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 brown 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". He later performed 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.
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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 34 to 55 rats in a 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.
In the 1962 study, Calhoun described the behavior as follows: