[zargon]

Significant outdoor exposure is essentially the only relevant factor for preventing myopia. I would be interested in seeing any studies that showed any meaningful relevance of low light reading, while controlling for time spent outdoors.

[avadodin]

Main factor is genetics.

I was gaming on a blurry CRT and reading on a dim light while hardly ever going outside during my teens and I only have light myopia on one eye.

I don't know if it has ever been studied, but I suspect eye socket morphology from my own family anecdata.

large eyes + small socket = squashed eyes.

It could be different genetic factors for different populations too.

[zargon]

I agree that genetics can protect you from developing myopia. For people who are genetically susceptible, outdoor exposure is critically important. Most people won't know which group their children are in.