There are two kinds of eugenics. The first is positive eugenics, related to selective breeding. One frequently cited example is the Nazi SS matching its members with “worthy” Aryan women to improve the German stock. But positive eugenics is also inherent in opposition to immigration, race mixing and desegregation. White leaders like Theodore Roosevelt, for example, worried about the “mongrelization” of the Anglo-Saxon race.

The other eugenics, the negative kind, involves actively preventing certain individuals from living or reproducing — in other words, removing them from the gene pool. The Holocaust is an extreme example, but the basic concept was embraced in the United States early in the 20th century. Laws permitting forced sterilization of women, the mentally ill, gay people and those considered child rapists were passed across the country; they intended to purify the population.