There are many theories on this. The most accepted is genetics. Other people point at nutrition and at hunter-gatherer societies where there are no problems with eyesight. There is Bates, who thought that near sightedness was caused by inability to relax the eye fully, and came up with some "exercises", mostly for relaxing the eyes. Finally, I think one of the most interesting theories was put forth by Robin Parsons, who has a Yahoo! group called Visionsenz. He says that myopia is caused by a bad habit of focusing the eyes too broadly, a technique commonly used for reading more quickly.
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