During the 1960s and 70s, ethologist John B. Calhoun conducted a number of infamous social experiments on rats and mice. These experiments attempted to determine how the rodents would behave in crowded, but resource abundant environments. In one particular experiment, dubbed "Universe 25", he allowed it to last 600 days, out of morbid curiosity perhaps... Despite having all they need material-wise for survival, the rodent population nose-dived towards extinction following a collapse of the family unit that had previously enabled successful mating, breeding, and the nurturing of the young. The rise and fall of Universe 25's population is eerily similar to current human population growth projections, which are estimated to level off at around 11 billion according to a news report by the United Nations due to declining birth rates.
Near the end of the experiment, mothers abandoned and frequently cannibalized their babies, roaming gangs of males indiscriminately mated with each other, and last, but not least, a segment of the population opted-out of socialization altogether (the "Beautiful Ones"). All of this despite utopian environmental conditions, designed to maximize the longevity of the rodent population inside.