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.

习惯国际法（Customary International Law, CIL）是国际法的核心渊源。然而学界对习惯国际法的认知仍较为模糊，且缺乏针对其运作机制的系统性统计分析。既有理论认为，习惯国际法是一种可通过互惠维系的合作均衡。但习惯国际法缺乏支撑互惠的制度性特征，且普遍被认为具有普遍适用性，即便针对那些违背或拒绝某项规范的国家亦如此。由于习惯国际法的存续依赖于国家实践，只要各国重视该项合作规范，违背行为可能产生的先例效应便会激励各国维持合作。因此，一国是否适用习惯国际法规范，应取决于其互动的国家共同体对该规范的实践程度，而非单次互动中特定国家的过往行为。本研究依托新收集的数据集——该数据集记录了121个国家从绝对外国国家豁免转向限制外国国家豁免的时间与情况——对上述推论进行了检验。研究未发现直接互惠的相关证据：那些最为重视绝对豁免、且其违背行为对他国影响最大的国家，反而最不易违背该项规范；但当对该国决策影响最大的他国选择违背时，该国自身违背的概率反而会提升。