On day 560, a little more than eighteen months into the experiment, the population peaked at 2,200 mice and its growth ceased. A few mice survived past weaning until day six hundred, after which there were few pregnancies and no surviving young. As the population had ceased to regenerate itself, its path to extinction was clear. There would be no recovery, not even after numbers had dwindled back to those of the heady early days of the Universe. The mice had lost the capacity to rebuild their numbers—many of the mice that could still conceive, such as the “beautiful ones” and their secluded singleton female counterparts, had lost the social ability to do so. In a way, the creatures had ceased to be mice long before their death—a “first death,” as Calhoun put it, ruining their spirit and their society as thoroughly as the later “second death” of the physical body.

Calhoun had built his career on this basic experiment and its consistent results ever since erecting his first “rat city” on a quarter-acre of land adjacent to his home in Towson, Maryland, in 1947. The population of that first pen had peaked at 200 and stabilized at 150, when Calhoun had estimated that it could rise to as many as 5,000—something was evidently amiss. In 1954, Calhoun was employed by the National Institute of Mental Health in Rockville, Maryland, where he would remain for three decades. He built a ten-by-fourteen-foot “universe” for a small population of rats, divided by electrified barriers into four rooms connected by narrow ramps. Food and water were plentiful, but space was tight, capable of supporting a maximum of forty-eight rats. The population reached eighty before succumbing to the same catastrophes that would afflict Universe 25: explosive violence, hypersexual activity followed by asexuality, and self-destruction.

In 1962, Calhoun published a paper called “Population Density and Social Pathology” in Scientific American, laying out his conclusion: overpopulation meant social collapse followed by extinction. The more he repeated the experiment, the more the outcome came to seem inevitable, fixed with the rigor of a scientific equation. By the time he wrote about the decline and fall of Universe 25 in 1972, he even laid out its fate in equation form:

Mortality, bodily death = the second death

Drastic reduction of mortality

= death of the second death

= death squared

= (death)2

(Death)2 leads to dissolution of social organization

= death of the establishment

Death of the establishment leads to spiritual death

= loss of capacity to engage in behaviors essential to species survival

= the first death

Therefore:

(Death)2 = the first death

This formula might apply to rats and mice—but could the same happen to humankind? For Calhoun, there was little question about it. No matter how sophisticated we considered ourselves to be, once the number of individuals capable of filling roles greatly exceeded the number of roles,

only violence and disruption of social organization can follow. ... Individuals born under these circumstances will be so out of touch with reality as to be incapable even of alienation. Their most complex behaviors will become fragmented. Acquisition, creation and utilization of ideas appropriate for life in a post-industrial cultural-conceptual-technological society will have been blocked.