Firstborn children in a family are more likely to be nearsighted than younger siblings are, and parents may be to blame for pushing their oldest child harder to hit the books, researchers say.

The firstborn child is 10 percent more likely to by myopic than subsequent siblings, and 20 percent more likely to be classified as severely nearsighted, researchers are reporting.

When the data was adjusted to account for a family's level of education, the link between birth order and nearsightedness shrank, leading to the possibility that "reduced parental investment in education of children with later birth order may be partly responsible," the researchers wrote in the journal JAMA Ophthalmology.

In other words, firstborns may come under parental pressure to succeed educationally, which may have them spending more time peering at books and computer screens.

"In the current study we set out to test whether the link between birth order and myopia might have arisen because first-born individuals tend to spend slightly longer in full-time education than later-born individuals," says study lead author Jeremy Guggenhem of Cardiff University in Wales.

Time spent doing "near" work including educational activities such as reading is a known risk factor for myopia, which is increasingly prevalent in younger generations in many parts of the world, the researchers note.

The results could explain why people from more studious cultures may be prone to near-sightedness, borne out by recent studies that found a significant rise in the incidence of myopia in Asian countries.

"Greater educational exposure in earlier-born children may expose them to a more myopiagenic [factors causing myopia] environment; for example, more time doing near work and less time spent outdoors," the researchers wrote.

Short-sightedness in firstborns is not a new phenomenon, they note.

"The cause of the birth order-myopia association is widespread and has been in existence for several decades," they wrote.

Still, the UK researchers acknowledge, the association found in their study was fairly small, whereas the increase in Asia in recent decades has been much greater, suggesting there may be other environmental factors in play.

In some part of Asia the rate of myopia has reached 90 percent, compared with 20 percent to 50 percent in other parts of the world.

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