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.