Shrug, this problem with the prevalence of myopia ( what people call short sightedness ) has so many hypothesis practically spawning over the past twenty years that I am not sure where to begin. ( By the way, I wear glasses, and I am seriously considering laser surgery, even though my opthamology consultant tells me the technology is still too primitive as it involves mucking around with the cornea unnecessarily ).
The popular theory is reading and watching too much TV can cause myopia. This have evidence for and against it. The evidence for is mostly epidemiological, where there are evidence to indicate that people who do watch more TV and read too much ( eye strain ) has a higher prevalence rate of myopia over people who don't.
The problem with this theory is that there are major anomalies in terms of population base studies like in some affluent parts of Africa where people do read and watch TV a lot but has a large portion of the population still having 20/20 visions. Also, though there are statistical evidence, physiological evidence is rather scanty. The worse thing, it turns out that myopia has a strong racial tendency with lesser influence per hour and intensity basis compared to other races.
This leads to the second theory which is slightly unPC but has a stronger evidence behind this. The theory goes that the longer a population or race has left behind their hunter-gatherer past and mantains a long period of peace where vision no longer becomes a problem of life and death due to a lack of war and reliance more on an established system, as the generations goes by, myopia becomes more and more a problem as myopia is not selected against.
This is particularly evident among the Oriental populations, and most acute among the Japanese and Chinese, where the percentage of people with myopia reaching 98% in some cities like Taipei and Shanghai, and never reaches below 50% even in the village areas. ( Do note myopia among male Caucasians in London reaches 60% of the population surveyed only in 1990 ). The other interesting population spike is in Sri Lanka. After some time, it is noted that what distinguishes this two population from the rest is that vision was never really a survival factor for both groups for nearly 4000 years in the case of the Orientals and 2000 years in the case of the Sinhalese. Oppose this to the urban Zulu and Bantu races of Africa who have a myopia rate of less than 10% and in many cases, have only left their hunter-gatherer past behind for at most 8 generation.