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CronoDAS comments on Two Truths and a Lie - Less Wrong
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According to the WP article's section on epidemiology, possibly more than half of all people have a very weak form of myopia (0.5 to 1 diopters). The general amount of prevalence (as much as a third of population for significant myopia) is much bigger than could be explained solely by the proposed correlations (genetic or environmental).
To me this high prevalence and smooth distribution (in degree of myopia) suggests that it should just be treated as a weakness or a disease. We shouldn't act surprised that such exist. It doesn't even mean that it's not selected against, as CronoDAS suggested (it would only be true within the last 50-100 years). Just that the selection isn't strong enough and hasn't been going on long enough to eliminate myopia. (With 30-50% prevalence, it would take quite strong selection effects.)
Why are you surprised that such defects exist? The average human body has lots of various defects. Compare: "many humans are physically incapable of the exertions required by the life of a professional Roman-era soldier, and couldn't be trained for it no matter how much they tried."
Maybe we should be surprised that so few defects exist, or maybe we shouldn't be surprised at all - how can you tell?
The prevalence of myopia has increased dramatically since 1970.
The two factors this suggests to me, over that time period, are "increase in TV watching among young children" and "change in diet toward highly processed foods high in carbohydrates". This hypothesis would also predict the finding that myopia increased faster among blacks than among whites, since these two factors have been stronger in poorer urban areas than in wealthier or more rural ones.
Hypotheses aside, good find!
Has this happened since 1970?
(The article suggests "computers and handheld devices.")
It didn't begin then, but it certainly continued to shift in that direction. IIRC from The Omnivore's Dilemma, it was under Nixon that massive corn subsidies began and vast corn surpluses became the norm, which led to a frenzy of new, cheap high-fructose-corn-syrup-based products as well as the use of corn for cow feed (which, since cows can't digest corn effectively, led to a whole array of antibiotics and additives as the cheap solution).
Upshot: I'd expect that the diet changes in the 1970s through 1990s were quite substantial, that e.g. sodas became even cheaper and more ubiquitous, etc.