1 Experimental jurisprudence in international law Benedikt Pirker, Izabela Skoczeń and Veronika Fikfak Penultimate version to appear in the Cambridge Handbook of Experimental Jurisprudence, edited by Kevin Tobia Abstract: The present chapter presents an overview of the recent applications of experimental methods to jurisprudential issues in international law as well as potential future developments of the field. We discuss how experimental methods can helpfully contribute to the understanding of the different moments of international law: the making of new rules, the interpretation of these rules and finally, their application in practice. We also present three main topics that constitute the studies of rule-application in practice, namely, the mechanisms behind human judgment, policy making and, finally, the decision taking itself. We identify some future potential lines of development based on the example of the current disagreements over the definition of an international crime of ‘ecocide’ and discuss some potential limitations of the experimental approach. Keywords: international law; experimental jurisprudence; treaty interpretation; legal rules; human judgment and decision taking; policy 1. Introduction In comparison to domestic law, in the absence of comparable enforcement mechanisms, international law relies largely on voluntary state and individual compliance. Consequently, the behavioral aspect of international law – the ways in which it is being observed or misapplied – are a crucial aspect of the study of international law. In order to develop a set of rules which will be followed and applied internationally, it seems vital to have a good grasp of the sociological and psychological mechanisms underlying states’ and individuals’ behaviour and specifically compliance with international legal norms. 1 The application of empirical methods is a nascent area in international law and there are numerous benefits for pursuing this type of work. For example, survey experiments can help uncover the thinking of elites, which is one key factor shaping the rules of international law. This chapter aims to map out an overview of experimental studies that have been carried out in international law as well as to provide an outline for future paths of inquiry. We discuss how experimental methods can helpfully contribute to the understanding of the different moments of international law: the creation of new rules, the interpretation of these rules and finally, their application in practice. We also present three main topics within the examination of rule- application in practice, namely, the mechanisms behind human judgment, policy making and, finally, the decision taking process itself. We also identify some future potential lines of development based on the example of the current disagreements over the definition of an 1 See for an early claim Adam Chilton and Dustin Tingley, 'Why the Study of International Law Needs Experiments' (2013) 52 Columbia Journal of Transnational Law 173.