Jon Adams is a former BBC New Generation Thinker and author of Interface patterns: literary studies, scientific knowledge and disciplinary autonomyPreviously, he worked at the London School of Economics.
Edmund Ramsden is a historian of science and medicine at Queen Mary, University of London. Before joining Queen Mary, he worked at the universities of Manchester, Exeter and the London School of Economics.
Below, co-authors Jon and Edmund share five key insights from their new book, Rat City: Overpopulation and Urban Confusion in the Rodent Universes of John B. Calhoun. Listen to the audio version – read by Jon – on the Next Big Idea app.
1. Rats practice a form of birth control.
John Calhoun was an ecologist turned psychologist who spent four decades, from the 1940s to the early 1980s, researching the link between crowding and behavior in rats and humans.
Originally tasked with combating a rodent infestation in post-war Baltimore, Calhoun discovered that rats are herd animals that form complex, hierarchical societies. To study rodent society more closely, he built an artificial city block, a “rodent utopia,” where all of his animals’ needs would be met—except for space.
As the population density in the “rat city” increased, the rats became stressed by the confinement and exhibited increasingly severe behavioral problems. The strictly ordered family units broke down, mothers neglected their young, and young males became violent and hypersexual.
Rats reproduce so quickly that it is theoretically possible to turn two rats into nearly two thousand in just over twelve months. But this kind of fertility never occurs in the real world. Calhoun wondered: Why not?
During studies of rats in Baltimore in the late 1940s, Calhoun discovered that population numbers were fixed at about 150 animals per city block. While residents were appalled at how many rats lived in the city, ecologists were surprised that there were so few. 150 rats per city block is far less than environmental resources allow; the back alleys of downtown had no shortage of garbage to eat and plenty of nesting sites.
“As social density increased, things started to fall apart.”
The population cap was strict: if a block’s rat population was reduced by poisoning or trapping, it soon recovered but never exceeded the limit of 150 rats. To Calhoun’s amazement, it seemed as if the rats were exercising their own form of birth control. But how?
Further research revealed that rat society was kept in a delicate balance, and raising young after weaning required a surprising level of domestic harmony. If a nursing mother is disturbed, she may attempt to relocate her young or abandon her nest and young altogether. Under normal circumstances, a dominant male would stand guard to prevent nest invasions, but even the toughest leaders would eventually be overwhelmed if the population grew too large.
As social density increased, things began to fall apart as squabbles destabilized family units and eventually caused reproduction rates in the overpopulated areas to drop until they settled back down to optimal levels. The mysterious mechanism behind the spontaneous regulation of population size turned out to be a limit set not by available food but by available space. And, crucially, a habitat was “socially full” long before that space ran out.
2. Crowding is a communication problem (not a proximity problem).
When we think of crowding, we often think of the number of people in the same space. But it’s not just the proximity that makes crowding uncomfortable. Crowding is stressful not only because we are physically close to one another, but also because we may have to interact with those other people. What we don’t fear so much about physical contact is social contact.
Luckily, we’re really good at ignoring each other. City dwellers are masters at looking at each other from a distance. On their daily commute, they carefully avoid eye contact and certainly avoid conversation. When everyone agrees to participate in this antisocial contract, a collective illusion of invisibility is created.
Rats are unfortunately unable to ignore each other. In the wild, rats would migrate to a new habitat as population density increased. But trapped in his rat cities, Calhoun’s rodents soon found the frequency of social contact disturbing. They were constantly forced to greet other rats, gauge their dominance rank, and undergo a never-ending process of threat assessment. The social environment so distressed them that they lost their good manners and became hostile. Over time, this led to more serious behavioral problems, escalating violence, and a population collapse.
Calhoun’s work with rats and mice made him realize that humans have a remarkable ability to avoid social obligations. This talent for voluntary social withdrawal enabled cities to grow. Although we often cite human communication skills as the secret to our success, learning how to ignore one another is just as important.
