120 Yale L.J. 202 (2010). 

Treaties are negotiated, usually written down, and often subject to cumbersome domestic ratification processes. Nonetheless, nations often have the right to withdraw unilaterally from them. By contrast, the conventional wisdom is that nations never have the legal right to withdraw unilaterally from the unwritten rules of customary international law (CIL), a proposition that we refer to as the “Mandatory View.” It is not obvious, however, why it should be easier to exit from treaties than from CIL, especially given the significant overlap that exists today between the regulatory coverage of treaties and CIL, as well as the frequent use of treaties as evidence of CIL. In this Article, we consider both the intellectual history and functional desirability of the Mandatory View. We find that a number of international law publicists of the eighteenth and nineteenth centuries thought that CIL rules were sometimes subject to unilateral withdrawal, at least if a nation gave notice about its intent. We also find that the Mandatory View did not come to dominate international law commentary until sometime in the twentieth century, and, even then, there were significant uncertainties about how the Mandatory View would work in practice. Moreover, we note that there are reasons to question the normative underpinnings of the shift to the Mandatory View, in that it may have been part of an effort to bind “uncivilized” states to the international law worked out by a small group of Western powers. After reviewing this history, we draw on theories developed with respect to contract law, corporate law, voting rules, and constitutional design to consider whether it is functionally desirable to restrict opt-out rights to the extent envisioned by the Mandatory View. We conclude that, although there are arguments for restricting opt-out in select areas of CIL, it is difficult to justify the Mandatory View as a general account of how CIL should operate.