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）是国际法的基本渊源之一。但学界对习惯国际法尚未形成清晰认知，亦未对其运作机制开展系统性的统计分析。现有理论提出，习惯国际法是一种可通过互惠机制维系的合作均衡。然而，习惯国际法缺乏助力互惠的制度性特征，且通常被认为具有普遍适用性，即便针对违反或拒绝接受某一规范的国家亦如此。由于习惯国际法的存续依赖于国家实践，只要各国重视该合作规范，违反行为所产生的潜在先例效力便会激励各国维持合作。因此，一国是否适用某一习惯国际法规范的决策，应取决于该规范在其互动的国家共同体中得到践行的程度，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对上述理论推论进行检验，该数据集记录了121个国家是否以及何时从绝对外国国家豁免转向限制性外国国家豁免。研究未发现直接互惠的相关证据。那些最重视绝对外国国家豁免、且其违反行为对他国影响最大的国家，违反规范的可能性反而最低；但当对其实践影响最大的国家违反规范时，该国自身违反规范的概率则会上升。