A classic series of experiments to determine the effects of overpopulation on communities of rats was conducted by a psychologist, John Calhoun．In each experiment, an equal number of male and female rats were placed in an enclosure．The rat populations were allowed to increase．Calhoun knew from experience approximately how many rats could live in the enclosures without experiencing stress due to overcrowding．He allowed the population to increase to approximately twice this number．Then he stabilized the population by removing offspring that were not dependent on their mothers．At the end of the experiments, Calhoun was able to conclude that overcrowding causes a breakdown in the normal social relationships among rats, a kind of social disease．The rats in the experiments did not follow the same patterns of behavior．as rats would in a community without overcrowding.
The females in the rat population were the most seriously affected by the high population density．For example, mothers sometimes abandoned their pups, and, without their mothers' care, the pups died．The experiments verified that in overpopulated communities, mother rats do not behave normally．Their behavior．may be considered diseased, pathological (病理学的).
The dominant males in the rat population were the least affected by overpopulation．Each of these strong males claimed an area of the enclosure as his own．Therefore, these individuals did not experience the overcrowding in the same way as the other rats did．However, dominant males did behave pathologically at times．Their antisocial behavior．consisted of attacks on weaker male, female, and immature rats．This deviant (不正常的) behavior．showed that even though the dominant males had enough living space, they too were affected by the general overcrowding.
Non-dominant males in the experimental rat communities also exhibited deviant social behavior．Some withdrew completely, avoiding contact with other rats．Other non-dominant males were hyperactive, chasing other rats and fighting each other.
The behavior．of the rat population has parallels in human behavior．People in densely populated areas exhibit deviant behavior．similar to that of rats in Calhoun's experiments．In large areas, such as New York City, London, and Cairo, there are abandoned children．There are cruel, powerful individuals, both men and women．There are also people who withdraw and people who become hyperactive．Is the principal cause of these disorders overpopulation? Calhoun's experiments suggest that it might be．In any case, social scientists and city planners have been influenced by the results of this series of experiments.
Calhoun stabilized the rat population ______.