Customary International Law in the 21st Century

Andrew T. Guzman, Boalt Hall, University of California, Berkeley
Timothy L. Meyer

Abstract

This chapter argues that rules of CIL support “harder” legal agreements by reducing the costs associated with such agreements. Beyond this supporting role for CIL, this chapter also argues that formal legal institutions provide ways in which states can create more credible rules of CIL. Under standard rational choice assumptions, states create legal obligations to maximize their cooperative gains, taking into account transaction costs. In an environment in which states exercise a veto over legal obligations flowing from explicit agreements, transaction costs can prevent the creation, by way of agreement, of an otherwise beneficial legal obligation. CIL can bridge this gap, allowing the creation of less credible but still valuable legal obligations. As is argued below, states have found ways to use formal legal institutions to increase the credibility of particularly valuable rules of CIL in situations in which transaction costs might prevent the creation of a treaty-based regime.