3. A good fence is a fence that you can climb over.
In Robert Frost’s 1913 poem “Mending Wall,” two neighbors are busy repairing the dry stone wall between their properties. When the poet asks why they need a wall at all, his neighbor stubbornly quotes the old aphorism: “Good fences make good neighbors.” It is easy to imagine a good neighbor, but what is a good fence?
“‘Good’ fences mark boundaries while allowing communication on both sides.”
In his early experiments, Calhoun divided his rat cities into individual cells. He used high walls with electric wires over them to tightly control which areas of the enclosure the rats could move between. The rats were conditioned to crowd into a single cell, leading to severe overcrowding, social breakdown, and brutal violence. In later experiments, these barriers were replaced with low partitions that the rats could climb over but, crucially, adequately protected them from unwanted contact.
A high fence with electric wire on top is not a neighborhood fence. “Good” fences mark boundaries and at the same time enable communication between the two sides. They are socially permeable. At least on the street side of the property, a picket fence at waist or even knee height serves its purpose. It signals a boundary and assumes that the passerby is decent enough not to cross it. If it is higher, the fence tacitly assumes that there is an intention to cross.
In “Mending Wall,” Frost was skeptical that a physical boundary was even necessary. And yet, the whole time they were repairing their low stone wall, he seemed to be deep in conversation with his neighbor. Good neighbors make good fences.
4. More privacy is not always better.
While working in a remote Canadian mental hospital in the 1950s, psychologists Robert Sommer and Humphry Osmond found that the cave-like, open-plan wards made the already disturbed residents even more insecure. An open space was more stressful than a closed cell. They discovered that they could make the wards more peaceful simply by putting up screens and rearranging the chairs. From this work, Sommer developed the concept of Personal area. While a certain amount of privacy was an advantage, one could have too much of it.
In the 1970s, the architect Oscar Newman borrowed the idea of Territoriality and applied it to public housing. His theory of defensible space was intended to give residents of public housing a sense of ownership. But instead, the focus on privacy and personal security fostered distrust and hostility. This logic led to gated communities and billionaires building bunkers for the apocalypse.
When sociologist Herb Gans lived in Boston’s West End in the early 1950s, he learned: Build a porch, a veranda, a place to sit that bridges the private and the public. Building communal spaces creates community.
The same was true for rodents. Calhoun designed his later enclosures so that there was a central area where rats from different colonies could meet and socialize without invading each other’s territory. Being able to control the extent and frequency of social contact made the lives of his rodents far less stressful, and they lived longer and could tolerate higher population densities. Privacy is desirable, but its value does not increase with its quantity.
5. A rat is a symptom of poor husbandry, not the cause.
New York City Mayor Eric Adams recently appointed a “rat czar” to reduce the city’s rodent population, declaring, “I hate rats and I’m sure most of our city’s residents do too.” We demonize the rat as a totem of urban decay, but rats only thrive where they are not taken care of.
“It turns out that the most sustainable method of rat control is good property maintenance.”
In the 1940s, the city of Baltimore made a major effort to improve its housing stock, and the city’s rodent population was drastically reduced. It turned out that the most sustainable method of rat control was good building maintenance. Regular garbage collection, repair of broken pipes and fixtures, and draft-proof buildings prevented rats from staying in the same room as residents.
Poor buildings fostered rat colonies, and poor buildings led to social problems. As one activist slogan from the 1960s put it: “RATS CAUSE RIOTS.” But the tail doesn’t really wag the dog, and you couldn’t stop the riots simply by removing the rats.
Throughout his career, Calhoun maintained that rat eradication was a purely cosmetic operation and that any program to eradicate the rodents would produce only short-term success if the underlying structural problems were neglected.
As one colleague put it, “Rats are a symptom of poor housing, not the cause.” The rat was a watchdog, its presence signaling that the infested neighborhood needed maintenance and repairs. So if you focus on killing the rats, you’re actually killing the bearer of bad news.
To listen to the audio version read by co-author Jon Adams, download the Next Big Idea app today:
