In 1920, most psychologists believed in the existence of mental differences between races; by 1940, they were searching for the sources of "irrational prejudice." In a few decades, a dramatic reversal of the dominant paradigm for the study of groups and group relations had occurred. Although this shift can be seen as a victory of objective-empirical research, there were other contributing factors: passage of the Immigration Restriction Law of 1924, which shifted the political problem from justification of differential exclusion to conflict resolution in this country; the influx of ethnics into the originally rather lily-white profession of psychology; the Great Depression and the leftward shift among psychologists; and finally, the need to unite the country against a dangerous enemy proclaiming racial superiority.