The proliferation of international courts and tribunals in the international system over the past century poses an interesting puzzle for exploration. Why has the number of international courts and the power that they wield increased despite the significant intrusion such courts can have on state sovereignty? The ICC, for example, requires mandatory jurisdiction for signatories to the Rome Statute and allows for an independent prosecutor to initiate proceedings regarding crimes against humanity when member states are unwilling or unable to do so. The ECJ is similar in that members of the EU are required to accept the court's jurisdiction. Even in situations where states can place reservations on their commitments to international courts, it is difficult to anticipate all future situations that might arise that could disadvantage states, as the United States realized in 1986 in light of the Nicaragua case before the ICJ.

The expansion of international adjudication is puzzling from a traditional realist perspective, where the existence of an anarchic system implies that courts might not work well globally because they are not backed by a coercive authority. One possible solution to this quandary is for a global or regional hegemon to step in and help create an international or regional court and then act as an enforcer for the court. For example, the Central American Court of Justice was arguably most effective when regional powers, such as the United States and Mexico, were willing to support the court's rulings (Allain 2000).

The ICC, created by the Rome Statute in 1998, constitutes a truly unique international adjudicative body. It exemplifies the process of global legalization, it points to the strong connection between domestic and international law, and it embodies a true compromise between two domestic legal traditions. During the Rome negotiations, state representatives pushed for rules and procedures that were familiar to them based on their domestic legal backgrounds, which resulted in the creation of a sui generis international court, an interesting hybrid between common law and civil law principles. This design process was not only rational from the perspective of the Court's supporters, it also had unintended consequences in that states considering whether to join the ICC at a later date would be influenced by the original design of the court.

In this chapter, we apply the theory articulated in Chapter 3, which contends that states' domestic legal traditions influence their preferences regarding the legal design of international courts. States involved in creating a new international adjudicative body seek to design a court with familiar legal rules and principles, anticipating that an initial commitment to the court will be durable and have long-term consequences. We argue that the ICC originators pushed for rules and procedures that mimicked those of their domestic legal systems to help reduce uncertainty regarding the Court's future decision-making processes.

In this chapter, we lay the groundwork for our theoretical argument about the creation of new international courts and the expansion of state support for pre-existing courts by focusing on the characteristics of the three major domestic legal traditions in the world: civil law, common law, and Islamic law. We provide a brief historical account of the origins of these three legal families and we describe the major distinctions between them, including differences in the source of law, distinctions in the law of contracts, and variations in legal procedures (e.g. litigation). We then discuss how these features of domestic legal traditions influence foreign policy decision-making processes, especially as they impinge upon the creation of new international institutions and the signature and design of interstate commitments. We conclude by describing some of the convergences that have occurred over time between these legal families, especially between civil law and common law, and the significance of this temporal trend for our research.

Introduction to legal systems

Each state in the international system, as a political entity, possesses its own legal system, which is manifested at any point in history by a system of rules, norms, and principles. There are several definitions of the term “legal system.” Merryman (1985, 1) defines it as “an operating set of legal institutions, procedures, and rules.”

In his book How International Law Works: A Rational Choice Theory, Andrew T. Guzman writes:

The goal of this book is to contribute to both aspects of this transformation – theory and social science methodology. We develop a rational legal design theory of international adjudication, which attempts to explain an important part of cooperation in the international system: the creation and expansion of international courts. In particular, we ask why states create new international courts and why states join pre-existing international courts. We argue that an interesting intersection of domestic and international law occurs when international courts are formed. After accepting a basic premise that states can benefit from bargaining with the potential assistance of an adjudicator, we contend that not all adjudicators are created equal. Different courts are created to suit the interests of a particular group of states. States have incentives to create international courts in their own legal image to reduce uncertainty in future bargaining situations. Thus, the initial negotiators of new courts, who we call the “originators,” design institutions in ways that are optimal from both a political and legal standpoint.

International courts have proliferated significantly in the international system, growing from only a handful of courts a century ago, to over 100 judicial or quasi-judicial bodies today. Prominent international courts include the International Court of Justice (ICJ), the International Criminal Court (ICC), the European Court of Justice (ECJ), and the World Trade Organization's (WTO) Dispute Settlement Understanding. International courts operate at the regional and global levels and cover a wide variety of issues such as territorial disputes, human rights, the law of the sea, trade, investments, and the use of military force.

While the number of international courts has increased significantly over time, there is considerable variation across courts. First, some international courts receive much stronger and broader state support than other courts. The Rome Statute, which recognizes the jurisdiction of the ICC, has currently been ratified by 111 countries, or over 55% of all states in the world. The World Trade Organization's adjudication mechanism receives a high level of international support as well, with 153 states (75%) belonging to the organization today. Other courts receive significantly less international support, such as the ICJ, where only one third of states in the world accept the compulsory jurisdiction of the Court (Alexandrov 1995).

Second, there is considerable variation in the design of international courts. Some courts, such as the ECJ, have a limited regional membership scope, while other courts, like the ICJ and the ICC, are more global and universal in their orientation.