When Americans talk about racism, we are almost always referring to white discrimination against blacks. This is natural enough, since prejudice against African-Americans is the original and longest-lasting form of American race hatred. But racism, in its early-20th-century heyday, was about more than simple hatred, though a particularly insidious form of inhumanity always lay at its heart. As the word itself suggests, racism, like communism, originally purported to be a science, or at least a scientific ideology – that is, a way of ordering human life based on alleged facts about nature.

Those facts, as they emerged in the writings of 19th-century racial theorists, seemed to fit perfectly into the world picture advanced by Charles Darwin, who revealed the merciless truth about the survival of the fittest. As with species, so too with human races, thought people as different as the founder of eugenics, Darwin’s cousin Francis Galton, and the head of the SS, Heinrich Himmler. The future of humanity depended on ensuring that the best races multiplied and the worst died out. The problem was that, in modern Western societies, a foolish compassion preserved the fit and the unfit alike, and even encouraged the poor and “feeble-minded” to continue propagating their inferior protoplasm (to use one of the racists’ favorite demi-scientific terms). Herbert Spencer, the Victorian sage who helped to popularize Darwin’s ideas, put it very bluntly in his 1865 book “Social Statics”:

Blind to the fact that under the natural order of things, society is constantly excreting its unhealthy, imbecile, slow, vacillating, faithless members, these unthinking, though well-meaning, men advocate an interference which not only stops the purifying process, but even … encourages the multiplication of the reckless and incompetent by offering them an unfailing provision.

The solution, to several generations of reformers in America and Europe, was obvious: “Unhealthy” human specimens had to be stopped from multiplying. Opinions on how this was to be accomplished, and what a perfected humanity would look like, varied widely among believers in eugenics. But one thing remained constant: The advocates of scientific racism, whether progressive American doctors or Nazi murderers, were always certain that they themselves belonged to the best racial stock.

Two new books tell complementary parts of this horrifying, deeply important story. In “Better for All the World” (Alfred A. Knopf, 402 pages, $30), Harry Bruinius focuses on the main American advocates of eugenics, who in the first third of the 20th century led the effort to sterilize people, mainly women, whom they deemed feeble-minded, moronic, or generally undesirable. Mr. Bruinius reminds us of a historical irony we are generally happy to forget: that in the early 20th century, sterilization joined Prohibition and suffragism as one of the favorite causes of the progressive movement. In each case, Protestant progressives believed themselves to be carrying the banner of reason and justice, against the ignorant Catholic and Jewish immigrants who threatened to ruin the republic. Mr. Bruinius prints a shocking letter from no less a progressive hero than Theodore Roosevelt, the great trust-buster and conservationist, showing that he enthusiastically embraced the principles of eugenics:

It is really extraordinary that our people refuse to apply to human beings such elementary knowledge as every successful farmer is obliged to apply to his own stock breeding. Any group of farmers who permitted their best stock not to breed, and let all the increase come from the worst stock, would be treated as fit inmates for an asylum. … Someday we will realize that the prime duty, the inescapable duty, of the good citizen of the right type is to leave his or her blood behind him in the world; and that we have no business to permit the perpetuation of citizens of the wrong type.

Mr. Bruinius’s approach is to focus on the biographies of a few leading theorists of eugenics. Chief among them is Charles Davenport, the addressee of Roosevelt’s letter. Davenport, the scion of an old Puritan family, transposed his father’s religious zeal into the new key of modern science, working with all the passion of a true believer to convince America of the need to safeguard its protoplasm. (Margaret Sanger recalled him murmuring, “Protoplasm. We want more protoplasm,” like a genetic Dr. Strangelove.) As the head of a research station at Cold Spring Harbor, Long Island, and a member of the American Breeders’ Association, Davenport helped introduce the principles of Mendelian genetics to American farmers.

But his interest in breeding didn’t stop with plants and animals. He wrote the major textbook on eugenics, “Heredity in Relation to Eugenics” (“the human babies born each year constitute the world’s most valuable crop”), and established a Eugenics Record Office to compile family histories of as many Americans as he could get to answer his ludicrously vague questionnaires. Davenport’s deputy Harry Laughlin, another of Mr. Bruinius’s major subjects, argued that the “bottom” 10% of the American population should be sterilized.

Laughlin fell well short of his goal. But before World War II, Mr. Bruinius writes, more than 65,000 Americans were forcibly sterilized, mainly women who had been sent to mental hospitals on account of “feeble-mindedness.” This flexible category, like “moron,” could hold anyone a doctor or judge considered too poor or too promiscuous to deserve the right to bear children. Many women were sterilized not just without their consent but without their knowledge, having been told that they were only getting an appendectomy.

