What's the cliff notes version, some of us are stoned, immaculate.

A dude named Calhoun did some environment and population studies with rats when he was part of the NIMH. He built some rather elaborate rat cities and airdropped unlimited food into several predetermined locations, and put some rats inside. He expected the rats to quickly multiply and overcrowd their environment with access to unlimited food.

Some rather curious and unexpected things happened during these experiments.

The rats did multiply rapidly in the very early stages, but the population never came close to filling the rat cities.

Rat societies have defined social roles, and in this environment of unlimited food that didn't require work or cooperation, there were not very many social roles. Within a few generations the younger mice would do strange things. Some formed roaming gangs that would do homosexual assaults on other rats, other beta mice holed up in little rooms with the corridors guarded by alpha family members, and only came out to get food when the other mice slept. These mice completely lost interest in caring for their young, and eventually in mating altogether - even after the population dwindled (these spaces were never filled). All the rat societies eventually stopped breeding and died. In the last experiment he put the last few living mice back in with the normal mice, they never recovered.

TLDR: He brought about complete social degeneration and civilizational collapse in rat societies by creating sociological problems for the rats purely by putting them in a specific environment