Originally Posted by Wikipedia
In his 1988 book "The Collapse of Complex Societies" Tainter presents the view that for given technological levels there are implicit declining returns to complexity, in which systems deplete their resource base beyond levels that are ultimately sustainable. Tainter argues that societies become more complex as they try to solve problems. Social complexity can include differentiated social and economic roles, reliance on symbolic and abstract communication, and the existence of a class of information producers and analysts who are not involved in primary resource production. Such complexity requires a substantial "energy" subsidy (meaning resources, or other forms of wealth). When a society confronts a "problem," such as a shortage of or difficulty in gaining access to energy, it tends to create new layers of bureaucracy, infrastructure, or social class to address the challenge.
For example, as Roman agricultural output slowly declined and population increased, per-capita energy availability dropped. The Romans "solved" this problem by conquering their neighbours to appropriate their energy surpluses (metals, grain, slaves, etc). As a consequence of the growing Empire, the cost of maintaining communications, garrisons, civil government, etc. increased. Eventually, this cost grew so great that any new challenges such as invasions and crop failures could not be solved by the acquisition of more territory. At that point, the empire fragmented into smaller units.
We often assume that the collapse of the Roman Empire was a catastrophe for everyone involved. Tainter points out that it can be seen as a very rational preference of individuals at the time, many of whom were actually better off (all but the elite, presumably[citation needed]). Archeological evidence from human bones indicates that average nutrition actually improved after the collapse in many parts of the former Roman Empire. Average individuals may have benefited because they no longer had to invest in the burdensome complexity of empire.
In Tainter's view, while invasions, crop failures, disease or environmental degradation may be the apparent causes of societal collapse, the ultimate cause is diminishing returns on investments in social complexity.