From Nature in 2015:
The myopia boom
Short-sightedness is reaching epidemic proportions. Some scientists think they have found a reason why.
18 March 2015
… 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.
Other parts of the world have also seen a dramatic increase in the condition, which now affects around half of young adults in the United States and Europe — double the prevalence of half a century ago. …
They are challenging old ideas that myopia is the domain of the bookish child and are instead coalescing around a new notion: that spending too long indoors is placing children at risk. …
For many years, the scientific consensus held that myopia was largely down to genes. Studies in the 1960s showed that the condition was more common among genetically identical twins than non-identical ones, suggesting that susceptibility is strongly influenced by DNA1. Gene-finding efforts have now linked more than 100 regions of the genome to short-sightedness.
But it was obvious that genes could not be the whole story. One of the clearest signs came from a 1969 study of Inuit people on the northern tip of Alaska whose lifestyle was changing2. Of adults who had grown up in isolated communities, only 2 of 131 had myopic eyes. But more than half of their children and grandchildren had the condition. Genetic changes happen too slowly to explain this rapid change — or the soaring rates in myopia that have since been documented all over the world (see ‘The march of myopia’). “There must be an environmental effect that has caused the generational difference,” says Seang Mei Saw, who studies the epidemiology and genetics of myopia at the National University of Singapore.
There was one obvious culprit: book work. That idea had arisen more than 400 years ago, when the German astronomer and optics expert Johannes Kepler blamed his own short-sightedness on all his study. The idea took root; by the nineteenth century, some leading ophthalmologists were recommending that pupils use headrests to prevent them from poring too closely over their books.
… In the 1990s, for example, they found that teenage boys in Israel who attended schools known as Yeshivas (where they spent their days studying religious texts) had much higher rates of myopia than did students who spent less time at their books. On a biological level, it seemed plausible that sustained close work could alter growth of the eyeball as it tries to accommodate the incoming light and focus close-up images squarely on the retina.
Attractive though the idea was, it did not hold up. In the early 2000s, when researchers started to look at specific behaviours, such as books read per week or hours spent reading or using a computer, none seemed to be a major contributor to myopia risk5. But another factor did. In 2007, Donald Mutti and his colleagues at the Ohio State University College of Optometry in Columbus reported the results of a study that tracked more than 500 eight- and nine-year-olds in California who started out with healthy vision6. The team examined how the children spent their days, and “sort of as an afterthought at the time, we asked about sports and outdoorsy stuff”, says Mutti.
… Close work might still have some effect, but what seemed to matter most was the eye’s exposure to bright light.
… And Ian Flitcroft, a myopia specialist at Children’s University Hospital in Dublin, questions whether light is the key protective factor of being outdoors. He says that the greater viewing distances outside could affect myopia progression, too.
Are there state-by-state differences in myopia? Are kids in cold, wet Massachusetts more nearsighted than kids in balmy California or Hawaii?
Baseball batters benefit from sharp eyesight. David Epstein reported in The Sports Gene that when the Los Angeles Dodgers hired an optometrist to check their players, he had to create new, harder eye charts because most of the hitters could easily read the smallest print on the standard charts. Many big league batters scored 20-12 and one 20-9.
This might help explain why so many major leaguers these days come from Sunbelt states like Arizona and Florida: they are sunny.
That perhaps ties into the often discussed conundrum of how Mike Trout of New Jersey was still around for the Angels to get him with the 25th pick in the first round of the amateur draft. He was the best high school player in New Jersey, but how good are New Jersey high school players? In the case of Trout, who has finished first or second in A.L. MVP voting eight times in his first nine seasons, really good.
This also raises the question of how good old time baseball players were. In a nation of farm boys who grew up working outside, maybe a larger percentage of young men had excellent eyesight. (There might be data on this from conscription in WWI, WWII, and up into the 1970s.)