Adherents included liberals and conservatives, progressives and libertarians. In the early decades of this century the emphasis on encouraging reproduction among those assumed to possess a superior genetic endowment became known as positive eugenics . The goal of negative eugenics has throughout the century been to limit child bearing by undesirable individuals, presumably because of a strong likelihood that their children too would be unfit. During the first quarter of the twentieth century, particularly in the United States, ardent supporters of negative eugenics emerged in many quarters, and by the late 1920s social policies to implement negative eugenics were widespread. That policy was not significantly altered until 1968.
Most nations have a history of eugenic thought or practice.
Some have tried to keep gene pools separate by forbidding legitimate unions between members of different social groups. There is no evidence that a racial caste system had any impact on stratifying the population by ability or other characteristics. Immigration laws have also been used to attempt to insulate gene pools . The Untied States immigration policy that was erected in the 1920s and dismantled in 1968 favored immigrants from northern and western Europe over all other peoples. The United States never enacted a federal sterilization statute, but about 30 states did, many after the Supreme Court upheld a 1924 Virginia law that permitted state officials to sterilize institutionalized retarded persons whom a physician had deemed likely to become the parents of children with similar defects.