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"Population declines following economic collapse" should be "Population declines following sustained drop in birthrate." The population is going to decline regardless of what the future economy does.

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I'm not very familiar with their model, but if the birthrate asymptotes to above the replacement rate, or goes back up, the population won't decline. So the economy might be a factor.

scottw is referring to the well-studied demographic transition.

http://en.wikipedia.org/wiki/Demographic_transition

TL;DR The world is getting older, hence less babies born.

Downvote? For quickly summarizing a wikipedia link? I guess we all have time to waste skimming wikipedia articles, don't we?

The problem is that it's wrong.

A better summary would be, "The world is getting richer, hence lower infant mortality, hence birth rates drop."

The world is graying extremely quickly (Japan, Western Europe, the US, and soon China).

I knew that already.

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