Comment by dragonwriter

> Genetics are likely a factor too, but light exposure seems to have a huge effect: "The prevalence of myopia in 6- and 7-year-old children of Chinese ethnicity was significantly lower in Sydney (3.3%) than in Singapore (29.1%), while patterns of daily outdoor light exposure showed that children living in Singapore were exposed to significantly less daily outdoor light than Australian children."

So, it's not even an individual correlation but a population-frequency-correlation between populations in different physical environments, which is so even more likely than an individual correlation to represent an effect other than causation in either direction, such as any other environmental (or population-genetic-distribution) difference.