I remember years back reading one of Loren Cordain's paper on myopia. He hypothesized that insulin resistance created via diet was a primary factor.
While diet may be a contributing factor, it appears that sunlight exposure is now being viewed as the primary factor. As children are spending more time indoors, myopia rates are on the rise. A number of studies over the past 5 years have analyzed this phenomenon.
An objective discussion on sunlight exposure and myopia can be found here. The researcher being interviewed believes that 10-14 hours per week spent outside is the minimum threshold needed to lessen the odds of myopia.