In the hygienic era, national populations were conceptualized as biological entities, that is, as races. Encompassing much more than skin color, the notion of race reflected social characteristics (such as courage or honesty) as well as physical ones (such as longevity and intelligence) that were considered to be hereditary. Races were thought about in Darwinian terms as organisms that could evolve or degenerate, win or become extinct, according to changes in their membership and in competition with others. Accordingly, at the beginning of the twentieth century many Western physicians and social reformers were concerned that their race would "degenerate" and become devitalized by the reproduction of "unfit" specimens, that is, those with undesirable physical and social characteristics thought to be heritable, including insanity, alcoholism, Down's syndrome, epilepsy, criminality, and poor eyesight. As new tests for fitness, such as the Binet IQ test, were developed and distributed across populations, ever increasing numbers of such "unfit" were discovered and anxieties worsened. Racial hygienists sought to improve their racial stock by encouraging the propagation of the fit, sterilizing the unfit, and forbidding racial "dilution" by intermarriage. These policies were written into law in most Western countries in one form or another, such as in the antimiscegenation and sterilization laws passed in a majority of the United States, which by 1941 had caused 36,000 individuals to have been compulsorily sterilized. The ultimate expression of these ideas was of course found in Nazi Germany, where euthanasia of "unfit" children and of the inmates of psychiatric institutions was carried out in the 1930s, a precursor to the horrors of the mass genocide of Jews, Gypsies, and others designated unfit during World War II.
In the aftermath of war, many racial hygiene associations, where couples had been encouraged to seek medical testing and confirmation of fitness before marrying, gradually became planned parenthood organizations with different social goals in mind.