German study

[Bifocal]

I have been saying this over,and over.
In the years prior to the 1970's,high myopia prescriptions were rare. Minus 3 or lower,was common for people that read ,or did close work frequently.
But since the advent of computers,and people spending many hours a day on it ,there has been an explosion of people required to wear high minus prescriptions,often into the double digits.
http://www.philly.com/philly/health/Why_...asses.html

[wtfgod]

To play the devils advocate, before the 1970's maybe there were not many opticians or reasons for people to need 20/20 vision. For example, if I was on a farm milking a cow or sowing it would not necessarily bother me to the degree that I would spend money to go into a town to find an optician. (Fictional argument)

So maybe there was always low grade myopia in a large portion of the population and we are just now fully proliferating the population with easy access to vision care. (Differences in countries could be accounted for by genetics)

Disclaimer: I do not believe this argument, but it's best to view all sides of the coin.