Certainly in some cases it is caused by genetics, but from what I've read, the evidence suggests that some other factor is at play for the overwhelming majority of cases. Here is an excerpt from a nature article which leads me to believe that this is the case.
> East Asia has been gripped by an unprecedented rise in myopia, also known as short-sightedness. Sixty years ago, 10–20% of the Chinese population was short-sighted. Today, up to 90% of teenagers and young adults are. In Seoul, a whopping 96.5% of 19-year-old men are short-sighted.
I don't know how that change is possible in one generation if genetics are to blame. It's possible I'm missing something though.
What proportion of the population had their eye sight measured 60 years ago in China (or anywhere)?
Is it certain that we are comparing like for like? Many more people everywhere do work that requires a lot of reading now than 60 years ago even in the northern Europe so perhaps myopia is more often diagnosed now simply because it is more of a problem in the modern environment.
You don't necessarily have to have measured the prevalence of myopia among children 60 years ago to extrapolate from the prevalence of myopia among 70yo today (since myopia is, by and large, a progressive condition and not typically reversed spontaneously).
Sure, you have to be worried about selection biases of all sorts including survivorship bias (eg. suppose myopic people mostly died before they reached 50), but in general, if myopia is as high or higher among 20yo than 60yo today, it tells you something pretty darn significant.
https://www.nature.com/news/the-myopia-boom-1.17120
> East Asia has been gripped by an unprecedented rise in myopia, also known as short-sightedness. Sixty years ago, 10–20% of the Chinese population was short-sighted. Today, up to 90% of teenagers and young adults are. In Seoul, a whopping 96.5% of 19-year-old men are short-sighted.
I don't know how that change is possible in one generation if genetics are to blame. It's possible I'm missing something though.