This essay addresses the conflict between international rules formed through global multilateral agreements, like the WTO, and customary international law. It challenges the venerable view that no absolute hierarchy between global multilateral agreements and customary international law exists. In my view, global multilateralism takes precedence over customary international law, because the multilateral treaty process is more likely than customary international lawmaking to produce legitimate and beneficent norms. Multilateral agreements, like a legislative process, can generate precise rules and facilitate compromises by permitting tradeoffs among different provisions. Moreover, multilateral agreements are generally ratified in a manner that provides better assurance that they represent the views of nations' citizens, providing more robust legitimacy and reducing agency costs. In contrast, the process of discovering customary international law is fraught with difficulty and uncertainty, resulting in principles with vague contours. Moreover, the legal academics and international courts that announce the content of customary international law can provide little assurance that their views represent the consensus of the states, let alone the peoples of the world. Thus, unless clearly otherwise stated in the text of a multilateral agreement, the agreement should trump customary international law. I also provide a concrete illustration of the appropriate priority of multilateral agreements over customary international law by discussing the relation between the WTO regime and the precautionary principle. The precautionary principle generally empowers government to regulate risks to prevent quite speculative harms. Some have suggested that the precautionary principle can be used to supplement - indeed to override - otherwise applicable principles of the WTO. But the process from which the WTO emerges has advantages over the customary law process from which the precautionary principle emerges, providing us with greater confidence in the beneficence of these rules. The multilateral trade treaty process generates relatively fixed rules whose ratification in a domestic process focuses public attention on their content, thereby reducing agency costs. Moreover, these rules have real consequences for countries because of the WTO's dispute resolution mechanism, and those consequences make nations treat their assent as a serious matter. In contrast, the precautionary principle has not emerged from a deliberative process with real consequences. Moreover, the evidence from which publicists infer the precautionary principle is much less likely to reveal the preferences of nations' citizens
