universe 25 experiment documentary

Distillations magazine

Mouse heaven or mouse hell.

Biologist John Calhoun’s rodent experiments gripped a society consumed by fears of overpopulation.

Officially, the colony was called the Mortality-Inhibiting Environment for Mice. Unofficially, it was called mouse heaven.

Biologist John Calhoun built the colony at the National Institute of Mental Health in Maryland in 1968. It was a large pen—a 4½-foot cube—with everything a mouse could ever desire: plenty of food and water; a perfect climate; reams of paper to make cozy nests; and 256 separate apartments, accessible via mesh tubes bolted to the walls. Calhoun also screened the mice to eliminate disease. Free from predators and other worries, a mouse could theoretically live to an extraordinarily old age there, without a single worry.

But the thing is, this wasn’t Calhoun’s first rodent utopia. This was the 25th iteration. And by this point he knew how quickly mouse heaven could deteriorate into mouse hell.

John Calhoun grew up in Tennessee, the son of a high school principal and an artist, and was an avid birder when young. After earning his PhD in zoology, he joined the Rodent Ecology Project in Baltimore in 1946, whose purpose was to eliminate rodent pests in cities. The project had limited success, partly because no one could figure out what aspects of rodent behavior, lifestyle, or biology to target. Calhoun set up his first utopia, involving Norway rats, in the woods behind his house to monitor rodents over time and figure out what factors drove their population growth.

Eventually Calhoun grew fascinated with the rodent behavior for its own sake and began crafting ever more elaborate and carefully controlled environments. It wasn’t just the behavior of rats that interested him. Architects and civil engineers at the time were having vigorous debates about how to build better cities, and Calhoun imagined urban design might be studied in rodents first and then extrapolated to human beings.

Calhoun’s most famous utopia, number 25, began in July 1968, when he introduced eight albino mice into the 4½-foot cube. Following an adjustment period, the first pups were born 3½ months later, and the population doubled every 55 days afterward. Eventually this torrid growth slowed, but the population continued to climb, peaking at 2,200 mice during the 19th month.

That robust growth masked some serious problems, however. In the wild, infant mortality among mice is high, as most juveniles get eaten by predators or perish of disease or cold. In mouse utopia, juveniles rarely died. As a result, there were far more youngsters than normal, which introduced several difficulties.

Rodents have social hierarchies, with dominant alpha males controlling harems of females. Alphas establish dominance by fighting—wrestling and biting any challengers. Normally a mouse that loses a fight will scurry off to some distant nook to start over elsewhere.

But in mouse utopia, the losing mice couldn’t escape. Calhoun called them “dropouts.” And because so few juveniles died, huge hordes of dropouts would gather in the center of the pen. They were full of cuts and ugly scars, and every so often huge brawls would break out—vicious free-for-alls of biting and clawing that served no obvious purpose. It was just senseless violence. (In earlier utopias involving rats, some dropouts turned to cannibalism.)

Alpha males struggled, too. They kept their harems in private apartments, which they had to defend from challengers. But given how many mice survived to adulthood, there were always a dozen hotshots ready to fight. The alphas soon grew exhausted, and some stopped defending their apartments altogether.

As a result, apartments with nursing females were regularly invaded by rogue males. The mothers fought back, but often to the detriment of their young. Many stressed-out mothers booted their pups from the nest early, before the pups were ready. A few even attacked their own young amid the violence or abandoned them while fleeing to different apartments, leaving the pups to die of neglect.

Eventually other deviant behavior emerged. Mice who had been raised improperly or kicked out of the nest early often failed to develop healthy social bonds, and therefore struggled in adulthood with social interactions. Maladjusted females began isolating themselves like hermits in empty apartments—unusual behavior among mice. Maladjusted males, meanwhile, took to grooming all day—preening and licking themselves hour after hour. Calhoun called them “the beautiful ones.” And yet, even while obsessing over their appearance, these males had zero interest in courting females, zero interest in sex.

Intriguingly, Calhoun had noticed in earlier utopias that such maladjusted behavior could spread like a contagion from mouse to mouse. He dubbed this phenomenon “the behavioral sink.”

Between the lack of sex, which lowered the birth rate, and inability to raise pups properly, which sharply increased infant mortality, the population of Universe 25 began to plummet. By the 21st month, newborn pups rarely survived more than a few days. Soon, new births stopped altogether. Older mice lingered for a while—hiding like hermits or grooming all day—but eventually they died out as well. By spring 1973, less than five years after the experiment started, the population had crashed from 2,200 to 0. Mouse heaven had gone extinct.

Universe 25 ended a half century ago, but it continues to fascinate people today—especially as a gloomy metaphor for human society. Calhoun actively encouraged such speculation, once writing, “I shall largely speak of mice, but my thoughts are on man.” As early as 1968, journalist Tom Wolfe titled an essay about New York “O Rotten Gotham—Sliding Down into the Behavioral Sink.” Oddly, though, none of the prognosticators could agree on the main lesson of Universe 25.

The first people to fret over Universe 25 were environmentalists. The same year the study began, biologist Paul Ehrlich published The Population Bomb , an alarmist book predicting imminent starvation and population crashes due to overpopulation on Earth. Pop culture picked up on this theme in movies, such as Soylent Green , where humans in crowded cities are culled and turned into food slurry. Overall, the idea of dangerous overcrowding was in the air, and some sociologists explicitly drew on Calhoun’s work, writing: “We . . . take the animal studies as a serious model for human populations.” The message was stark: Curb population growth—or else .

More recently scholars saw similarities to the Industrial Revolution and the rise of modern urban society. The 19th and 20th centuries saw population booms across the world, largely due to drops in infant mortality—similar to what the mice experienced. Recently, however, human birth rates have dropped sharply in many developed countries—often below replacement levels—and young people in those places have reportedly lost interest in sex. The parallels to Universe 25 seem spooky.

