Joseph Tainter - Wikipedia, the free encyclopedia:
"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). However, as the Empire grew, the cost of maintaining communications, garrisons, civil government, etc. grew with it. 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). 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."