They called for improved diagnostic tests, and larger population studies to test the theory further.

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One theory is that as the population grew, the people quickly exhausted all of the natural resources.

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This lends weight to the theory that a viable hybrid population was sustained by interbreeding over a long time.

ECONOMIST: Humans could have interbred with chimpanzees

In theory the park's population should be self-sustaining by 2004.

ECONOMIST: Wildlife conservation: Animal farm | The

To test this theory, the Glasgow Centre for Population Health examined 20 regions in Europe that went through rapid deindustrialisation.

ECONOMIST: Unhealthy Glaswegians

But across the population the number of children born with disorders is likely to increase if this theory holds true.

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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

The latest theory to gain sway: We don't want our unemployment rate kept down by an exodus of population, so we must stop that tide, too.

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