States join international institutions and sign treaties with other states frequently in world politics. The depth of cooperation and design of these commitments varies considerably. Some treaties are extremely detailed and span hundreds of pages, such as the United Nations Convention on the Law of the Sea (UNCLOS), while others spell out the terms of the treaty in a few hundred words, such as the original North Atlantic Treaty Organization (NATO) agreement. International relations scholars have pointed to a variety of factors that may explain the rich variation in the design and depth of cooperation in states' international commitments, including relative capabilities, number of signatories, and regime types of the negotiating parties.

In this chapter, we focus on a factor that has received less attention in the rational design literature: domestic legal systems. As we explained in Chapter 2, there is considerable variation in the form and design of contracts in civil law, common law, and Islamic law systems. Contracts signed in these legal traditions differ in terms of their attention to detail, their length, and their inclusion of general principles. We assert that these contractual differences in the domestic realm carry over onto the international arena, where states make commitments with international institutions and with other states. International negotiators bring their legal backgrounds to the negotiating table, which influences both their willingness to sign treaties and the design of the resulting agreements.