In 1968, John B. Calhoun, a researcher at the National Institute of Mental Health, conducted an influential experiment known as Universe 25 to study the effects of overcrowding on mouse behavior.
He created an enclosed “utopia” for mice, with unlimited food, water, and nesting materials but limited physical space. Starting with just four pairs of mice, the population initially grew rapidly but then began to decline sharply as overcrowding set in.
As density increased, the mice exhibited disturbing behavioral changes: increased aggression, abnormal sexual behavior, neglect or even harm to their offspring, and social withdrawal. Mice born into this environment failed to develop normal social bonds and remained in an infantile, isolated state even when later introduced to normal mice. Ultimately, the colony died out, and no recovery was observed.
Calhoun coined the term “behavioral sink” to describe the collapse of social behavior due to overcrowding. He anthropomorphized his findings with labels like “juvenile delinquents” and “social dropouts,” which led to popular but controversial comparisons between his rodent results and human societies.
Later analysis and contemporary science raise criticisms of the experiment that don’t consider, in its benchmark with human societies, complex psychological and social factors, personal autonomy and social roles.
Calhoun himself emphasized the importance of architectural design to mitigate overcrowding effects in prisons, mental hospitals, and other dense environments. Additionally, the ethical standards of today would not permit such experiments due to the distress and harm caused to animals.