Behavioral biologists have echoed the eugenics movement in blaming the strange behaviors of the mice on a lack of natural selection, which in their view culls those they consider weak and unfit to breed. This lack of culling resulted in supposed “mutational meltdowns” that led to widespread mouse stupidity and aberrant behavior. (The researchers argued that the brain is especially susceptible to mutations because it’s so intricate and because so many of our genes influence brain function.)

Extrapolating from this work, some political agitators warn that humankind will face a similar decline. Women are supposedly falling into Calhoun’s behavioral sink by learning “maladaptive behaviors,” such as choosing not to have children, which “destroy[s] their own genetic interests.” Other critics agonize over the supposed loss of traditional gender roles, leaving effete males and hyperaggressive females, or they deplore the undermining of religions and their imperatives to “be fruitful and multiply.” In tandem, such changes will lead to the “decline of the West.”

Still others have cast Universe 25’s collapse as a parable illustrating the dangers of socialist welfare states, which, they argue, provide material goods but remove healthy challenges from people’s lives, challenges that build character and promote “personal growth.” Another school of thought viewed Universe 25 as a warning about “the city [as] a perversion of nature.” As sociologists Claude Fischer and Mark Baldassare put it, “A red-eyed, sharp-fanged obsession about urban life stalks contemporary thought.”

Most critics who’ve fretted over Calhoun’s work cluster on the conservative end of the political spectrum, but self-styled progressives have weighed in as well. Advocates for birth control repeatedly invoked Calhoun’s mice as a cautionary tale about how runaway population growth destroys family life. More recent interpretations see the mice collapse in terms of one-percenters and wealth inequality; they blame the social dysfunction on a few aggressive males hoarding precious resources (e.g., desirable apartments). In this view, said one critic, “Universe 25 had a fair distribution problem” above all.

Given these wildly varying (even contradictory) readings, it’s hard to escape the suspicion that personal and political views, rather than objective inquiry, are driving these critics’ outlooks. And indeed, a closer look at the interpretations severely undermines them.

When forecasting population crashes among human beings, Population Bomb –type environmentalists invariably predicted that overcrowding would lead to widespread shortages of food and other goods. That’s actually the opposite of what Universe 25 was like. The mice there had all the goods they wanted. This also undermines arguments about unfair resource distribution.

Perhaps, then, it was the lack of struggles and challenges that led to dysfunction, as welfare critics claimed. Except that the spiral of dysfunction began when hordes of “dropout” mice lost challenges to alpha males, couldn’t escape elsewhere, and began brawling in the middle of the pen. The alpha males in turn grew weary after too many challenges from youngsters. Indeed, most mice faced competition far in excess of what they would encounter in the wild.

The appearance of the sexless “beautiful ones” does seem decadent and echoes the reported loss of interest in sex among young people in developed countries. Except that a closer look at the survey data indicates that such worries might be overblown. And any comparison between human birth rates and Universe 25 birth rates is complicated by the fact that the mouse rates dropped partly due to infant neglect and spikes in infant mortality—the opposite of the situation in the developed world.

Then there are the warnings about the mutational meltdown and the decline of intelligence. Aside from echoing the darkest rhetoric of the eugenics movement, this interpretation runs aground on several points. The hermit females and preening, asexual males certainly acted oddly—but in doing so, they avoided the vicious, violent free-for-alls that beset earlier generations. This hardly seems dumb. Moreover, some of Calhoun’s research actually saw rodents getting smarter during experiments.

This evidence came from an earlier utopia involving rats. In that setup, dropout rats began digging new burrows into the dirt floor of their pen. Digging produces loose dirt to clear away, and most rats laboriously carried the loose dirt outside the tunnel bit by bit, to dump it there. It’s necessary but tedious work.

But some of the dropout rats did something different. Instead of carrying dirt out bit by bit, they packed it all into a ball and rolled it out the tunnel in one trip. An enthused Calhoun compared this innovation to humankind inventing the wheel. And it happened only because the rats were isolated from the main group and didn’t learn the dominant method of digging. By normal rat standards, this was deviant behavior. It was also a creative breakthrough. Overall, then, Calhoun argued that social strife can sometimes push creatures to become smarter, not dumber.

(Incidentally, after Universe 25’s collapse, Calhoun began building new utopias to encourage creative behavior by keeping mice physically and mentally nourished. This research, in turn, inspired a children’s book named after Calhoun’s workplace— Mrs. Frisby and the Rats of NIMH , wherein a group of rats escape from a colony designed to stimulate their intelligence.)

So if all these interpretations of Universe 25 miss the mark, what lesson can we draw from the experiment?

Calhoun’s big takeaway involved status. Again, the males who lost the fights for dominance couldn’t leave to start over elsewhere. As he saw it, they were stuck in pathetic, humiliating roles and lacked a meaningful place in society. The same went for females when they couldn’t nurse or raise pups properly. Both groups became depressed and angry, and began lashing out. In other words, because mice are social animals, they need meaningful social roles to feel fulfilled. Humans are social animals as well, and without a meaningful role, we too can become hostile and lash out.

Still, even this interpretation seems like a stretch. Humans have far more ways of finding meaning in life than pumping out children or dominating some little hierarchy. And while human beings and mice are indeed both social creatures, that common label papers over some major differences. Critics of Calhoun’s work argued that population density among humans—a statistical measure—doesn’t necessarily correlate with crowding —a feeling of psychological stress. In the words of one historian, “Through their intelligence, adaptability, and capacity to make the world around them, humans were capable of coping with crowding” in ways that mice simply are not.

