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This is for everyone whose parents said, “Sitting too close to the TV is going to ruin your eyes.” In other words, pretty much all of us. Now as parents you are concerned about your children and the amount of time they spend looking at various devices that make up our digital world.

Sitting too close to the TV doesn’t predict nearsightedness, according to a study that tracked the vision of thousands of children over 20 years. Nor does doing a lot of close work i.e. tablets and smart phones.

Instead, as early as age 6 a child’s refractive error or prescription – best predicts the risk.

One-third of adults are nearsighted, and the problem typically develops between ages 8 and 12.

Children are not great about telling parents that they can’t see the board in class, and the letter-chart screening tests used by some schools and pediatricians are less than ideal, according to Karla Zadnik, dean of the College of Optometry at Ohio State University and lead author of the a new study. It was published Thursday in JAMA Ophthalmology.

“Just measuring how well they can read the chart won’t capture that key piece of information,” she says.

In this analysis, the researchers looked at 13 potential risk factors for nearsightedness, or myopia.

Having nearsighted parents increases the risk for myopia, the study found, but it’s not as strong a predictor as is refractive error. Doing close work or watching TV close up weren’t risk factors.

Earlier work by this group found that children who spent more time outdoors were less likely to be nearsighted, but it’s unclear why that would be. One theory is that being exposed to sunlight or higher vitamin D levels could make a difference. The study was funded by the National Institutes of Health.

In the study, children whose refractive error was less than +0.75 diopters (which is slightly farsighted) in first grade were most likely to become nearsighted. As a child gets older that number drops, so that by sixth grade a child with no refractive error is at risk. Myopia is defined as a refractive error of -0.75 diopters or more.

There’s no way to prevent nearsightedness, but finding out if your child is at risk is an important first step.