The rapid growth in investor–state dispute settlement has sparked a decades-long legitimacy crisis in the international investment regime. The upshot is that concerns over foreign investor success in arbitration proceedings, from levels of compensation to lack of arbitral diversity and independence, has prompted wide-ranging reform efforts and even calls to abolish this treaty-based system. In the context of this historical and contemporary debate, the authors describe the importance of assessing empirically the claims and counter-claims about the regime’s absence of legitimacy. The chapter begins with an overview of the different types of legitimacy (normative, sociological, legitimation) and discusses how and to what extent empirical research can contribute to assessing legitimacy claims. The different chapters of the book, which is structured largely according to legitimacy categories, are then introduced and the piece concludes with some reflections on the overall themes and way forward for empirical research.

Any assessment of the international investment regime and its legitimacy crisis requires a preliminary understanding of their important and relevant features. However, the sprawling nature of both defies most doctrinal and qualitative attempts at description. The regime is based on a decentralized network of legal instruments, different procedural mechanisms and ad hoc proceedings, while the accompanying chorus of critique and counter-critique is populated with multiple actors and interests across the world. This chapter seeks to capture this distinct and fragmented universe. First, the authors map consent to arbitration, not on a generic per signed bilateral investment treaty basis, but rather by tracking multilateral, bilateral and unilateral consents in force. Second, they provide a description and overview of the over 1,100 registered cases up to January 2020, focusing inter alia on case outcomes, rules, cases types, institution, parties, economic sector and legal basis. Third, they trace discontent with regime, charting the origin of legitimacy crisis and its maturing over time. It ends by discussing both state-led efforts at reform and the extent to which arbitrators themselves have adjusted reflexively to the backlash.

The lack of geographic diversity among arbitrators is a common critique of investor–state dispute settlement. This has emerged as a major legitimacy problem as 80% of ISDS cases are against non-Western respondent states. In this chapter, the authors map the existing level of diversity with new methods (tracking both nationality and residence) and examine whether greater diversity would make a difference in outcomes. The descriptive statistics reveal that only a third of arbitral appointments have gone to non-Western individuals, and that this falls to 25% when residence is taken into account. However, the issue becomes more complicated when examining the effect on outcomes. The regression analysis indicates that the absence of geographic representativeness can favour Western home and host states, especially when the Chair is from the West. However, possibly due to a high degree of institutionalization and socialization of arbitrators in the system, it does not appear at present that arbitrator nationality has a significant effect on outcomes.