Now we can look back and see the festering presence of social Darwinism lurking throughout the whole postwar era, like a monstrous Forrest Gump. The United States’ enduring attachment to empire and racial domination helped ensure the survival of social Darwinist ideas, just as the imperialist expansion of the late nineteenth century gave the ideology its initial purchase. In turn, these ideas, repackaged and sanitized by popular psychologists and libertarian economists, eventually helped to sanctify inequality as the foundation of the neoliberal order—­to create a dog-­eat-­dog political economy that only strengthened the appeal of social Darwinism as an explanation of how society operates.

W. D. Hamilton, one of the most influential evolutionary biologists of the second half of the twentieth century. Hamilton helped develop new theoretical tools that allowed scientists to postulate a genetic basis for a range of complex social behaviors. Among these behaviors, in Hamilton’s view, was genocide, which was a natural if morally regrettable response to population growth among a competing “tribe.”

After World War II, when the United States’ embrace of the anti­fascist struggle raised the possibility that its own apartheid system would be the next target, a vanguard of white nationalists and Nazi sympathizers rallied around a private philanthropy called the Pioneer Fund, which had been established shortly before the war. The fund’s grants helped ensure that scientists who shared the founders’ enthusiasm for the Third Reich’s “selective breeding” policies could pursue their work under an all-­American aegis.

Reasoning along similar lines, the historian Carl N. Degler asserted in 1991 that the struggle against Nazism had at last left social Darwinism “definitely killed, not merely scotched.”

In Social Darwinism in American Thought, the 1944 book that popularized the term, Richard Hofstadter writes that the ideology, at least “as a conscious philosophy,” had “largely disappeared” in the United States by the end of World War I, thanks to its uncomfortably Teutonic overtones.

last October, Jonathan Chait argued that the social Darwinist commitments Trump shares with the rest of the Republican Party distinguish it not only from the Democrats but “from conservative parties in other industrialized democracies.”

If old-school Social Darwinists like Herbert Spencer viewed nature as a marketplace, albeit an unusually cutthroat one, the new version was outright capitalist. The neo-Darwinists assumed not just a struggle for survival, but a universe of rational calculation driven by an apparently irrational imperative to unlimited growth.

We all know the eventual answer, which the discovery of genes made possible. Animals were simply trying to maximize the propagation of their own genetic codes. Curiously, this view—which eventually came to be referred to as neo-Darwinian—was developed largely by figures who considered themselves radicals of one sort or another.

Mutual Aid grew from a series of essays written in response to Thomas Henry Huxley, a well-known Social Darwinist, and summarized the Russian understanding of the day, which was that while competition was undoubtedly one factor driving both natural and social evolution, the role of cooperation was ultimately decisive.

An alternative school of Darwinism emerged in Russia emphasizing cooperation, not competition, as the driver of evolutionary change. In 1902 this approach found a voice in a popular book, Mutual Aid: A Factor of Evolution, by naturalist and revolutionary anarchist pamphleteer Peter Kropotkin.