Ultimately Calhoun’s work functions like a Rorschach blot—people see what they want to see. It’s worth remembering that not all lab experiments, especially contrived ones such as Universe 25, apply to the real world. In which case, perhaps the best lesson to learn here is a meta-lesson: that drawing lessons itself can be a dangerous thing.

Sam Kean is a best-selling science author. His latest book is  The Icepick Surgeon: Murder, Fraud, Sabotage, Piracy, and Other Dastardly Deeds Perpetrated in the Name of Science .

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This Old Experiment With Mice Led to Bleak Predictions for Humanity’s Future

From the 1950s to the 1970s, researcher John Calhoun gave rodents unlimited food and studied their behavior in overcrowded conditions

Maris Fessenden ; Updated by Rudy Molinek

What does utopia look like for mice and rats? According to a researcher who did most of his work in the 1950s through 1970s, it might include limitless food, multiple levels and secluded little condos. These were all part of John Calhoun’s experiments to study the effects of population density on behavior. But what looked like rodent paradises at first quickly spiraled into out-of-control overcrowding, eventual population collapse and seemingly sinister behavior patterns.

In other words, the mice were not nice.

Working with rats between 1958 and 1962, and with mice from 1968 to 1972, Calhoun set up experimental rodent enclosures at the National Institute of Mental Health’s Laboratory of Psychology. He hoped to learn more about how humans might behave in a crowded future. His first 24 attempts ended early due to constraints on laboratory space. But his 25th attempt at a utopian habitat, which began in 1968, would become a landmark psychological study. According to Gizmodo ’s Esther Inglis-Arkell, Calhoun’s “Universe 25” started when the researcher dropped four female and four male mice into the enclosure.

By the 560th day, the population peaked with over 2,200 individuals scurrying around, waiting for food and sometimes erupting into open brawls. These mice spent most of their time in the presence of hundreds of other mice. When they became adults, those mice that managed to produce offspring were so stressed out that parenting became an afterthought.

“Few females carried pregnancies to term, and the ones that did seemed to simply forget about their babies,” wrote Inglis-Arkell in 2015. “They’d move half their litter away from danger and forget the rest. Sometimes they’d drop and abandon a baby while they were carrying it.”

A select group of mice, which Calhoun called “the beautiful ones,” secluded themselves in protected places with a guard posted at the entry. They didn’t seek out mates or fight with other mice, wrote Will Wiles in Cabinet magazine in 2011, “they just ate, slept and groomed, wrapped in narcissistic introspection.”

Eventually, several factors combined to doom the experiment. The beautiful ones’ chaste behavior lowered the birth rate. Meanwhile, out in the overcrowded common areas, the few remaining parents’ neglect increased infant mortality. These factors sent the mice society over a demographic cliff. Just over a month after population peaked, around day 600, according to Distillations magazine ’s Sam Kean, no baby mice were surviving more than a few days. The society plummeted toward extinction as the remaining adult mice were just “hiding like hermits or grooming all day” before dying out, writes Kean.

Calhoun launched his experiments with the intent of translating his findings to human behavior. Ideas of a dangerously overcrowded human population were popularized by Thomas Malthus at the end of the 18th century with his book An Essay on the Principle of Population . Malthus theorized that populations would expand far faster than food production, leading to poverty and societal decline. Then, in 1968, the same year Calhoun set his ill-fated utopia in motion, Stanford University entomologist Paul Ehrlich published The Population Bomb . The book sparked widespread fears of an overcrowded and dystopic imminent future, beginning with the line, “The battle to feed all of humanity is over.”

Ehrlich suggested that the impending collapse mirrored the conditions Calhoun would find in his experiments. The cause, wrote Charles C. Mann for Smithsonian magazine in 2018, would be “too many people, packed into too-tight spaces, taking too much from the earth. Unless humanity cut down its numbers—soon—all of us would face ‘mass starvation’ on ‘a dying planet.’”

Calhoun’s experiments were interpreted at the time as evidence of what could happen in an overpopulated world. The unusual behaviors he observed—such as open violence, a lack of interest in sex and poor pup-rearing—he dubbed “behavioral sinks.”

After Calhoun wrote about his findings in a 1962 issue of Scientific American , that term caught on in popular culture, according to a paper published in the Journal of Social History . The work tapped into the era’s feeling of dread that crowded urban areas heralded the risk of moral decay.

Events like the murder of Kitty Genovese in 1964—in which false reports claimed 37 witnesses stood by and did nothing as Genovese was stabbed repeatedly—only served to intensify the worry. Despite the misinformation, media discussed the case widely as emblematic of rampant urban moral decay. A host of science fiction works—films like Soylent Green , comics like 2000 AD —played on Calhoun’s ideas and those of his contemporaries . For example, Soylent Green ’s vision of a dystopic future was set in a world maligned by pollution, poverty and overpopulation.

Now, interpretations of Calhoun’s work have changed. Inglis-Arkell explains that the main problem of the habitats he created wasn’t really a lack of space. Rather, it seems likely that Universe 25’s design enabled aggressive mice to stake out prime territory and guard the pens for a limited number of mice, leading to overcrowding in the rest of the world.

However we interpret Calhoun’s experiments, though, we can take comfort in the fact that humans are not rodents. Follow-up experiments by other researchers, which looked at human subjects, found that crowded conditions didn’t necessarily lead to negative outcomes like stress, aggression or discomfort.

“Rats may suffer from crowding,” medical historian Edmund Ramsden told the NIH Record ’s Carla Garnett in 2008, “human beings can cope.”

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Maris Fessenden is a freelance science writer and artist who appreciates small things and wide open spaces.

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Rudy Molinek is  Smithsonian  magazine's 2024 AAAS Mass Media Fellow.

