His famous experiments were to set up rodent pens where a population of rodents could live with all of their needs being met, except space. They had an infinite amount of food, water, protection from predators, etc. But the pens were a finite space as the only limitation.
What Calhoun was expecting was that the population would explode, reach a peak, then level off to be sustainable within the finite space, but that was not what happened. Every time he ran this experiment, there were catastrophic changes in behavior in the rodents and a near complete collapse in the population.
Initially the population grew rapidly, doubling every 55 days. The population reached 620 by day 315, after which the population growth dropped markedly. The last surviving birth was on day 600. This period between day 315 and day 600 saw a breakdown in social structure and in normal social behavior.
Among the aberrations in behavior were the following: expulsion of young before weaning was complete, wounding of young, inability of dominant males to maintain the defense of their territory and females, aggressive behavior of females, passivity of non-dominant males with increased attacks on each other which were not defended against. After day 600, the social breakdown continued and the population declined toward extinction. During this period females ceased to reproduce. Their male counterparts withdrew completely, never engaging in courtship or fighting. They ate, drank, slept, and groomed themselves – all solitary pursuits. Sleek, healthy coats and an absence of scars characterized these males. They were dubbed “the beautiful ones”.
The conclusions drawn from this experiment were that when all available space is taken and all social roles filled, competition and the stresses experienced by the individuals will result in a total breakdown in complex social behaviors, ultimately resulting in the demise of the population.
From: John B. Calhoun
Many [female rats] were unable to carry pregnancy to full term or to survive delivery of their litters if they did. An even greater number, after successfully giving birth, fell short in their maternal functions. Among the males the behavior disturbances ranged from sexual deviation to cannibalism and from frenetic overactivity to a pathological withdrawal from which individuals would emerge to eat, drink and move about only when other members of the community were asleep. The social organization of the animals showed equal disruption. [...]
In the experiments in which the behavioral sink developed, infant mortality ran as high as 96 percent among the most disoriented groups in the population.[4]
From Behavioral sink
The Left is enamored with Malthus, who predicted that the population will always expand to fill up the available resources, and will create misery if the population is not controlled. This has been falsified, especially by Calhoun. People can have all they want, and still not want to reproduce. In fact, by having all they want, they view children and the inconvenience of raising them as burdens. The left will continue to try and control us to prevent "overpopulation" well into our Calhoun colony collapse.
Calhoun's critics have claimed that his work has no relevance to humans, but I think it does. For instance it completely explains the left/right political divide in this country. Democrats are the party of high density populations, and they have the behaviors of a Calhoun "behavioral sink," the anti-child policies. Republicans are the party of low population density, and hold to traditional pro-human policies.
Every developed country, and every developing country, is urbanizing into high density cities. It is this, simply the stress of high density life, which can cause our population to collapse. The economy will just add a new layer onto it, and will be self reinforcing. As the population declines, the economy will shrink, and people will continue to choose not to have children.
These Mayan ruins were not built in a forest, this used to be a city more densely populated than Los Angeles is today. It functioned for hundreds of years, then the Mayan city states started collapsing. The greatest collapse happened over only two hundred years, when the population dropped 90%. The populations and culture never recovered. There were no catastrophes, no all out wars that really explain this. The current fad is to blame climate change, but I suspect there have been civilizational collapses where the population just stopped having enough children, for economic and other reasons. Scientists don't see it, because they are still under the spell of Malthus, who claimed it can't happen.