Abstract Customary international law is typically defined as the general and consistent practices of states that they follow from a sense of legal obligation (Restatement 1987, § 102[2]). This definition contains two elements: there must be a widespread and uniform practice of states, and states must engage in the practice out of a sense of legal obligation. This second requirement, often referred to as opiniojuris, is the central concept of customary international law. Because opinio juris refers to the reason a state acts in accordance with a behavioral regularity, it is often described as the “psychological” element of customary international law (Brownlie 1960, 7–9; D’Amato 1971, 47–55, 66–73). Opinio juris is what distinguishes a state act done out of interest or comity from one that a state performs because it is required to do so by law. Courts and scholars say that a long-standing practice among states “ripens” or “hardens” into customary international law when it becomes accepted by states as legally binding (The Paquete Habana 1900, 686).