• May 2022, Issue 1
• Foundations

Universe 25 Experiment

A series of rodent experiments showed that even with abundant food and water, personal space is essential to prevent societal collapse, but universe 25's relevance to humans remains disputed..

Stephanie "Annie" Melchor is a freelancer and former intern for The Scientist .

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J une 22, 1972. John Calhoun stood over the abandoned husk of what had once been a thriving metropolis of thousands. Now, the population had dwindled to just 122, and soon, even these inhabitants would be dead. 

Calhoun wasn’t the survivor of a natural disaster or nuclear meltdown; rather, he was a researcher at the National Institute of Mental Health conducting an experiment into the effects of overcrowding on mouse behavior. The results , laid bare at his feet, had taken years to play out.

Universe 25 Experiment Explained

In 1968, Calhoun had started the experiment by introducing four mouse couples into a specially designed pen—a veritable rodent Garden of Eden—with numerous “apartments,” abundant nesting supplies, and unlimited food and water. The only scarce resource in this microcosm was physical space, and Calhoun suspected that it was only a matter of time before this caused trouble in paradise.

Calhoun had been running similar experiments with rodents for decades but had always had to end them prematurely, ironically because of laboratory space constraints, says Edmund Ramsden, a science historian at Queen Mary University of London. This iteration, dubbed Universe 25, was the first crowding experiment he ran to completion.

As he had anticipated, the utopia became hellish nearly a year in when the population density began to peak, and then population growth abruptly and dramatically slowed. Animals became increasingly violent, developed abnormal sexual behaviors, and began neglecting or even attacking their own pups. 

Eventually Universe 25 took another disturbing turn. Mice born into the chaos couldn’t form normal social bonds or engage in complex social behaviors such as courtship, mating, and pup-rearing. Instead of interacting with their peers, males compulsively groomed themselves; females stopped getting pregnant. Effectively, says Ramsden, they became “trapped in an infantile state of early development,” even when removed from Universe 25 and introduced to “normal” mice. Ultimately, the colony died out. “There’s no recovery, and that’s what was so shocking to [Calhoun],” says Ramsden. 

Debunking Popular Interpretations of Universe 25

Calhoun wasn’t shy about anthropomorphizing his findings, binning rodents into categories such as “juvenile delinquents” and “social dropouts,” and others seized on these human parallels. Population growth in the 1970s was swelling, and films such as Soylent Green  tapped into growing fears of overpopulation and urban violence. In a 2011 article , Ramsden writes that Calhoun’s studies were brandished by others to justify population control efforts largely targeted at poor and marginalized communities. 

But Ramsden notes that Calhoun didn’t necessarily think humanity was doomed. In some of Calhoun’s other crowding experiments, rodents developed innovative tunneling behaviors, while in others, adding more rooms allowed the animals to live in the high-density environment without being forced into unwanted contact with others, largely minimizing the negative social consequences. According to Ramsden, Calhoun wanted these findings to influence the architectural design of prisons, mental hospitals, and other buildings prone to crowding. Writing in a report summary in 1979, Calhoun noted that “no single area of intellectual effort can exert a greater influence on human welfare than that contributing to better design of the built environment.” 

Relevance and Criticisms of the Universe 25 Experiment

Looking back on the Universe 25 experiment with present day scientific perspective, the limits of its interpretations are evident. The research was largely observational and subjective. Calhoun described his study as “not normal science,” referring to it instead as an “observation and reconstruction of a process.” 2 Observational studies have a higher risk of bias and confusing correlation with causation. 3 Scientists have suggested that Universe 25 suffers from inaccurate interpretation of experimental outcomes, methods, and potentially confounding variables, 4 which reflect information bias . 3 For instance, at the time that Calhoun presented and published Universe 25’s results, his peers inquired about unsanitary animal husbandry and a lack of quantitative stress hormone measurements as potential confounding or missing information pertinent to Calhoun’s conclusions. 2

Importantly, despite popular interpretations of Universe 25 deeming it informative about urban crowding, many human studies on crowding and population density have yielded inconsistent results. 4 Behavioral scientists today largely acknowledge that how humans experience and respond to crowding is governed by a range of individual-specific social and psychological factors , including personal autonomy and social roles or contexts. 4 In some ways, this aligns with how Calhoun discussed his Universe 25 findings, not as effects of population density per se but effects of altered social interactions . 2 Additionally, the Universe 25 experiment did not address systemic determinants of well-being at the time, nor does it reflect present-day systems that are endemic to the human experience. The societal implications of increased population density and its effects on human beings are a far throw from Universe 25’s experimental design and the behavioral changes that Calhoun observed in his caged rodent experiments. 2,4

Finally, from an ethical standpoint, Calhoun’s experiments would not be permitted today. The mouse universes that Calhoun created intentionally placed its study subjects into constructed environments that caused harm. The study conditions were maintained despite evident animal distress, and many preventable casualties ensued. 2 This goes against current regulatory safety standards for animal research. 5

Which scientist conducted the Universe 25 experiment?

• John B. Calhoun led the Universe 25 experiment , which examined the long-term effects of increasing population density and resulting social stressors on mice living in a constructed environment. 2

How many times was the Universe 25 experiment repeated?

• Universe 25 was one long-term experiment in a series of mouse studies. The entire research series involved Calhoun’s constructed Mortality-Inhibiting Environment for Mice , and each universe examined separate mouse populations and conditions. Calhoun stated that the Universe 25 experiment involved the largest mouse population and longest follow up period. 2

What is a behavioral sink?

• In overcrowding rat studies that Calhoun performed before the Universe 25 experiment, he observed that individual rats began to associate feeding with the company of other rats, which led to the learned behavior of voluntary crowding despite insufficient resources at a crowded site and available resources elsewhere. He termed this specific voluntary crowding a behavioral sink . 1 Calhoun also observed this learned behavior in mice during the Universe 25 experiment. 2

Were the results of the Universe 25 experiment reproduced by other scientists?

