Custom, Reconsidered: Lessons on Unwritten Rules from Professional Sports.

This article examines the unwritten but obligatory rules in professional sports as a novel framework to better understand the nature, origins, and evolution of customary international law (CIL). It analyzes four prominent unwritten sports rules—such as baseball's "one for one" pitcher retaliation and soccer's practice of stopping play for injured players—to illustrate how emergent, widely accepted norms can generate binding obligations and enforcement mechanisms akin to those in CIL. The study highlights key lessons for CIL scholarship, including the foundational role of basic norms (grundnormen), the informal and evolutionary legislative process of customary rules, the critical importance of enforcement, and the variability of rules over time and context. It further suggests that, like professional sports, CIL's customary rules demonstrate resilience alongside written law, the potential for new customary norms to arise, and the expanding role of diverse actors in law-creation beyond traditional state-centric models.