A study involving 20,000 children in China said Thursday that poor kids are far less likely to be nearsighted than their wealthier peers.

Nearsightedness, or myopia, was twice as prevalent in the middle-income province of Shaanxi -- where students have greater access to books that require close-up focusing -- compared to the poorer neighboring province of Gansu, said research in Ophthalmology, the journal of the American Academy of Ophthalmology.

Researchers said the findings are based on one of the largest population-based studies ever conducted on nearsightedness in children.

The scientific team included Chinese government agencies and universities and experts from Stanford University in California.

They tested the vision of fourth and fifth grade students, those in the nine- to 11-year-old range.

More than 9,400 students were in Shaanxi, and more than 10,100 students in Gansu, the second poorest province in China.

Nearly 23 percent of youths in Shaanxi has clinically significant myopia, nearly twice that of the lower-income province of Gansu, which had a 12.7 percent prevalence rate of myopia, said the findings.

"Living in the middle-class area was associated with a 69 percent increased risk for nearsightedness, even after adjusting for other risk factors, such as time spent reading, outdoor activity and whether the student's parents wore glasses," said the study.

Doing well at math, which requires intensive focus on a written page, was among the factors linked to higher incidence of myopia.

Girls were also more likely than boys to be nearsighted.

One idea researchers had is that the poorer schools' use of blackboards as a teaching tool may have had some protective effect against myopia, but this association disappeared when researchers controlled for other factors such as family history and outdoor play.

"We're still on the hunt for a plausible explanation and think the results merit more study into whether using blackboards versus books may be partially responsible for protecting eyes against nearsightedness, and what other factors may play a role," said lead investigator Professor Nathan Congdon of the Zhongshan Ophthalmic Center at Sun Yat-sen University in Guangzhou, China.

"What's important is that we figure out how lower-income Chinese students have avoided nearsightedness so we can use those same strategies to prevent more childhood myopia cases across Asia and perhaps even the world."

Myopia has been rising rapidly in some developed parts of East Asia, where the prevalence of myopia has reached 80 to 90 percent of the population.

Studies in Asia and Denmark have shown that nearsightedness is less common in children who spend more time outdoors and are exposed to more natural daylight.