• The social effects of population density vary between organisms and populations. Calhoun’s work inspired many scientists to focus on behavioral studies , but the specific experiment has not been replicated. 4,6

What is the criticism of Universe 25?

• The Universe 25 experiment faces several scientific limitations, including experimental biases inherent to observational studies, misinterpretation and unsubstantiated extrapolation to human experiences, and ethical concerns related to animal care . 2-5  

This article was originally published on May 2, 2022.  It was updated on May 28, 2024 by  Deanna MacNeil , PhD .

• Calhoun JB. Population density and social pathology . Scientific American Magazine . 1962;206(2):139. 
• Calhoun JB. Death squared: the explosive growth and demise of a mouse population . Proc R Soc Med . 1973;66(1P2):80-88. 
• Boyko EJ. Observational research opportunities and limitations . J Diabetes Complications . 2013;27(6):642-648. 
• Ramsden E. The urban animal: population density and social pathology in rodent and humans . Bull World Health Organ . 2009;87(2):82.
• Animal Research Advisory Committee (ARAC) Guidelines . OACU. Accessed May 28, 2024.
•  Ramsden E, Adams J. Escaping the laboratory: The rodent experiments of John B. Calhoun and their cultural influence . J Soc Hist . 2009;42(3):761-797. 

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The Behavioral Sink

The mouse universes of john b. calhoun.

How do you design a utopia? In 1972, John B. Calhoun detailed the specifications of his Mortality-Inhibiting Environment for Mice: a practical utopia built in the laboratory. Every aspect of Universe 25—as this particular model was called—was pitched to cater for the well-being of its rodent residents and increase their lifespan. The Universe took the form of a tank, 101 inches square, enclosed by walls 54 inches high. The first 37 inches of wall was structured so the mice could climb up, but they were prevented from escaping by 17 inches of bare wall above. Each wall had sixteen vertical mesh tunnels—call them stairwells—soldered to it. Four horizontal corridors opened off each stairwell, each leading to four nesting boxes. That means 256 boxes in total, each capable of housing fifteen mice. There was abundant clean food, water, and nesting material. The Universe was cleaned every four to eight weeks. There were no predators, the temperature was kept at a steady 68°F, and the mice were a disease-free elite selected from the National Institutes of Health’s breeding colony. Heaven.

Four breeding pairs of mice were moved in on day one. After 104 days of upheaval as they familiarized themselves with their new world, they started to reproduce. In their fully catered paradise, the population increased exponentially, doubling every fifty-five days. Those were the good times, as the mice feasted on the fruited plain. To its members, the mouse civilization of Universe 25 must have seemed prosperous indeed. But its downfall was already certain—not just stagnation, but total and inevitable destruction.

Calhoun’s concern was the problem of abundance: overpopulation. As the name Universe 25 suggests, it was not the first time Calhoun had built a world for rodents. He had been building utopian environments for rats and mice since the 1940s, with thoroughly consistent results. Heaven always turned into hell. They were a warning, made in a postwar society already rife with alarm over the soaring population of the United States and the world. Pioneering ecologists such as William Vogt and Fairfield Osborn were cautioning that the growing population was putting pressure on food and other natural resources as early as 1948, and both published bestsellers on the subject. The issue made the cover of Time magazine in January 1960. In 1968, Paul Ehrlich published The Population Bomb , an alarmist work suggesting that the overcrowded world was about to be swept by famine and resource wars. After Ehrlich appeared on The Tonight Show with Johnny Carson in 1970, his book became a phenomenal success. By 1972, the issue reached its mainstream peak with the report of the Rockefeller Commission on US Population, which recommended that population growth be slowed or even reversed.

But Calhoun’s work was different. Vogt, Ehrlich, and the others were neo-Malthusians, arguing that population growth would cause our demise by exhausting our natural resources, leading to starvation and conflict. But there was no scarcity of food and water in Calhoun’s universe. The only thing that was in short supply was space. This was, after all, “heaven”—a title Calhoun deliberately used with pitch-black irony. The point was that crowding itself could destroy a society before famine even got a chance. In Calhoun’s heaven, hell was other mice.

So what exactly happened in Universe 25? Past day 315, population growth slowed. More than six hundred mice now lived in Universe 25, constantly rubbing shoulders on their way up and down the stairwells to eat, drink, and sleep. Mice found themselves born into a world that was more crowded every day, and there were far more mice than meaningful social roles. With more and more peers to defend against, males found it difficult and stressful to defend their territory, so they abandoned the activity. Normal social discourse within the mouse community broke down, and with it the ability of mice to form social bonds. The failures and dropouts congregated in large groups in the middle of the enclosure, their listless withdrawal occasionally interrupted by spasms and waves of pointless violence. The victims of these random attacks became attackers. Left on their own in nests subject to invasion, nursing females attacked their own young. Procreation slumped, infant abandonment and mortality soared. Lone females retreated to isolated nesting boxes on penthouse levels. Other males, a group Calhoun termed “the beautiful ones,” never sought sex and never fought—they just ate, slept, and groomed, wrapped in narcissistic introspection. Elsewhere, cannibalism, pansexualism, and violence became endemic. Mouse society had collapsed.

On day 560, a little more than eighteen months into the experiment, the population peaked at 2,200 mice and its growth ceased. A few mice survived past weaning until day six hundred, after which there were few pregnancies and no surviving young. As the population had ceased to regenerate itself, its path to extinction was clear. There would be no recovery, not even after numbers had dwindled back to those of the heady early days of the Universe. The mice had lost the capacity to rebuild their numbers—many of the mice that could still conceive, such as the “beautiful ones” and their secluded singleton female counterparts, had lost the social ability to do so. In a way, the creatures had ceased to be mice long before their death—a “first death,” as Calhoun put it, ruining their spirit and their society as thoroughly as the later “second death” of the physical body.

