Informal international lawmaking, a phenomenon involving new actors, processes, and outputs, is an important and increasingly common phenomenon of contemporary international life. It has not, however, entirely replaced traditional international lawmaking. In fact, informal international law exists alongside treaties and custom. But while the interaction between treaties and informal law has been consistently addressed in the academic literature, the relationship between customary international law and informal law remains largely neglected, despite the complex questions it raises. First, what explains the rise of informal international law to the detriment of custom? What place is there still for custom in a world of increasing informality? Second, how do custom and informal law interact in practice? Do they merely serve as alternatives in states’ cooperation endeavours, or do they also interact in a complementary manner?

This chapter will attempt to answer these questions. We first examine and explain the rise of informal international lawmaking and the functions it serves. Next, we turn to the place of custom in an age of informality. We then focus on the complementary interaction between custom and informal international law. Finally, we examine the possibility of stretching the boundaries of the traditional international system in order to incorporate informal international lawmaking.

THE RISE OF INFORMAL INTERNATIONAL LAWMAKING

Many authors have recently indicated that international law finds itself in impasse. Indeed, there is a rather broad acknowledgement among international lawyers that traditional forms of multilateralism are facing a deep crisis, and that new multilateral processes have emerged. “What crisis?” one might curiously ask. Contemplate, for instance, all the great challenges to modern international relations: globalization, economic development, global environmental protection, international public health, technology, cybersecurity, and, why not, financial cataclysms. Would you imagine that customary international law (CIL) could effectively address these challenges? Arguably not. Treaty-based hard law, on the other hand, might fit the bill much better. But even the development of treaties is in decline.

The limited scope of Article 38 (1) of the Statute of the International Court of Justice (ICJ Statute) has become ill-suited to address the complexities of global governance. Therefore, in order to circumvent the rigidities and inadequacies of the traditional framework of international law, states have been progressively turning toward informal cooperative fora, radically transforming the international legal order. The ever-changing demands of international society, it seems, have brought into the limelight new, nontraditional types of international lawmaking.