John B. Calhoun, a research psychologist at the National Institute of Mental Health, displays some of his test subjects in 1971. (Douglas Chevalier/The Washington Post)

John B. Calhoun loved rats. He designed elaborate colonies for the creatures that became a kind of paradise, free of predators and disease, with an unlimited supply of food.

But paradise soon became a crowded hell, and that’s why his work half a century ago has had such a profound impact on our understanding of humans.

Calhoun, a research psychologist at the National Institute of Mental Health for 40 years, discovered that severe crowding produced horrific behavioral changes among animals. The changes were so profound that social order broke down, and ultimately the entire rodent population collapsed.

His findings led to the concept of the “behavioral sink” and suggested that evolution had given animals, perhaps including humans, an innate and irreversible self-destruct button to prevent a species from overpopulating its habitat. He created a doomsday model of what might happen if human beings failed to slow their population growth.

A rat colony in Calhoun’s study. (National Institute of Mental Health)

Calhoun’s work captured the public’s imagination in the mid-1960s, just as awareness was growing of the population explosion and the destruction people had already done to their environment. A 1962 edition of Scientific American on the early results of his experiments became one of the most widely cited papers in psychology, included in anthologies such as “Forty Studies That Changed Psychology.”  Calhoun and his rodents also became the stuff of dystopian fantasy and popular culture, inspiring books, comic books and movies from “Logan’s Run” to “Mrs. Frisby and the Rats of NIMH.” Calhoun wasn’t shy about publicizing his findings or their ominous parallels to human behavior — a practice that created plenty of critics in the scientific community. But Calhoun felt the implications of his work required a wide audience and a sense of urgency.

“Rats and mice, of course, are not perfect models of humans,” he told The Washington Post in a 1971 profile. “But the disaster they represent is so compelling that the world cannot wait for proof of every step in the equation.”

Calhoun, who died in 1995 at the age of 78, left NIMH in less than happy circumstances in the 1980s, about the time the agency’s focus was shifting from animal studies to pharmacology. But he had also achieved the recognition attained by only handful of other social scientists, such as Pavlov and Skinner. His work caught on just as a generation was extolling the virtues of suburbs and fleeing the perceived depravity of the cities, and it propelled the movement for zero population growth and informed our understanding of how people behave in dorms, prisons and other crowded spaces. It can still be felt in architectural and urban design.

“The significance of Calhoun’s work for social thought was unquestionable,” Edmund Ramsden wrote in 2011 for the History of Science Society in the University of Chicago Press.

From Calhoun’s papers at the National Library of Medicine: a graphic of a “rat utopia” or “mouse paradise.”(Fredrick Kunkle)

Calhoun’s calculations regarding human population growth. (Fredrick Kunkle)

It all started in 1947 when Calhoun asked a neighbor if he could build a rat colony on a quarter acre of vacant land behind his house in Towson, Md., according to an 2009 report in the Journal of Social History. Calhoun, who was then at Johns Hopkins University studying ways to control rodent populations, calculated that his “rat city” could have accommodated as many as 5,000 rats, but instead had leveled off at about 150 for reasons that were not obvious.

So Calhoun decided to tweak these colonies to create a rat utopia or a mouse paradise. In an experiment that lasted from 1965 until 1973, he built a mouse colony with 256 “apartments” in towers that resembled high-rises. Ample feeding stations became gathering points for socializing. Steps were taken to improve the colony’s hygiene and reduce disease. Mice that exhibited unusual behavior were marked with paint. Data was collected and coded onto 750,000 punch cards for computers to analyze.

From eight original pairs of mice, the population skyrocketed, until the colony became choked with animals and dysfunctional behaviors appeared.

Dozens of young male mice, unable to find a place in groups dominated by others, became marauding gangs that attacked female and young mice. Sexual and maternal behaviors also underwent dramatic change. Some male mice became exclusively homosexual or hypersexual. Mothers abandoned their pups or sometimes attacked them. Infant mortality soared to as high as 96 percent.

Other behaviors seemed even more bizarre. Groups of female mice — “Pied Pipers,” Calhoun called them — blindly followed foreign objects, such as his shoes, no matter how many times the mice had encountered them; it was as if they were unable to learn. Other mice became inert lumps of fur, “dropouts” that withdrew from society altogether. Oddest of all perhaps were those Calhoun called “the beautiful ones” that spent their days obsessively grooming. Violence and agitation became commonplace, until hardly a mouse could be found that wasn’t speckled with blood, its tail bitten and chewed.

Amid such profound squalor and chaos, the mice forgot how to be mice. They ceased to breed, and their population collapsed.

It was a disturbing vision that seemed to echo the experience of millions in America’s cities. Calhoun fanned the pessimism, making specific predictions that if humans failed to slow their exponential rate of population growth, a similar extinction could befall them by the year 2027.

But what was lost among some was also Calhoun’s optimism, and his belief in the resiliency of human beings. He discovered that some deviant behaviors, in a different light, could be seen as creative activity: One group of submissive males began to dig burrows in a way that reduced social contact but also became more efficient.

And just as Calhoun had altered the animals’ behavior by tinkering with the colony’s physical design, he believed that humans could counter the effects of overcrowding by modifying their environment. Through technology and culture, people could enlarge the “conceptual space” that allows them to live in peace among a multitude. By the late 20th century, he prophesied, the world would be knit together into a single network, and scientists would have unprecedented means to collaborate, perhaps using interconnected computers to form a “World Brain.” New concepts and smarter design would not only allow people to live in proximity but to thrive.

“It was with through this growth in conceptual space — enabled by the design of new buildings, new technologies, new social and intellectual networks — that humanity was presented with a more desirable future,” the Journal of Social History authors said.

And that, according to Calhoun, would be “Dawnsday.”

Calhoun’s papers at the National Library of Medicine include a curious illustration suggesting that he also had a sense of humor. (Fredrick Kunkle)

Read more of Retropolis:

“Great God, he is alive!” The first man executed by electric chair died slower than Thomas Edison expected.

Blood in the water: Four dead, a coast terrified and the birth of modern shark mania

Virginia Tech was not the worst school massacre in U.S. history. This was. 

When Henrietta Lacks had cervical cancer, it was a ‘death sentence.’ Her cells would help change that.

Guinea pigs or pioneers? How Puerto Rican women were used to test the birth control pill.