During the Nazi regime in Germany, he protested against the dismissal of Jewish professors from universities, as well as the requirement that professors give a Nazi salute at the beginning of their classes. In 1935 he left the country for the United States, where Swarthmore College in Pennsylvania offered him a professorship. He taught with its faculty for 20 years, and did continuing research. A Review of General Psychology survey, published in 2002, ranked Köhler as the 50th most cited psychologist of the 20th century.
