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

Customary international law (CIL) is a fundamental source of international law. But scholars lack a clear understanding of customary international law, as well as systematic statistical analyses of its workings. Existing theories posit that CIL is a cooperative equilibrium that can be sustained through reciprocity. Yet, CIL lacks institutional features that facilitate reciprocity and is commonly understood to apply universally, even to states that defect or reject a norm. Because the continued existence of CIL depends on state practice, the potential precedential effect of defection encourages cooperation as long as states value the cooperative norm. Consequentially, a state's decision to apply a CIL norm should be a function of the extent to which the norm is practiced in the community of states it interacts with rather than the past behavior of the specific state in an interaction. We test the implications with newly-collected data documenting if and when 121 states switched from absolute to restrictive foreign state immunity. We find no evidence of direct reciprocity. States that most valued absolute immunity and whose defection would most affect others were least likely to defect, but states became more likely to defect as the states whose practice most affected them defected.