Four pairs of mice placed in a 'paradise' with unlimited food. Scientists watched in disbelief as it unraveled

Four pairs of mice entered what looked like an ideal world: food and water were almost unlimited, nesting materials were available, and predators could not enter. The one resource that eventually became scarce was space.

The enclosure formed the basis of John B. Calhoun’s Universe 25 experiment, which began in 1968. He wanted to observe how rising population density and changing social relationships affected the animals’ behavior. At first, the population grew rapidly. As numbers increased, however, conflicts and disruptions in social behavior began to appear.

Some mothers neglected or attacked their young. Certain males withdrew from social interaction and spent much of their time grooming themselves. Reproduction slowed. After reaching a peak of roughly 2,200 mice, the population stagnated and then fell. Only 122 mice remained in 1972, and the colony ultimately became extinct.

Calhoun described some of these patterns as a “behavioral sink.” The experiment attracted intense attention during a period of public concern about world population growth and overcrowded cities, and its findings were often treated as a warning about the future of human society.

That interpretation requires caution, however. The mice lived in an artificial environment from which they could not leave, change social groups or reorganize their surroundings as humans can. Later researchers also pointed to methodological limitations, including the observational nature of much of Calhoun’s work, possible sources of bias and the absence of measurements such as stress hormones. Human research does not show a simple link between population density and social behavior. Privacy, control over personal space, social relationships and the surrounding context all shape how people experience crowding. Calhoun himself did not necessarily conclude that growing populations doomed human civilization.