People explore the National Museum of African American History and Culture in Washington, D.C. (Matt McClain/The Washington Post)

The mysterious and chronic sickness had been afflicting slaves for years, working its way into their minds and causing them to flee from their plantations.

Unknown in medical literature, its troubling symptoms were familiar to masters and overseers, especially in the South, where hundreds of enslaved people ran from captivity every year.

On March 12, 1851, the noted physician Samuel A. Cartwright reported to the Medical Association of Louisiana that he had identified the malady and, by combining two Greek terms, given it a name: Drapetomania.

Drapetes, a runaway, and mania, madness.

He also announced that it was completely curable.

Negroes, with their smaller brains and blood vessels, and their tendency toward indolence and barbarism, Cartwright told fellow doctors, had only to be kept benevolently in the state of submission, awe and reverence that God had ordained.

“The Negro is [then] spellbound, and cannot run away,” he said.

Cartwright’s presentation a decade before the Civil War was part of the long, insidious practice of what historians call scientific racism — the spread of bogus theories of supposed black inferiority in an attempt to rationalize slavery and centuries of social and economic domination and plunder.

Here, enslaved people were beneath even the human desire for freedom. They had to be diseased.

This thinking would thrive in the 18th and especially the 19th centuries. It would mutate, vary in perversion and persevere for 400 years right up to the present day. Starting with theories of physical and intellectual inferiority that likened blacks to animals — monkeys and apes especially — or helpless children, it would evolve to infer black cultural and then social inferiority.

“What black inferiority meant has changed in every generation . . . but ultimately Americans have been making the same case,” said historian Ibram X. Kendi.

Such thought exists today with pernicious assumptions about the current nature of black life and black people, still featuring age-old racist references to blacks as animals. It persists despite the advent of modern DNA science, which has shown race to be fundamentally a social construct. Humans, as it turns out, share about 99.9 percent of their DNA with each other, and outward physical characteristics such as hair texture and skin color, about which racists have long obsessed, occupy just a tiny portion of the human genome.

Even so, many Americans, blind to the origins of racist notions, “think that there’s such things as black blood and black diseases and that black people are by nature predisposed to dancing and athletics,” Kendi said. “These are common ideas.”

Modern examples — sometimes overt, sometimes seemingly springing from the collective American subconscious — underscore the insidiousness of pseudoscientific ideas about race that were first promoted in earlier centuries.

Consider comedian Roseanne Barr’s use of an ape analogy in a tweet about Valerie Jarrett, an African American adviser to President Barack Obama, which led to the cancellation of Barr’s ABC television show.

Now consider Cartwright’s claims in 1851 that, among other things, a Negro withstood the rays of the sun better because of an eye feature like one found in apes.

Cartwright also speciously observed that the black man’s neck was shorter than a white person’s, his “bile” was a deeper color, his blood blacker, his feet flatter, his skull different.

Yet, in addition to his keen eyesight, he had other animal-like senses, smelling better and hearing better than the white man.

“Like children, [Negroes] require government in everything . . . or they will run into excesses,” Cartwright said. Slavery, he concluded unsurprisingly, was for the enslaved person’s own good.

The twisted vestiges of scientific racism continue to inspire white hatred of and violence toward blacks today.

“Anyone who thinks that White and black people look as different as we do on the outside, but are somehow magically the same on the inside, is delusional,” mass murderer Dylann Roof wrote in the crude manifesto that he posted on the Internet in 2015. “Negroes have lower Iqs, lower impulse control, and higher testosterone levels in generals. These three things alone are a recipe for violent behavior.”

On June 17, 2015, Roof went into an African American church in Charleston, S.C., and shot nine black worshipers to death. He was convicted of murder and sentenced to death.

'More sensation than reflection'

Self-interested justifications for atrocities against and the oppression of African Americans go back to the 1400s and an early Portuguese defense of slave trading written by Gomes Eanes de Zurara, wrote Kendi, a professor at American University in Washington, in his book, “Stamped From the Beginning: The Definitive History of Racist Ideas in America,” which won the 2016 National Book Award.

Zurara wrote that captured Africans had “lived like beasts, without any custom of reasonable beings . . . [and] only knew how to live in bestial sloth.” Once enslaved, their souls could be saved and their lives improved, he said.

On this side of the Atlantic, Thomas Jefferson played an early and highly influential role in the establishment of pseudoscientific ideas about black racial inferiority.

On Feb. 27, 1787, more than a decade after he helped write the Declaration of Independence, future president Jefferson published his book “Notes on the State of Virginia,” an extensive study of subjects including his state’s geography, climate, religion and its enslaved black population.

The book made clear that when the revered Founding Father said it was “self-evident, that all men are created equal,” he was not including black people.

“In general, their existence appears to participate more of sensation than reflection,” wrote Jefferson, whose livelihood depended on the existence of slavery. “In imagination they are dull [and] tasteless. . . . This unfortunate difference of colour, and perhaps of faculty, is a powerful obstacle to the emancipation of these people.”

“Deep rooted prejudices . . . real distinctions which nature has made . . . and many other circumstances will probably never end but in the extermination of the one or the other race,” he wrote.

It was perhaps the most damaging and enduring instance of scientific racism in American history, Kendi said.

“This was one of the . . . best selling nonfiction books in early America,” he said. “And black and other anti-racist activists were arguing against Jefferson’s theory of black intellectual inferiority into the 1830s.”

'Pervading darkness'

In 1849, Samuel Cartwright was engaged by a Louisiana medical committee to investigate “the diseases and physical peculiarities of our negro population.”

He seemed well qualified. The 57-year-old native of Fairfax County, Va., had practiced in Natchez, Miss., for 25 years, and his patients included his friend Jefferson Davis, the future president of the Confederacy. The same year the report was issued, he was appointed professor of “diseases of the Negro” at what is now Tulane University.

He began his report for the Louisiana committee by reviewing “the anatomical and physiological differences between the negro and the white man.”

Skin color was obvious.

But “there are other differences more deep, durable and indelible,” he wrote. “The membranes, the muscles, the tendons . . . even the negro’s brain and nerves . . . are tinctured with a shade of pervading darkness.”

Then there was the true cause of the enslaved person’s “debasement of mind,” he wrote.

“It is . . . [the] defective hematosis, or atmosperization of the blood, conjoined with a deficiency of cerebral matter in the cranium . . . [that] has rendered the people of Africa unable to take care of themselves,” he claimed.

Although Cartwright’s ideas were actually part of a long racist tradition, by the time he rendered them they had a new urgency, said Khalil Muhammad, professor of history, race and public policy at Harvard University’s Kennedy School of Government.

The rise of the movement to abolish slavery “created a crisis of knowledge about . . . who people of African descent were in the hierarchy of man, and what precisely were they capable of,” he said.

Until then, he said, despite pronouncements like Jefferson’s, science wasn’t essential to justifying slavery. Now, under threat, the then-250-year-old institution was direly in need of a “scientific” rationale.

“There is a great convulsion before us,” Josiah C. Nott, a South Carolina physician, anthropologist and a future medical director in the Confederate army, told a Southern Rights Association meeting in 1851.

“It is time that we should arouse from our lethargy and prepare for the crises,” he said.

Nott offered his pseudoscientific rationale: “Look around you . . . at the Negro races,” Nott said. “Their physical type is peculiar; their grade of intellect is greatly inferior; they are utterly wanting in moral and physical energy.”

Embedded within his speech was a not-so-hidden motive: The institution of slavery, he said, “has grown up with us from our infancy, it has become part of our very being; our national prosperity and domestic happiness are inseparable from it.”

Evolutionary ladder

Around that time, in 1850, an enslaved man named Jack stood before a camera in Columbia, S.C. His face was deeply creased, perhaps from age, perhaps from long exposure to the weather.

He was described as a “driver” — of what was not specified. Livestock? Wagons? People?

Originally from Guinea, in West Africa, he was owned by one B.F. Taylor Esq., who had a plantation in Columbia, S.C.

That March, Jack, who looked about 40, and six other enslaved men and women, were brought to the studio to have their photographs taken. They were specimens, and for the most part they were pictured naked.

The images were made at the behest of Louis Agassiz, the famous Swiss American scientist and Harvard professor, who was studying what was called “polygenism.”

This was the latest “scientific” tool applied to the idea of supposed black inferiority: the now-discredited notion that man sprang from numerous sources and that “races” could therefore be categorized and ranked.

It would carry well into the 20th century.

The photographs were “designed to analyze the physical differences between European whites and African blacks, but at the same time . . . prove the superiority of the white race,” photography scholar Brian Wallis wrote in a 1995 essay on the pictures.

“In nineteenth-century anthropology, blacks were often situated along the evolutionary ladder midway between a classical ideal and the orangutan,” he wrote.

Such thinking went with the rise in the early 1900s of modern eugenics — the idea that a “race” could and should be purified by selective breeding and the elimination of flawed peoples.

In 1916, a New York lawyer and racial theorist named Madison Grant wrote a notorious book called “The Passing of the Great Race.”

Grant, whose father, a Union army doctor, had earned the Medal of Honor in the Civil War, believed in a rigid racial hierarchy, with “nordics” at the top and blacks and others at the bottom.

“Negroes have demonstrated throughout recorded time that they are a stationary species and that they do not possess the potentiality of progress or initiative from within,” Grant wrote.

His book was translated into several languages.

One reader in Germany was especially admiring. He, too, mused about extermination, but of a different “race.” His name was Adolf Hitler, and he reportedly referred to his copy of the book as his Bible.

In 1936, African American sprinter Jesse Owens smashed the ideas of Hitler and Madison when he won four gold medals at the Berlin Olympics.

But Owens’s own track coach belittled the success of black runners: “It was not long ago that his ability to sprint and jump was a life-and-death matter to him in the jungle.”

The old notion lived on, and so have many white social and economic advantages.

Even when “Americans have discarded old racist ideas, new racist ideas have constantly been produced for their renewed consumption,” Kendi wrote.

Some day, he hoped, the time will come “when Americans will realize that the only thing wrong with black people is that they think there is something wrong with black people.”

Magda Jean-Louis contributed to this report.