Behavioral sink

"Behavioral sink" is a term invented by ethologist John B. Calhoun to describe a collapse in behavior that can result from overpopulation. The term and concept derive from a series of over-population experiments Calhoun conducted on Norway rats between 1958 and 1962. In the experiments, Calhoun and his researchers created a series of "rat utopias" – enclosed spaces where rats were given unlimited access to food and water, enabling unfettered population growth. Calhoun coined the term "behavioral sink" in a February 1, 1962, Scientific American article titled "Population Density and Social Pathology" on the rat experiment. He would later perform similar experiments on mice, from 1968 to 1972.

Calhoun's work became used as an animal model of societal collapse, and his study has become a touchstone of urban sociology and psychology in general.

Experiments
In the 1962 study, Calhoun described the behavior as follows:

Calhoun's early experiments with rats were carried out on farmland at Rockville, Maryland, starting in 1947.

While Calhoun was working at the National Institute of Mental Health (NIMH) in 1954, he began numerous experiments with rats and mice. During his first tests, he placed around 32 to 56 rats in a 10 by(x) cage in a barn in Montgomery County. He separated the space into four rooms. Every room was specifically created to support a dozen matured brown Norwegian rats. Rats could maneuver between the rooms by using the ramps. Since Calhoun provided unlimited resources, such as water, food, and also protection from predators as well as from disease and weather, the rats were said to be in "rat utopia" or "mouse paradise", another psychologist explained.

Following his earlier experiments with rats, Calhoun later created his "Mortality-Inhibiting Environment for Mice" in 1968: a 101 by(x) cage for mice with food and water replenished to support any increase in population, which took his experimental approach to its limits. In his most famous experiment in the series, "Universe 25", population peaked at 2,200 mice even though the habitat was built to tolerate a total population of 4000. Having reached a level of high population density, the mice began exhibiting a variety of abnormal, often destructive, behaviors including refusal to engage in courtship, and females abandoning their young. By the 600th day, the population was on its way to extinction. Though physically able to reproduce, the mice had lost the social skills required to mate.

Calhoun retired from NIMH in 1984, but continued to work on his research results until his death on September 7, 1995.

Explanation
The specific voluntary crowding of rats to which the term "behavioral sink" refers is thought to have resulted from the earlier involuntary crowding: individual rats became so used to the proximity of others while eating that they began to associate feeding with the company of other rats. Calhoun eventually found a way to prevent this by changing some of the settings and thereby decreased mortality somewhat, but the overall pathological consequences of overcrowding remained.

Further, researchers argued that "Calhoun's work was not simply about density in a physical sense, as number of individuals-per-square-unit-area, but was about degrees of social interaction." "Social density" appears to be key.

Applicability to humans
Calhoun had phrased much of his work in anthropomorphic terms, in a way that made his ideas highly accessible to a lay audience.

Calhoun himself saw the fate of the population of mice as a metaphor for the potential fate of humankind. He characterized the social breakdown as a "spiritual death", with reference to bodily death as the "second death" mentioned in the Biblical verse.

Controversy exists over the implications of the experiment. Psychologist Jonathan Freedman's experiment recruited high school and university students to carry out a series of experiments that measured the effects of density on human behavior. He measured their stress, discomfort, aggression, competitiveness, and general unpleasantness. He declared to have found no appreciable negative effects in 1975.

The 1962 Scientific American article came at a time when overpopulation had become a subject of great public interest, and had a considerable cultural influence. However, such discussions often oversimplified the original findings in various ways. It should however be noted that the work has another message than, for example, Paul Ehrlich's now widely disputed  book The Population Bomb.

Calhoun's worries primarily concerned a human population surge and a potentially independent increase in urbanization as an early stage of rendering much a given society functionally sterile. Under such circumstances society will move from some modality of overpopulation towards a much more irredeemable underpopulation. This has been seen in urban populations that have long been noted to have lower fertility than their rural counterparts, but growing use of especially digital media is likely to end up depressing rural population growth as well. As of today this primarily concerns elite population decline, but can due to positive feedback in social diffusion have Calhoun's empirical predictions apply to a much wider segment of society as well. For one, all this seems to hold up in the case of South Korea's unsustainably low total fertility rate, having already led to societal aging comparable with Calhoun's final cohort.