When faced with the inevitable task of interpreting customary international law (CIL), what should a court do and what should it consider? Should it engage in an ‘inductive’ process of sifting through available evidence of state practice and opinio juris or a deductive process designed to reason logically from principles embedded in the rule? Should the court invoke something like the rules of treaty interpretation with their focus on good faith, ordinary meaning, context, and object and purpose? International law doctrine falls short here. Figuring out how to interpret and apply custom requires a theory of custom, a focus on the normative stories we tell. This reveals that there is not just one story explaining why custom should be a source of law, but multiple stories. What we call ‘custom’ may represent or draw from at least three different sources of law: Negotiated Law, Legislated Law and Adjudicated Law. This chapter aims to show that the non-treaty rules derived from each draw on different sources of legitimacy, operate according to different logics, dictate different methods of interpretation, and favour different methods for resolving disputes.

Cohen explores the origin of realism and argues that a wide range of schools of American legal thought drew from various aspects of legal realism. He notes that these various strands have converged and have become mutually reinforcing. International relations scholarship has been refracted through legal realism; American international law scholarship has borrowed from constitutional law scholarship already suffused with realism. He concludes that American legal realism has always had counterparts in other parts of the world, and one could question whether the thick lines between realism and positivism look much thinner and more porous in actual practice.

Over the past few decades, scholars in a variety of fields – economics, psychology, sociology, anthropology, and international relations, among others – have made enormous strides studying the behavioral roots of international law by exploring individual motivations, describing organizational cultures, and mapping communities of practice. Taken together, the work of these scholars presents a complex, nuanced understanding of how international law works. However, these projects are rarely considered together and are generally separated by academic enclosures and focused on different subfields within international law, thereby unfortunately restricting communication among scholars who are using different methodologies. The goal of this book is to break down some of these barriers and provide a glimpse of what an international law more focused on behavior and more engaged with these other fields might look like; this chapter aims to provide a roadmap in this effort by describing international law’s long interest in behavior and the past attempts to explore that relationship, exploring the book’s approach and laying out the contributions in each chapter, and beginning the process of bringing these insights together and outlining a series of takeaways for future study of international law as behavior.