Pierres-Hugues Verdier

How Does Customary International Law Change? The Case of State Immunity

CO-AUTHORS Erik Voeten
International Studies Quarterly


Customary international law (CIL) is a widely accepted source of international law, but it is poorly understood and has eluded systematic empirical analysis. Existing theories model CIL rules as cooperative equilibria in repeated prisoner’s dilemmas sustained primarily by reciprocity and retaliation. We argue that several features of CIL reduce the effectiveness of such mechanisms. Instead, we propose a theory under which the potential precedential effects of defection influence the stability of CIL rules. We test the observable implications of this theory with newly collected data on the CIL rule of sovereign immunity, under which states are immune from lawsuits in other states. The data document if and when 121 states switched to restrictive immunity, which allows foreign states to be sued for their commercial activities. We find no evidence that defectors were singled out for punishment. States generally followed a single policy for all foreign states and switched only once. Our regressions find that states that exported more were less likely to switch, but that states became more likely to switch as their export destinations switched and as foreign economic activity within their borders increased. We find no consistent correlations between change and democracy, legal origins or colonial history.


Pierre-Hugues Verdier & Erik Voeten, How Does Customary International Law Change? The Case of State Immunity, 59 International Studies Quarterly 209-222 (2015).

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