International economics, where he influenced the development of two important international trade models: the Balassa–Samuelson effect, and the Heckscher–Ohlin model (with the Stolper–Samuelson theorem).
Inom internationell ekonomi bidrog han till utvecklingen inom två handelsteorier, Balassa–Samuelson effekten och Heckscher–Ohlin-modellen (med Stolper–Samuelson teoremet).