Koopmans moved to the United States in 1940. There he worked for a while for a government body in Washington, D.C., where he published on the economics of transportation focusing on optimal routing, then moved to Chicago where he joined a research body, the Cowles Commission for Research in Economics, affiliated with the University of Chicago. In 1946, he became a naturalized citizen of the United States and in 1948 director of the Cowles Commission. Also in 1948, he was elected as a Fellow of the American Statistical Association.View/Search Fellows of the ASA, accessed 2016-07-23. In 1950 he became a corresponding member of the Royal Netherlands Academy of Arts and Sciences. Rising hostile opposition to the Cowles Commission by the department of economics at University of Chicago during the 1950s led Koopmans to convince the Cowles family to move it to Yale University in 1955 (where it was renamed the Cowles Foundation). He continued to publish, on the economics of optimal growth and activity analysis.
