This chapter examines the emergence, circulation, and institutionalisation of IQ testing and conceptions of intelligence in Norway, Sweden, and Denmark during the interwar period. Drawing on prosopographic methods and historical source criticism, it analyses exchanges among key educators, psychologists, psychiatrists, and medical doctors who helped shape a transnational Scandinavian arena for special education. Using the Nordic Special School Association and its journal (1923–1940) as a central point of departure—along with archival correspondence, policy documents, and contemporary newspaper articles—the chapter shows how Scandinavian actors used one another as reference societies when developing new practices of educational differentiation. Through Bruno Latour’s concept of the "centre of calculation," the chapter conceptualises this inter-Scandinavian arena as a hub that accumulated, codified, and redistributed knowledge across national boundaries. It concludes that the interwar Scandinavian experience illuminates enduring dilemmas in inclusive education: balancing universal provision with targeted differentiation, managing professional boundaries, and governing the politics of measurement in the service of social reform.