For example, if productivity falls, current returns decline, says the theory, so workers and firms choose to work less and take more leisure.

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The theory is that reversing that decline before surgery may help patients be mobile and active afterwards.

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In theory, the region should provide it with boundless opportunities to offset the decline in its population.

ECONOMIST: Friends and neighbours

Insofar as civilization's collapse is something we're bringing upon ourselves with a little help from world powers, Martin and his readers might relate to radical environmentalists or peak oil environmentalists, who subscribe to the theory that there is a date when petroleum production will max out and decline rapidly.

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Other possible causes include hitters muscling themselves up as never before during the off-season, the creation of smaller baseball parks and a general decline in the quality of pitching (every time there is an expansion of the leagues, the theory goes, the pitching talent pool is stretched thin, and batters have a field day, so to speak).

FORBES: Fact And Comment