The most famous victim of this policy, and the figure with whom Mr. Bruinius begins his story, was Carrie Buck, a 21-year-old Virginia woman whose case went all the way to the Supreme Court. In Buck v. Bell, a decision that deserves to be as infamous as Plessy or Dred Scott, the court upheld the states’ right to sterilize people without their consent. As Oliver Wendell Holmes Jr. put it, in the opinion from which Mr. Bruinius takes his title: “It is better for all the world if instead of waiting to execute degenerate offspring for crime, or let them starve for their imbecility, society can prevent those who are manifestly unfit from continuing their kind.”

But who are the “manifestly unfit,” and more important, who decides? For the Nazi “experts” on race profiled in Heather Pringle’s “The Master Plan” (Hyperion, 464 pages, $24.95), the answers seemed no less self-evident than they were to Davenport and Laughlin. The supreme race was the Aryan, as Hitler, Himmler, and other leading Nazis never tired of repeating. The problem, as Ms. Pringle shows in her lucid, deeply researched book, is that the word Aryan had no determinate meaning. Originally a linguistic term invented in the 18th century to describe the hypothetical early language now called Indo-European, it was swiped by German racists to designate the primeval warrior race from which modern Germans purportedly sprung.

But for Himmler, a man whose credulity and intellectual feebleness would be laughable if their consequences hadn’t been so monstrous, the honor of the Aryan name demanded more. All of human history had to be rewritten so that every important accomplishment could be credited to the Aryans. Aryans were the conquerors of India and Greece; Aryans founded an empire on the Black Sea; Aryans had occult knowledge of electricity thousands of years before it was discovered; Aryans stemmed from the lost continent of Atlantis, and before that possibly from outer space. No theory was too ludicrous for Himmler, whose SS was meant to reincarnate the Aryan warrior-farmer nobility. Even Hitler lost patience with his deputy’s obsessions, preferring a more conventional worship of Greco-Roman antiquity. All of these fairy tales, however, had a deadly serious purpose: By establishing the permanent superiority of the German race, they meant to justify the annihilation of the Nazis’ racial enemies, especially the Jews.

The evolution of Nazi racial “science,” from comic idiocy to horror and tragedy, is cleverly told by Ms. Pringle through a focus on one bizarre institution: the Ahnenerbe, a research foundation under the aegis of the SS. Before the war began, the Ahnenerbe (a word Ms. Pringle roughly translates as “something inherited from the forefathers”) specialized in sending archaeological expeditions to sites where the ancient Aryans allegedly left traces – rock carvings in Sweden, temples in Tibet. Like the Nazi Party as a whole, this little niche of Himmler’s empire was extraordinarily hospitable to lunatics and charlatans. One of its “experts,” Karl-Maria Wiligut, claimed to be descended from the god Thor, though he was a psychotic and child molester who had spent time in a Salzburg mental hospital. More disturbing, however, was the Ahnenerbe’s ability to attract genuine scholars – archeologists, linguists, biologists – who eagerly prostituted themselves to Himmler for fancy titles and research grants.

It was during the war that the Ahnenerbe’s crackpot racial science showed its true face. The story of Nazi medical experiments on Jewish prisoners is well-known, but Ms. Pringle sheds new light on it, showing how such tortures were directly related to Nazi eugenic theories. In the most gruesome case, SS scientists selected prisoners from Auschwitz as exemplary Jewish racial types, then gassed them and stripped the flesh from their bones in order to preserve their skeletons for research. Such experiments were a natural conclusion of Nazi racial principles, which held that the survival of the fittest required extermination of the Jewish “bacillus.”

This language, of course, is frighteningly similar to that used by Theodore Roosevelt, when he warned against “the worst stock” and “citizens of the wrong type.” Indeed, when it came to eugenics, the Americans blazed the trail for the Germans. As Mr. Bruinius notes, the Nazi law for the prevention of hereditarily diseased offspring, passed by Hitler in 1934, was modeled after, and explicitly referred to, California’s sterilization law. Fittingly, then, it was World War II and the revelation of the Holocaust that discredited the eugenics movement in the United States – permanently, one would like to say, except that the discovery of increasingly powerful genetic tools makes its revival by no means unthinkable.

The lesson Ms. Pringle and Mr. Bruinius have to teach is a vital one for our technological age: that scientists are not the best guides to the use or implications of their own work. There will always be credentialed ideologues willing to misrepresent self-serving lies as scientific truths. To avoid the perversion of science, we need something more than science – we need wisdom, a quality that no amount of laboratory research can discover.