The overcrowding death spiral

And why it doesn't exist

It was a paradise built for rats. Or so he thought.

John Calhoun, not a behaviorist exactly, wanted to study the effects of overcrowding. In a barn, over about 10,000 square feet, he constructed a pen with nesting areas, stairs, ramps, and unlimited food and water. He estimated that perhaps 5,000 rats could eventually live there (that would be about 2 square feet per rat if they were at floor level, though as seen in the illustration, there were vertical rows of nests, like apartments). 

Calhoun witnessed a peculiar behavior: the rats chose to eat together in a large group. Despite four food hoppers spread across four quadrants (see above), about 75% of the rats tended to congregate at one hopper. They would eat for long periods due to the design of the hopper, increasing their proximity and interaction.

What happened next shocked Calhoun. 

The rats engaged in increasingly bizarre behavior. Some became aggressive, while some became overly passive. Some male rats “dropped out” of society and groomed themselves all day. Mothers stopped caring for children, leading to infant mortality of up to 96%.

After reaching a population of around 2,000 rats, the population dropped precipitously. It held stable at only 150 rats.

Calhoun himself was clear on this: humans could be subject to the same fate.

Calhoun was wrong. Both in a literal sense, and in a behavioral sense.

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The global population in 1972, when Calhoun published Death squared: the explosive growth and demise of a mouse population, was approximately 3.8 billion, while in 2025 it is over 8 billion. It’s easy enough to say that the predictions did not come true. (Though there are indications that the birth rate is recently dropping worldwide, this could not be attributed to “crowding” alone, as population density varies.)

Calhoun approvingly cites Malthus, who proposed that food production could not keep pace with population growth. Malthus wrote this around 1800, when the world population was estimated to be 1 billion.

Malthus was wrong. 

But Calhoun was in good company. Many scientists and politically liberal Americans held the view that unchecked population growth could destroy the world, and many approvingly cited Malthus. There was evidence for this proposition, as the Cuyahoga river famously caught fire every few years, culminating in the notorious 1969 fire. Dumping waste into the environment until it is not possible to produce food was a legitimate concern.

Skinner namedrops Malthus in Walden II:

Meanwhile, politically conservative Americans sometimes held the view that the “wrong people” were reproducing too quickly. Certainly there are echoes of this perspective when Calhoun characterizes behavior as “vice” and “spiritual death”; reminder: he is talking about rats. Calhoun at times quotes the Bible in his work. In America, religion was being abandoned at low rates in the 1970s, which increased rapidly through the 2000s. Some less sane mainstream conservatives continue to espouse similar views, which are not worth discussing here.

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Calhoun was literally wrong from the perspective of 2025. But what about from the perspective of, say, 1972?

Kowloon Walled City was an infamous semi-legal settlement in Hong Kong. Due to quirks of global politics, the settlement was ignored by British colonial settlers and Chinese government officials, leading to a benign/malignant neglect of impoverished residents. Independent towers of up to 11 stories were built haphazardly next to each other, leading to what amounted to an experiment in unregulated housing density. By the 1980s it was theorized to have the highest population density in the world.

In 1972, it was less dense, but marginally so. 

Of course, there was crime in Kowloon Walled City, particularly in the 50s and 60s. But the residents formed associations and security patrols. There was drug abuse, sex work, gambling, and other activities that largely happened without intervention; but reading personal stories of the residents provides evidence that this didn’t impact the majority.

This level of density, in some ways incomparable to anything before or since, persisted until the government demolished the settlement in 1993. Calhoun could have pointed to the crime in Kowloon Walled City as evidence for his hypothesis. However, there was no “behavioral sink,” no skyrocketing infant mortality, no widespread aggression or passivity. (One of his wackier ideas was that crowding would induce “universal autism.” Dear reader, this did not come to pass in Kowloon Walled City.)

So…what happened to the rats?

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Surely the rats did engage in high rates of unusual behavior. There are a few behavioral explanations. Testing would confirm these hypotheses. 

First, rats engage in certain phylogenetic behaviors, including running long distances. Running is adaptive in rats because they forage in the wild. This is why pet rats typically have a wheel. From his depictions of his utopia, Calhoun included nothing like a wheel. In fact, what activities could the rats engage in? The system was closed, precluding exploration. There was no possibility of foraging. 

Related to this first point is the concept of allocation. Allocation could be explained in the following way: your behavior must total 100% each day. In the case of the rats, when they no longer engaged in various behaviors – running, nesting, foraging, hiding from predators, etc. – other behaviors necessarily filled the leftover time. For various rats this could be grooming, aggression, hiding, etc.

A hypothetical rat forages for 70% of its day, cares for infants for 20%, nests for 5%, and grooms for 5%. Eliminate the foraging, and any of the remaining activities fill the space – perhaps the rat now grooms for that entire period, hence 75% of the day. Calhoun created “maladaptive” behavior not through crowding, but by altering the environment.

Second, it was found that modifying the environment could prevent the “behavioral sink” effect. However it is our opinion that Calhoun’s interpretation of his results is incorrect.

The rats, as mentioned previously, had unlimited food and water. Initially the food hopper was arranged so that eating was time-consuming: the pellets were held behind a wire mesh. Water was available in a gravity-fed trough. Neither of these required any response from the rat beyond the consummatory response. In Calhoun’s telling, the food hopper encouraged lengthy interactions, which created the behavioral sink effect. 

However, in later experiments the water source was changed in such a way that it required a bar press. Calhoun attributes the lack of behavioral sink effect in this experiment to the tendency of rats to drink just after waking, discouraging them from going out of their nest and eating together. 

We believe this explanation ignores the obvious: the response requirement. 

There is some evidence that animals (and humans) prefer response-dependent reinforcers to “noncontingent reinforcers” (potentially a misnomer). Similarly, response-dependent shock has little effect on animals, while response-independent shock can have significant deleterious effects. Giving the rats something to do, and imposing a response requirement, may have been the key to avoiding the “behavioral sink” effect. 

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Calhoun did believe that there was (to some extent) a 1:1 relationship between the behavior of rats and humans. He was incorrect about human behavior, as illustrated by dense settlements the world over. He was incorrect in ascribing changes in rat behavior in his experiments to crowding alone, as there were more impactful environmental stimuli (or lack thereof).

Looking closely at his experiment, did he discover something else?

The rats were kept in a closed system, with ad libitum food and water, and without most phylogenetic activities. This is not a good analogue for society at large – but it is similar to a prison.

Calhoun may have endeavored to build a rat utopia. But perhaps he modeled something far different.

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Epilogue:

Calhoun approvingly cited the following. Mark your calendars!