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Customary International Law in the 21St Century

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Chapter Summary

This chapter argues that rules of customary international law (CIL) support "harder" legal agreements by reducing the costs associated with such agreements. Beyond this supporting role for CIL, it 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. The chapter reviews traditional definitions of CIL, as well as criticisms of traditional approaches. It develops a model of CIL based on rational states. The chapter considers several ways in which CIL remains relevant to a world in which the predominant legal instrument of international relations is the treaty. It demonstrates that the rise of more formal legal institutions in the last century has reinforced and complemented the way in which CIL impacts state behavior, rather than rendering CIL irrelevant to international relations.

Keywords: customary international law (CIL); legal institutions; rational states



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