I seem to remember from a while back (maybe a couple of years) some research that addressed this.
Researchers in Australia looked at ethnically Chinese kids in Australia and in Singapore, so genetically almost identical populations. The kids in Singapore spent much more time indoors, whereas the kids in Australia spent at least two (IIRR) hours per day at school outdoors.
It seems that exposure to natural light, most likely to UV spectrum, signals to the eye when to stop growing. In the absence of this signal, a kid's eyes continue to grow after the skull has stopped growing. The result is that the eye bulges, increasing the distance between the lens and the retina, giving rise to myopia.
The eyes of the kids in Australia were getting enough light to stop growing, the eyes of the kids in Singapore were not getting enough light and continue to grow.