Calhoun had built his career on this basic experiment and its consistent results ever since erecting his first “rat city” on a quarter-acre of land adjacent to his home in Towson, Maryland, in 1947. The population of that first pen had peaked at 200 and stabilized at 150, when Calhoun had estimated that it could rise to as many as 5,000—something was evidently amiss. In 1954, Calhoun was employed by the National Institute of Mental Health in Rockville, Maryland, where he would remain for three decades. He built a ten-by-fourteen-foot “universe” for a small population of rats, divided by electrified barriers into four rooms connected by narrow ramps. Food and water were plentiful, but space was tight, capable of supporting a maximum of forty-eight rats. The population reached eighty before succumbing to the same catastrophes that would afflict Universe 25: explosive violence, hypersexual activity followed by asexuality, and self-destruction.

In 1962, Calhoun published a paper called “Population Density and Social Pathology” in Scientific American , laying out his conclusion: overpopulation meant social collapse followed by extinction. The more he repeated the experiment, the more the outcome came to seem inevitable, fixed with the rigor of a scientific equation. By the time he wrote about the decline and fall of Universe 25 in 1972, he even laid out its fate in equation form:

Mortality, bodily death = the second death Drastic reduction of mortality = death of the second death = death squared = (death) 2 (Death) 2 leads to dissolution of social organization = death of the establishment Death of the establishment leads to spiritual death = loss of capacity to engage in behaviors essential to species survival = the first death Therefore: (Death) 2 = the first death

This formula might apply to rats and mice—but could the same happen to humankind? For Calhoun, there was little question about it. No matter how sophisticated we considered ourselves to be, once the number of individuals capable of filling roles greatly exceeded the number of roles,

only violence and disruption of social organization can follow. ... Individuals born under these circumstances will be so out of touch with reality as to be incapable even of alienation. Their most complex behaviors will become fragmented. Acquisition, creation and utilization of ideas appropriate for life in a post-industrial cultural-conceptual-technological society will have been blocked.

If its growth continued unchecked, human society would succumb to nihilism and collapse, meaning the death of the species. Calhoun’s death-squared formula was for social pessimists what the laws of thermodynamics are for physicists. It was a sandwich board with “The End Is Nigh” written on one side, and “QED” on the other. Indeed, the plight of Calhoun’s rats and mice is one we easily identify with—we put ourselves in the place of the mice, mentally inhabit the mouse universe, and cannot help but see ways in which it is like our own crowding world.

This is precisely what Calhoun intended, in the design of his experiments and the language he used to describe them. Universe 25 resembles the utopian, modernist urban fantasies of architects such as Ludwig Hilberseimer. Calhoun referred to the dwelling places within his Universes as “tower blocks” and “walk-up apartments.” As well as the preening “beautiful ones,” he refers to “juvenile delinquents” and “dropouts.” This handy use of anthropomorphism is unusual in a scientist—we are being invited to draw parallels with human society.

And that lesson found a ready audience. “Population Density and Social Pathology” was, for an academic paper, a smash hit, being cited up to 150 times a year. Particularly effective was Calhoun’s name for the point past which the slide into breakdown becomes irretrievable: the “behavioral sink.” “The unhealthy connotations of the term are not accidental,” Calhoun noted drily. The “sink,” a para-pathology of shared hopelessness, drew in pathological behavior and exacerbated its effects. Once the event horizon of the behavioral sink was passed, the end was certain. Pathological behavior would escalate beyond any possibility of control. The writer Tom Wolfe alighted on the phrase and deployed it in his lament for the declining New York City, “O Rotten Gotham! Sliding Down into the Behavioral Sink,” anthologized in The Pump House Gang in 1968. “It got to be easy to look at New Yorkers as animals,” Wolfe wrote, “especially looking down from some place like a balcony at Grand Central at the rush hour Friday afternoon. The floor was filled with the poor white humans, running around, dodging, blinking their eyes, making a sound like a pen full of starlings or rats or something.” The behavioral sink meshed neatly with Wolfe’s pessimism about the modern city, and his grim view of modernist housing projects as breeding grounds for degeneration and atavism.

Wolfe wasn’t alone. The warnings inherent in Calhoun’s research fell on fertile ground in the 1960s, with social policy grappling helplessly with the problems of the inner cities: violence, rape, drugs, family breakdown. A rich literature of overpopulation emerged from the stew, and when we look at Calhoun’s rodent universes today, we can see in them aspects of that literature. In the 1973 film Soylent Green , based on Harry Harrison’s 1966 novel Make Room! Make Room! , the population of a grotesquely crowded New York is mired in passivity and dependent on food handouts which, it emerges, are derived from human corpses. In Stand on Zanzibar , John Brunner’s 1972 novel of a hyperactive, overpopulated world, society is plagued by “muckers,” individuals who suddenly and for no obvious reason run amok, killing and wounding others. When we hear of the death throes of Universe 25—the cannibalism, withdrawal, and random violence—these are the works that come to mind. The ultraviolence-dispensing, gang-raping, purposeless “droogs” of Antony Burgess’s novel A Clockwork Orange , which appeared in the same year as Calhoun’s Scientific American paper, are the very image of some of the uglier products of mouse utopia.

