International investment arbitration is inhabited by actors who theorize the law as scholars and decide on it when arbitrating, which raises critical questions concerning their normative role within the legal system. This article explores empirically the influence of legal scholars on the development of inter-national investment law through a combined legal-empirical and sociological approach designed to capture the multidirectional nature of influence. It relies on a novel database of all citations to legal scholarship in International Centre for the Settlement of Disputes (ICSID) case law, which is analysed quantitatively and qualitatively and contextualized alongside interviews with 16 leading arbitrators and lawyers. The article firstly introduces the arbitrator/academic as an agent of international law, and then details the methodology. It then turns to provide an overview of citations to legal scholarship in ICSID case law, before considering why legal actors find it useful. The following parts show how legal scholarship influenced the development of the definition of an ‘investment’ in ICSID arbitration through an analysis of legal decisions and scholarship authored by arbitrators. The final part goes into conversation with the actors to further unravel how they manage competing and complementary obligations towards theory and practice, and the article concludes by reflecting on the ethical implications of their law-making function.