules pertaining to the treatment of individuals in international law, assessing how reciprocity functions differently depending on who rights are owed to in different substantive areas of law. First looking at historical standards of treatment including those based on reciprocity and the use of the minimum standard of treatment, the chapter goes on to examine how reciprocity functions in national treatment and the most-favoured nation clause. The chapter then goes on to examine the treatment of individuals under human rights, international humanitarian law, and international investment law, analyzing the differences that arise in the role of reciprocity when the legal obligations in question are owed directly to individuals. The chapter ends with an examination of recent developments in diplomatic protection.