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Traditional accounts of international law in domestic courts focus on the distinction between monist and dualist legal systems. In monist systems, courts apply international law directly. In dualist systems, direct application is not an option, so courts apply international law indirectly, or not at all. Although this account is formally correct, it tells us very little about the functional role of domestic courts in the international legal system. In this chapter, we present a functional account that focuses on the distinctions among horizontal, vertical, and transnational legal obligations. Modern international law regulates horizontal relationships between states, vertical relationships between states and private parties, and transnational relationships between private parties whose interactions cross state lines. The role of domestic courts in interpreting and applying international law varies greatly, depending on whether the international rule at issue is horizontal, vertical, or transnational.

We demonstrate in this chapter that the willingness (or ability) of courts to apply international law—that is, the decision that a particular issue is “legal” as opposed to “political”—depends heavily on the nature of the international legal rule. Domestic courts rarely interpret or apply horizontal rules. Indeed, they often refer to treaties that regulate solely the horizontal relationship between states as “political” in nature. As a result, implementation of horizontal obligations typically involves executive, not judicial action. This is true for both monist and dualist states. In contrast, domestic courts routinely interpret and apply transnational rules, and executive branch officials play little or no role in implementing such rules. (Purely private law treaties such as the Convention on Contracts for the International Sale of Goods provide a clear example.) Again, this is true for both monist and dualist states.

It is more difficult to generalize about the role of domestic courts in interpreting and applying vertical rules, such as human rights treaties. However, the classic monist-dualist distinction does a poor job of explaining why domestic courts in some countries apply vertical rules aggressively, while domestic courts in other countries are reluctant to apply vertical (international) rules to regulate the conduct of government officers. It is in this dimension, therefore, that we seek to draw special insights into how particular political relationships may affect the willingness of domestic courts to implement international law in domestic law, and to effect legal change on their own initiative.

This chapter draws on materials from approximately two dozen countries to present an account of the role of domestic courts in interpreting and applying horizontal, transnational, and vertical international legal rules.


Forthcoming in "Handbook on the Politics of International Law".

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