Donald Trump speaks at a campaign rally in Bloomington, Illinois, on March 13.

At the 1934 public mock trial of Adolf Hitler, organized by the American Jewish Congress and held at Madison Square Garden in New York City, professor of medicine Lewellys F. Barker defined Hitlerism as “a ‘psychic epidemic’ … an abnormal emotional mass movement that reminds us of the Dark Ages.” Barker argued, to a crowd of 20,000: “To understand Hitler and Hitlerism, one is compelled to enter the domain of psychopathology.” At the bitter end of an election season full of armchair diagnoses—what’s wrong with Donald Trump? What’s wrong with his supporters?—the scene feels frighteningly familiar. But is a psychological diagnosis ever a useful way to view racism? Or does it merely absolve the racist of blame for his actions?

In a forthcoming book, Are Racists Crazy? How Prejudice, Racism, and Antisemitism Became Markers of Insanity, historian Sander Gilman and sociologist James M. Thomas look at the centurieslong project to pinpoint the psychological origins of racism. Over the years, psychologists, doctors, and sociologists have wrangled over the source of racial prejudice, with some arguing that this “madness” is inspired by the toxic influence of a crowd and others looking to an individual’s particular neurology for answers.

In the 18th and 19th centuries, people thinking about the relationship between psychology and race weren’t thinking about the source of racial prejudice. Instead, they emphasized nonwhite people’s supposed biological predisposition to mental illness. “With very few exceptions, everyone across the board in the West—England, the United States, Canada, France—working in the sciences, and in popular culture, believed that. It was a truism,” Gilman told me. One example: Using numbers from the 1840 census, which discovered a high rate of insanity among free black people in the North, pro-slavery mid-19th-century reformers and physicians argued that Southern black people needed slavery to thrive. (That these 1840 numbers were contested, notably by Edward Jarvis, a physician who wrote a critique of the census findings in 1844, meant little to people who were looking for medical evidence for black inferiority.)

Scientists have since taken a centurylong turn away from thinking about race as a biological category, instead investing in the idea that it’s a social construct, and the belief that minority groups might have innate psychological traits in common has lost its appeal. But at the same time, Gilman and Thomas argue, the quest to understand racism as a group sickness has accelerated. Theorists fixated on the actions of crowds in the late 19th and early 20th centuries, and this field of study was a gateway to a new analysis of racial prejudice. As World War I approached, observers outside Germany struggled to understand its politics, writing studies of its “national character.” Building on an 1895 work by French physician Gustave Le Bon, The Crowd: A Study of the Popular Mind, British neurosurgeon Wilfred Trotter wrote Instincts of the Herd in Peace and War (1916), a case study of German group consciousness. Peering in at Germany’s psychology, Trotter marveled at the hatred Germans ginned up for England, diagnosing it as primal or biological: “The fact that it was possible to organize so unanimous a howl shows very clearly how fully the psychological mechanisms of the wolf were in action.”

In the mid-1930s, as Hitler rose, Germans became prime examples of people infected by prejudice and object lessons for psychologists trying to understand the way racism operated as groupthink. Psychologist Wilhelm Reich’s The Mass Psychology of Fascism (1933) argued that racism was “a symptom of sexual repression in the mind of the authoritarian racist.” A racist, Reich believed, was likely raised in a kind of family that denied its children’s sexuality and created tractable individuals, who could easily be guided by a strong state.

During wartime, in the United States, the study of social psychology blossomed, as the threat from Hitler and his followers became all too clear. “If racism was a symptom of the psychopathology of fascism,” Gilman and Thomas write, “understanding the fascist mind had to be one of the operative undertakings of the war effort.” A project at the University of California at Berkeley worked on the question of susceptibility to authoritarianism, which resulted in the creation of a personality test called the F-scale and the 1950 volume The Authoritarian Personality. The researchers found that authoritarians tended to express prejudice against a diverse range of minority groups, to identify with the strong as opposed to the weak, and to be fascinated by the “deviant” sex lives of others.

These diagnostic efforts to understand German psychology had a purpose: avoiding the rise of other fascisms worldwide and proposing measures for German rehabilitation in the postwar period. The Institute for Social Research at Columbia University, host to exiled members of the Frankfurt School, looked at prejudice as a mass psychosis, fixable by certain measures of re-education. Psychiatrist Richard Brickner’s 1943 book Is Germany Incurable? proposed “a vast educational program” to correct Germany’s “paranoid contagion,” once the war should be won. If an occupying government were to school German children in tolerance, Brickner thought German adults could be reached, and their minds changed, in their own homes. Brickner’s proposal was not uncontroversial, Gilman and Thomas write, noting that psychologist Erich Fromm responded to Brickner by arguing that to cast Germany’s actions as a sickness served as “a substitute for valid ethical concepts,” tending to “weaken the sense for moral values, by calling something by a psychiatric term when it should be called plainly evil.”

(Gilman and Thomas are careful to point out that, amid these efforts to understand the racist mind, mass psychological diagnoses of the problems of nonwhite groups continued apace—resulting in postwar projects such as the Clark doll test, which diagnosed black children with endemic self-hatred, induced by experiences of segregation. While these postwar projects were generally sympathetic toward minority groups, the effect, Gilman and Thomas argue, was still to pathologize those affected by racism on the basis of their race, while absolving the racist majority by diagnosing racism as a collective madness.)

Between the civil rights era and today, psychologists in the United States have shifted from a crowd-based explanatory model to an attempt to identify individual cognitive explanations for racism—a neuroscientific approach that promises specific treatment plans, perhaps even a pill that doctors might one day prescribe to reduce implicit bias. While a pill for racism remains a thought experiment, the use of the term disease to describe racism is increasingly common in popular culture. When high-profile people are caught making racist or anti-Semitic statements, they often offer apologies that tap into the language of therapy: “I have begun an ongoing program of recovery,” Mel Gibson said in a formal statement after being caught on tape in an anti-Semitic rant during a DUI arrest in 2006. Pathological language around racism is also increasing, Gilman and Thomas write, in “scholarly articles, treatment protocols, academic conference presentations, and ‘shoptalk’ among behavioral and social scientists.”

After looking at the long history of psychology’s approach to racism, Gilman has come to the conclusion that we’ve gone around in circles. “There’s been a lot of discussion lately of ‘baskets of deplorables,’ ” Gilman said. “Of notions that groups of people, by definition, hold racist views.” But, he went on, “what that does is eliminates any responsibility for the individual. Racism is not an unconscious following of a leader—Trump, Hitler, whatever—but rather a whole set of individual people making choices.”

The disease metaphor of racism has given a century and a half of liberal anti-racists hope for a cure for prejudice. But it doesn’t work, and in the process, it gives too many people an out. “Let’s not avoid responsibility,” Gilman begged, echoing Fromm’s World War II–era plea. “Let’s make sure people who say evil things, who do evil things, who believe evil things have to take responsibility.”

John Blanding/The Boston Globe via Getty Images

Demonstrators gather to rally against Donald Trump at the Parkman Bandstand in Boston Common on Wednesday.