Pierres-Hugues Verdier
Mila Versteeg

International Law in National Legal Systems: An Empirical Investigation

American Journal of International Law


International legal scholars have long recognized the importance of the rules and processes by which states adhere to international legal obligations and "translate" them into their domestic legal systems. Yet, the lack of systematic data makes it difficult to assemble an overall picture of the relationship between international law and national legal systems. In this paper, we draw upon a new data set, which currently covers 101 countries for the period 1815-2013 and reflects numerous specific features of national approaches to international law, including treaty-making procedures, the status of treaties in domestic law, and the reception of customary international law. We find that national legal systems have become more likely to give treaties direct effect and hierarchical superiority over domestic law, but at the same time have steadily expanded the categories of treaties whose ratification requires prior legislative approval. With respect to CIL, we find that the vast majority of national legal systems now recognize custom as directly applicable, at least in principle, but generally consider it to be hierarchically inferior to domestic law. These findings have implications for the comparative international law project and for several other strands of scholarship, including research on the role of courts and other domestic institutions in implementing international law obligations.


Pierre-Hugues Verdier & Mila Versteeg, International Law in National Legal Systems: An Empirical Investigation, 109 American Journal of International Law 514-533 (2015).

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