A THEORY OF CUSTOMARY
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). 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 opinio juris, 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).
This standard account of customary international law suffers from well-known difficulties (D’Amato 1971; Fidler 1996). There is little agreement about what type of state action counts as state practice. Policy statements, legislation, and diplomatic correspondence are the least controversial sources. Treaties, especially multilateral treaties, but also bilateral ones, are often used as evidence of customary international law, but in an inconsistent way. The writings of jurists are a common but tendentious source of customary international law. Even more controversially, United Nations General Assembly resolutions and other nonbinding statements and resolutions by multilateral bodies are often viewed as evidence of customary international law. Those who study