In 1968, the scientist John Calhoun created a new kind of mouse. Inside a barn with sloping mock-Dutch roofs, Calhoun had designed a habitat he called Universe 25. For the colony of white mice he placed there, it was a perfect metropolis in which to eat, play, grow, and breed—except that there were no exits. Without predation or disease, the inhabitants of Universe 25 were soon at standing room only. What surprised Calhoun was that once they reached this level of overcrowding, the mice were calm. They didn’t fight or go into sexual frenzies, as other mice had in Calhoun’s previous experiments in low-level crowding. Instead, the colony went numb. Post-mortems on some female mice showed their embryos withdrawn back into the womb; other females lost the ability to rear young. Even if they survived infancy and were placed within a healthy population outside, mice born in the late stages of Universe 25 mice did not—could not—thrive. Some male mice, meanwhile, withdrew from society altogether.…