Eugenics, the idea of "race improvement" through social engineering measures and state intervention, was, in the late nineteenth century and the first half of the twentieth century, a subject of debate within the medical, scientific, and political communities of Europe and North America, until the Nazi Holocaust gave the topic an extremely negative reputation. Eugenicists based themselves on the evolutionary theory of natural selection, the analogy between the animal and human world, and metaphors concerning selection and competition, to try to limit quantitatively and qualitatively what they considered to be a "decaying" population and society, allegedly caused by biological factors that would give rise to hereditarily morbid nations. This idea, based on social Darwinism, found its way into the ideas for creating a "new man" and the nationalistic views of "regeneration" in the nations of European dictatorships, particularly in Germany, in the period between the wars of the twentieth century.