Vision problems in children take many forms. But the likely condition in Japan and the United States is nearsightedness as a result of a combination of genes, behavior and environment.

Students in Japan with 20/20 vision, which is considered ideal, constituted just 31.4 percent of the population in elementary school, 54.6 percent in junior high and nearly 66 percent in high school. Yet Japan is not alone.

For reasons still unclear, young people in East Asian countries are most at risk for myopia, which is the medical term for nearsightedness. A study published in the journal PLOS One in 2015 found that 80 percent of 4,798 teenagers in Beijing were nearsighted. In Seoul, 96.5 percent of 19-year olds were nearsighted.

Although the rate of myopia in the U.S. increased by nearly two-thirds over the past three decades to an estimated 41.6 percent, according to a National Eye Institute study, it is far lower than in Asian countries.

One theory for the difference in nearsightedness between East Asian countries and the U.S. is that learning to read and write Asian languages requires far more close work than learning to read and write English. As the eyes of children develop, any unusual strain has the potential to elongate the eyeball, causing images to fall short of the retina. Children in the U.S. don’t have to focus as intently on identifying writing characters as their counterparts in Japan.

For many years, genes were thought to play the predominant role in determining visual acuity. But new research has shown that the amount of time spent watching smartphones, video games and other electronic devices, coupled with a reduction in the number of hours spent outdoors, are also significant.

Less appreciated is how staring at electronic devices affects vision. To see clearly up close, the eye has to exert its focusing power. The fatigue caused by staring at devices for hours on end has been implicated in leading to changes within the eye in the form of nearsightedness.

That’s why ophthalmologists recommend taking frequent breaks using the 20-20-10 rule. Every 20 minutes of screen time should be followed by looking at an object at least 3 meters away for at least 10 seconds.

In light of the new evidence, it’s surprising that the education ministry has not given greater attention to the importance of spending more time outdoors in natural light. A dozen research studies have shown that the rate of nearsightedness has grown in tandem with the amount of time spent indoors.

For example, one preliminary study of 2,000 children in China showed a 23 percent reduction in nearsightedness for those spending an additional 40 minutes outside each day. But with pressure on children in China to pass the “gaokao” (higher education entrance exam), which largely determines their future, it’s unlikely that less time will be spent on homework and reading.

The same pressure on students to excel academically in Japan has led to too many hours spent indoors hunched over books. Although correlation is not causation, countries that rank high on tests of international competition also have the highest rates of nearsightedness.

Whether the price paid to excel is worth the risk of developing myopia is more than a rhetorical question. An increase in the number of children in early primary grades who now have the condition means that it has a longer time period to grow in severity.

Walt Gardner writes the Reality Check blog for Education Week in the U.S.

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