Customary International Law Adjudication as Common Law Adjudication

Abstract

The standard view 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 book chapter suggests that these difficulties stem in part from the effort to formulate one conception of “CIL” that applies across all institutional contexts. This chapter posits a particular account of CIL, considered from the perspective of international adjudication. The application of CIL by an international adjudicator, this chapter suggests, is best understood in terms similar to the judicial development of the common law: that is, as an approach whereby adjudicators look to past practice but necessarily make choices about how to describe it, which baselines to apply in evaluating it, and whether and when to extend or analogize it to new situations. These choices, moreover, are shaped by assessments of state preferences as well as social and moral considerations. Unlike the standard view of CIL, this common law account recognizes a significant element of judgment and creativity in determining the content of CIL. Understanding the adjudication of CIL in this way, the chapter contends, avoids many of the difficulties surrounding the standard view of CIL.

Keywords: customary international law, common law, international adjudication, opinio juris