From the BBC:
...there are two factors that matter: ecological strain and economic
stratification. The ecological category is the more widely understood
and recognized path to potential doom...
That economic stratification may lead to collapse on its own, on the
other hand, came as more of a surprise to Motesharrei and his
colleagues. Under this scenario, elites push society toward instability
and eventual collapse by hoarding huge quantities of wealth and
resources, and leaving little or none for commoners who vastly outnumber
them yet support them with labor. Eventually, the working population
crashes because the portion of wealth allocated to them is not enough,
followed by collapse of the elites due to the absence of labor...
According to Joseph Tainter, a professor of environment and society at Utah State University and author of The Collapse of Complex Societies, one of the most important lessons from Rome’s fall is that complexity
has a cost. As stated in the laws of thermodynamics, it takes energy to
maintain any system in a complex, ordered state – and human society is
no exception. By the 3rd Century, Rome was increasingly adding new
things – an army double the size, a cavalry, subdivided provinces that
each needed their own bureaucracies, courts and defenses – just to
maintain its status quo and keep from sliding backwards. Eventually, it
could no longer afford to prop up those heightened complexities. It was
fiscal weakness, not war, that did the Empire in...
Whether in the US, UK or elsewhere, the more dissatisfied and afraid
people become, Homer-Dixon says, the more of a tendency they have to
cling to their in-group identity – whether religious, racial or
national. Denial, including of the emerging prospect of societal
collapse itself, will be widespread, as will rejection of evidence-based
fact. If people admit that problems exist at all, they will assign
blame for those problems to everyone outside of their in-group, building
up resentment. “You’re setting up the psychological and social
prerequisites for mass violence,” Homer-Dixon says. When localized
violence finally does break out, or another country or group decides to
invade, collapse will be difficult to avoid...