I have bad myopia (around -5) both my parents have it too and to me it has always been evident that it isn't purely genetic. I think I could willfuly downgrade my vision if I concentrated.
In my family (n=basically_none), it seems that myopia correlates strongly with habits. The most bookish, indoors-y kids ended up in glasses - and the more so, the thicker the glasses - while folks who spent their childhoods running around outside tend to have 20/20 vision.
Assuming such an effect really exists, I expect it would be very difficult to statistically distinguish from straight heredity. Kids' early childhood habits tend to reflect family culture.
OTOH, perhaps those who are starting to have myopia aren't going to want to do outdoorsy things because they tend to require far-seeing. If I can't see what's going on far away, I'm not going to have a fun time with most sports, and I'd rather read a book.
The other hypothesis I've heard is that it's switching back and forth between focusing on close-up things and distant things. Which you naturally do a lot when engaging in outdoor activities, but basically not at all when staring at books or screens.
I have hypermyopia (-12.5 left, -13 right) and my sister has 20/15 vision. Same childhood environment as her and I needed glasses before I could even read, let alone before I ever played a video game.
It's well beyond a genetic predisposition, it was a genetic guarantee for me.
Not sure if it'll help, there's an older gentleman on Youtube who claims reading with glasses actually makes myopia worse with time, due to the way it affects the shape of the eyeballs. Look up Tod Becker.
