Document Type

Working Paper

Publication Date



customary international law, international adjudication, state preferences, opinio juris


The standard account today of customary international law (CIL) is that it arises from the widespread and consistent practice of states followed out of a sense of legal obligation. Although commonly recited, this account is plagued by evidentiary, normative, and conceptual difficulties, and it has been subjected to increasing criticism in recent years. This paper posits a different account of CIL, considered from the perspective of international adjudication. A fundamental problem with much of the theorizing about CIL, the paper contends, is that it fails to identify which decisionmaker it has in mind. Instead, the discussion proceeds as if CIL existed in the abstract without any particular human entity to interpret and apply it. The application of CIL by an international adjudicator is best understood, this chapter contends, as an effort to determine the preferences of the relevant community of states concerning the norms that should apply in the absence of a controlling treaty. Unlike the standard view of CIL, this state preferences account recognizes an element of judgment and creativity in determining the content of CIL, somewhat akin to the judicial development of Anglo-American common law. Understanding the adjudication of CIL in this way, the paper contends, avoids many of the difficulties surrounding the standard account of CIL.

Library of Congress Subject Headings

International customary law, International law