The Customary International Law Supergame


Customary international law is an enigma. It is produced by the decentralized actions of states, and it generally lacks centralized enforcement mechanisms. Political science realists and some rationalist legal scholars argue that customary international law cannot affect state behavior: that it is “epiphenomenal.”

This article develops a model of an n-person prisoner’s dilemma in the customary international law context that shows that it is plausible that states would comply with customary international law under certain circumstances. These circumstances relate to: (i) the relative value of cooperation versus defection, (ii) the number of states effectively involved, (iii) the extent to which increasing the number of states involved increases the value of cooperation or the detriments of defection, including whether the particular issue has characteristics of a commons problem, a public good, or a network, (iv) the information available to the states involved regarding compliance and defection, (v) the relative patience of states in valuing the benefits of long-term cooperation compared to short-term defection, (vi) the expected duration of interaction, (vii) the frequency of interaction, and (viii) whether there are also bilateral relationships or other multilateral relationships between the involved states.

One implication of this model is that it lends credence to customary international law. From a research standpoint, this model identifies a number of parameters for which data may be developed in order to test the model. From a policy standpoint, this model shows what types of contexts, including malleable institutional features, may affect the ability of states to reach stable and efficient equilibria in their customary international law relations.


International Law

Date of this Version

April 2004