Before Alaska became a state of the United States in 1959, the incidence of short-sightedness (also known as near-sightedness) among the native Aleuts was almost zero. After attaining statehood, the government opened schools and Aleut children became school students, spending large amounts of time reading which they had not done before. Almost immediately, the incidence of near-sightedness shot up to around 25%.
It is known that in a person's early years, an exquisitely fine-tuned chemical feedback loop is active in determining the final shape of the cornea, the part of the eye that performs most of the refraction of light necessary for a focused image on the retina. (The lens provides a secondary, variable, stage to adjust for distance.) It seems that in some fraction of people who spend a lot of time focusing on a nearby object such as a book or cell phone, that feedback loop decides that the best shape of the cornea is that which provides a default focusing for near objects, with the deformable lens unable to correct in the sense opposite to that for which it was intended.
It this is correct, having children spend more time outside would seem to be helpful but this probably does not recognize the underlying cause, which is not that kids spend too much time indoors per se but that they spend too much time reading. The increase in nearsightedness around the world in recent years quite possibly relates to increased time looking at smart phones. Of course there is likely a connection to how much different cultures encourage academic performance.
It is interesting to speculate that if susceptible children could be identified, they could be given "pre-corrective" lenses to wear only while reading or using a phone, tricking the feedback loop to settle on a more-normal cornea shape.
I'm sorry that I can't provide a reference for the Alaska story, but I am sure it sits somewhere on my bookshelf.