Complexity has a cost — in physical energy, administrative friction and other forms. At first, increases in complexity seem affordable and even useful. Eventually, however, a law of diminishing marginal returns on complexity sets in, and the costs, while difficult to see, become harder to bear. If such a complex society is then buffeted by external or internal shocks, it can collapse. The shocks have typically included droughts, famines, plagues, migrations, rebellions, civil wars and invasions