Rates of nearsightedness, or myopia, have skyrocketed around the world in recent years, especially among young people who spend their time indoors, Science News reports. In the U.S., 42 percent of people between the ages of 12 and 54 have the condition, compared with 25 percent 40 years ago. Researchers used to believe that myopia was primarily hereditary, but “the gene pool can’t change that much in a generation, not even in several,” says Ian Morgan of Australian National University. He and his colleagues now believe that too much time gazing at books and computer screens is damaging kids’ vision. Recent studies have shown that 95 percent of Shanghai college students are nearsighted, perhaps because Chinese children tend to spend more time studying indoors than students elsewhere in the world do. Rates of the condition also seem to be rising only among urban kids, as opposed to rural children, who spend more time outside. Researchers still aren’t sure exactly how working indoors harms eyes—or how being outdoors helps protect them—but it may be that bright, outdoor light stimulates the release of dopamine in the retina, which helps the eye to develop properly.