Calhoun’s research remains a touchstone for a particular kind of pessimistic worldview. And, in the way that writers like Wolfe and the historian Lewis Mumford deployed reference to it, it can be seen as bleakly reactionary, a warning against cosmopolitanism or welfare dependence, which might sap the spirit and put us on the skids to the behavioral sink. As such, it found fans among conservative Christians; Calhoun even met the pope in 1974. But in fact the full span of Calhoun’s research had a more positive slant. The misery of the rodent universes was not uniform—it had contours, and some did better than others. Calhoun consistently found that those animals better able to handle high numbers of social interactions fared comparatively well. “High social velocity” mice were the winners in hell. As for the losers, Calhoun found they sometimes became more creative, exhibiting an un-mouse-like drive to innovate. They were forced to, in order to survive.

Later in his career, Calhoun worked to build universes that maximized this kind of creativity and minimized the ill effects of overcrowding. He disagreed with Ehrlich and Vogt that restrictions on reproduction were the only possible response to overpopulation. Man, he argued, was a positive animal, and creativity and design could solve our problems. He advocated overcoming the limitations of the planet, and as part of a multidisciplinary group called the Space Cadets promoted the colonization of space. It was a source of lasting dismay to Calhoun that his research primarily served as encouragement to pessimists and reactionaries, rather than stimulating the kind of hopeful approach to mankind’s problems that he preferred. More cheerfully, however, the one work of fiction that stems directly from Calhoun’s work, rather than the stew of gloom that it was stirred into, is optimistic, and expands imaginatively on his attempts to spur creative thought in rodents. This is Robert C. O’Brien’s book for children, Mrs. Frisby and the Rats of NIMH , about a colony of super-intelligent and self-reliant rats that have escaped from the National Institute of Mental Health.

Cabinet and the author regret that a previous version of this article omitted its sources.

See press about “The Behavioral Sink” on Longreads.com and theatlantic.tumblr.com .

Will Wiles is a London-based author and journalist. He is deputy editor of Icon , a monthly architecture and design magazine. His debut novel, Care of Wooden Floors , will be published by HarperPress in February 2012.

Infamous Universe 25 'Rodent Utopia' Experiment Is Not a Sign of the Apocalypse

Many people have used john calhoun's "rodent utopia" experiments as scientific evidence for "social decay" in humans., published may 27, 2024.

People predicting the end of world generally make those predictions without scientific evidence to support them. So when an animal-behavior researcher ran experiments in the 1960s that described "utopian" rodent societies pushing themselves into extinction, scientists and the general public alike took notice. That attention never fully went away.

Ever since the original "Universe 25" rodent utopia experiment took place, countless people have discussed the study and its findings, with some suggesting it could be an apocalyptic prediction for the future of humanity. The story pops up online every so often, and Snopes readers have written many emails over the past few years asking us about the notorious rodent utopia experiment.

The Background

Before explaining the experiment, it's important to understand why it   was performed. While environmentalism as a political theory had been around in bits and pieces since the early days of the Industrial Revolution, it was not until just after World War II that people truly began to politically organize around the environment.

One of the largest fears at the time was overpopulation — sometimes called Malthusianism after an 18th-century demographer, Thomas Malthus , who proposed that population would eventually grow faster than food production, meaning that, eventually, humanity would be unable to feed everyone. Many early environmentalists proposed similar ideas.

In the 1950s, an animal behaviorist named John Calhoun started working at the National Institute of Mental Health. He had long worked with rats, the subject of his Ph.D. thesis, and was interested in studying how a rat society would develop over time when it was limited only   by space. In other words, he wanted to test the effects of overpopulation.

In order to run his experiment, Calhoun designed complexes, which he named "Universes," that would provide his rodent subjects all they needed to survive — food, water and protection from predators and disease. The only thing that would limit the population growth would be space. 

As he watched the rodent societies grow, he began noticing strange trends:

Pregnant females began having problems raising offspring. Dominant males became incredibly territorial and overactive, while subordinate males increasingly withdrew from the larger group, coming out "to eat, drink and move about only when other members of the community were asleep." Rats became so conditioned to eating with others that they would refuse to eat alone. Some males became hypersexual and attempted to mate with anyone and everyone. Fighting was frequent. Rats began cannibalizing other rats. At one point, the infant mortality rate reached an astonishing 96%.

As one of Calhoun's assistants put it, "utopia" had turned into a "hell."

The Experiments

Calhoun published the results of his early experiments in the February 1962 edition of Scientific American, with the title " Population Density and Social Pathology ," coining the term " behavioral sink " to describe the most-crowded spots, where he observed the highest rate of antisocial behavior. In the 1960s, at the height of political discourse about so-called "social decay" in American cities, the study was a natural discussion topic. In the meantime, Calhoun continued his work.

And now we arrive the 25th version of this study Calhoun ran, and the one he would become most well-known for: Universe 25. It was the only one of Calhoun's habitats fully studied from beginning to end. Universe 25 was populated with mice instead of rats, but most everything else remained the same. Mice had everything they needed to survive and were limited only by space.

Calhoun constructed a square box with a side length of 54 inches. He built nesting boxes, water bottles and food hoppers into the walls, with each side of the universe having 64 different nesting boxes located at various heights, 16 water bottles and four food hoppers. All of the "utilities" were accessible via a series of mesh tunnels running from the floor up the side of the wall.

