Abstract

This Essay identifies a fundamental but overlooked tension between international adjudication and the evolution of customary international law (CIL). According to the traditional understanding, the evolution of CIL requires one or more states to deviate from existing customary rules and engage in new conduct—a concept that I refer to as "custom breaking." A deviation's legal status is determined over time, as other states respond by deciding whether to follow the proposed break or adhere to the existing rule. Therefore, the deviation cannot be classified definitively as either legal or illegal at the time it occurs. During the period of state response, CIL necessarily contains some legal ambiguity and inconsistency. Because an important function of international adjudication involves resolving ambiguities in the law, a central tension emerges: international courts may be called upon to adjudicate a break with CIL before other states have had the opportunity to decide for themselves whether to follow the break. Given that most international courts will invalidate deviations from the status quo, international adjudication risks impeding the traditional process by which CIL evolves. More specifically, international adjudication of cases that involve custom breaking may have both a procedural and a substantive effect: procedurally, it may short-circuit state responses to the break with CIL, and substantively, it may deter states from following the custom breaker, even when the other states are not formally bound by the international judicial decision. To illustrate these constraining effects, this Essay discusses three departures by domestic courts from the foreign sovereign immunity rule. It concludes by proposing that in cases involving custom breaking, international courts should adopt, as Professor Cass Sunstein has argued for U.S. courts, a minimalist approach that produces narrow and shallow decisions. This judicial strategy would give states, including their domestic courts, the opportunity to determine for themselves whether a break with international custom is the beginning of a new legal rule.

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