The numbers are interesting, but not quite as important I think as the reasons why these civilizations, like all the rest, failed. I find it interesting that some of the early American civs knew for quite a while they were going to end. Were they smart enough to know they were doomed, but not smart enough to be able to do anything about it?

Sounds familiar.
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The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)