Calhoun published the results of Universe 25 in 1973 in a paper called " Death Squared: The Explosive Growth and Demise of a Mouse Population ." He broke down the development and collapse of the society into four phases:

• Phase A, consisting of the first 104 days, was the adjustment phase, with "considerable social turmoil" between the eight original mice placed into the habitat. Phase A ended once the mice had their first offspring.
• Phase B, which Calhoun named the resource exploitation phase, lasted from Day 105 to Day 315. During this phase, the population grew rapidly, reaching more than 600 mice before growth began slowing. Social stratification also began to happen, with different groups of mice living in certain areas and self-selecting into their own independent groups.
• Phase C, called the stagnation phase, lasted from Day 316 to Day 560. Male mice who were not able to find room in the pre-existing social structure began to withdraw from society, violently attacking one another. Their female counterparts retreated into the highest boxes, also isolating themselves. Socially dominant males began to lose control over their territory, leaving mothers to aggressively defend their young, sometimes even abandoning them. "For all practical purposes there had been a death of societal organization by the end of Phase C," Calhoun wrote.
• Phase D was the death phase. The death rate outpaced the birth rate, and the society began to shrink. Mothers raised newborn mice for a very short time, and Calhoun proposed that the young generation's strange behaviors were a direct result of a very abnormal social upbringing that did not allow some of the more "complex behaviors," including mating rituals, to develop. Females rarely gave birth, and a large group of males, which Calhoun named the "beautiful ones," did nothing other than eat, drink, sleep and groom themselves. During Phase D, a few mice were placed in newly established universes to see whether they would relearn those social behaviors. They did not.

Calhoun's 1973 paper was not subtle. "I shall largely speak of mice, but my thoughts are on man, on healing, on life and its evolution," he wrote. He made frequent references to the Book of Revelation in the Bible and almost all of his wording aimed to personify his rodent subjects. The mice in Universe 25 lived in "walk-up apartments," and Calhoun described subgroups like "somnambulists" or the "bar flies," terms that could easily be mapped to urban life.

The conclusions felt grim, and the fears of overpopulation made their way into pop culture , like the movie "Soylent Green." There's even a book for children very loosely based around Calhoun's mouse cities (although without the doom and gloom of societal breakdown): "Mrs. Frisby and the Rats of NIMH."

The Conclusions

In modern times, Calhoun's Universe 25 experiment is often used as a way to talk about some kind of " degradation of Western society ." These analyses look at Calhoun's experiments and say, "He predicted this would happen to humans, and look at all the cultural degeneracy we see today!" For instance, here's an excerpt from a comment about the experiment we've seen repeatedly on Facebook :

According to Calhoun, the death phase consisted of two stages: the "first death" and "second death." The former was characterized by the loss of purpose in life beyond mere existence — no desire to mate, raise young or establish a role within society. As time went on, juvenile mortality reached 100% and reproduction reached zero. Among the endangered mice, homosexuality was observed and, at the same time, cannibalism increased, despite the fact that there was plenty of food. Two years after the start of the experiment, the last baby of the colony was born. By 1973, he had killed the last mouse in the Universe 25. John Calhoun repeated the same experiment 25 more times, and each time the result was the same. Calhoun's scientific work has been used as a model for interpreting social collapse, and his research serves as a focal point for the study of urban sociology. We are currently witnessing direct parallels in today's society ... weak, feminized men with little to no skills and no protection instincts, and overly agitated and aggressive females with no maternal instincts.

Scientists have repeatedly pushed back against these ideas since Calhoun's research came out. Researchers who attempted to replicate Calhoun's studies in humans found mixed results, and other scientists chastised him for extrapolating rodent behavior to humans. While the popular conception of Universe 25 focused on the apocalyptic death of society because of overpopulation, other psychologists suggested otherwise. 

In an 2008 interview with the NIH Record , Dr. Edmund Ramsden, a science historian, explained the results of a similar 1975 experiment by a psychologist Jonathan Freedman:

Freedman's work, Ramsden noted, suggested that density was no longer a primary explanatory variable for society's ruin. A distinction was drawn between animals and humans. "Rats may suffer from crowding; human beings can cope… Calhoun's research was seen not only as questionable, but also as dangerous." Freedman suggested a different conclusion, though. Moral decay resulted "not from density, but from excessive social interaction," Ramsden explained. "Not all of Calhoun's rats had gone berserk. Those who managed to control space led relatively normal lives." Striking the right balance between privacy and community, Freedman argued, would reduce social pathology. It was the unwanted unavoidable social interaction that drove even fairly social creatures mad, he believed.

But what the modern critics often carelessly and conveniently leave out is how Calhoun's research evolved after Universe 25: Up until his death in 1995, Calhoun looked for solutions to the problem he had discovered, altering his designs and controls to try to   avoid the societal collapse of Universe 25. He described rodents coming up with creative solutions to daily tasks. And it was in this way that his experiments have actually proven more useful. Architects and urban designers have taken Calhoun's experiments into consideration when designing buildings and cities. Prison researchers and reformers have also found Calhoun's studies surprisingly helpful.

So yes, while Universe 25 and Calhoun's "rodent utopias" were real experiments, they're not the apocalyptic predictions that some people make them out to be. 

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By Jack Izzo

Jack Izzo is a Chicago-based journalist and two-time "Jeopardy!" alumnus.

Article Tags

• Experiments

Universe 25: An Experiment With Disturbing Results

All cited sources were thoroughly reviewed by our team to ensure their quality, reliability, currency, and validity. The bibliography of this article was considered reliable and of academic or scientific accuracy.

• Calhoun, J.B. (1971). Space and the Strategy of Life. In: Esser, A.H. (eds) Behavior and Environment. Springer, Boston, MA. https://doi.org/10.1007/978-1-4684-1893-4_25
• Calhoun JB. (1973). Death Squared: The Explosive Growth and Demise of a Mouse Population. Proceedings of the Royal Society of Medicine , 66 (1P2),80-88. doi:10.1177/00359157730661P202
• Freedman, J. L. (1975). Crowding and behavior. W. H. Freedman. https://psycnet.apa.org/record/1976-05842-000
• Ramsden E. (2011). From rodent utopia to urban hell: population, pathology, and the crowded rats of NIMH.  Isis; an international review devoted to the history of science and its cultural influences ,  102 (4), 659–688. https://doi.org/10.1086/663598

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