FRIDAY, Jan. 30 (HealthDay News) -- Kids who spend more time outside -- and away from the television set -- are less likely to develop myopia, the inability to see things clearly at a distance.

The new report, from researchers in Boston, doesn't determine whether too much indoor activity actually causes poor eyesight. And even if it does, researchers haven't pinpointed what the exact mechanism might be.

Still, "it would seem prudent to encourage outdoor activities -- not necessarily sports -- for all growing children and young adults in order to reduce the progression of myopia," said Howard C. Howland, a professor of neurobiology and behavior at Cornell University.

About a third of Americans suffer from myopia, said study author Jane Gwiazda, director of research at The New England College of Optometry. The rates are much higher in some parts of Asia.

The condition seems to be caused by both genetics and the environment, Gwiazda said. The condition is more common in people who engage in a lot of "near work" due to their jobs, she said.

The study authors gave questionnaires to the parents of 191 children who were at an average age of 13.3 years. Among other things, the researchers asked about the children's time spent using the computer, reading for pleasure and watching TV.

The children's eyesight was tested annually.

The findings were published in the January issue of Optometry and Vision Science.

The children who developed myopia -- also known as nearsightedness -- spent less time in outdoor activities, an average of 8.3 hours a week compared to 12.6 hours among the other children.