Treaty, Custom and the Cross-fertilization of International Law

¶1 The title of this timely and innovative new journal raises basic questions about the connection between human rights norms and development norms. What is their hierarchical relationship? How does the content of one inform that of the other, if at all? Is the international legal order a "bric-a-brac" or a "system"? Is it an aggregate of disparate elements haphazardly brought together or a systematically organized and coherent structure? If the latter, what organizing techniques, if any, exist to assure hierarchy? ¶2 In this Article, I set out some introductory thoughts on the relationship between and the hierarchy among different norms of international law. To be clear, the subject of relationship and hierarchy has several aspects. One is the general relationship between different sources of legal obligation, in particular between treaty and custom. A second is the relationship between different subject matter areas of international law: For example, which prevails in a conflict between treaty norms of the law of development and the law of human rights, or between the law of international trade and the law of the environment? A third aspect merges the first two: What is the relationship between a treaty norm arising in one area of international law and a customary norm arising in another? In this paper, I focus on the third issue, which has received surprisingly little attention, while touching also on the first aspect. ¶3 The relationship between treaty and custom—particularly across subject matter areas— may appear rather esoteric. In fact, it is a problematic subject that arises with increasing frequency. The world of international law is, in institutional terms at least, significantly transformed from that of a generation ago in at least two ways. First, there are more international "legislatures"—formal international institutions, conferences of the parties established by treaties, and so on—which are adopting ever