A study of twins suggests that a gene key to eye development plays a crucial role in people becoming short-sighted.
Scientists say too much TV is still a factor in short-sightedness
Researchers from St Thomas' Hospital in London said faulty versions of the PAX6 gene could make people more susceptible to the condition.
But they said sitting too close to the TV or playing too many computer games could exacerbate someone's risk.
Around a quarter of the UK population are short-sighted, but it has reached epidemic proportions in the Far East.
In Japan, two thirds of teenagers are already myopic.
The problem is said to
even worse in Singapore, where 80% of 18-year-old male army recruits are short
sighted, compared with 25% just 30 years ago.
The problem in Singapore is so bad that employers such as the police are
struggling to find people who meet their requirements.
Previous research looking at whether the causes of short-sightedness (myopia) were genetic or environmental had found inherited factors accounted for 89%, and the environment 11%.
'Looking at the mechanics'
The St Thomas' study scanned the DNA of 221 pairs of identical and non-identical twins who were on the hospital's Twin Research Unit database.
They found faults in the PAX6 gene, which affects how the eye develops. People who are short-sighted have 'longer' eyeballs than those with normal or long sight.
Dr Chris Hammond, an honorary consultant at the Twin Research Unit and consultant ophthalmologist at the Princess Royal University Hospital in Bromley, told BBC News Online environmental factors were likely to exacerbate the problem.
"If you do lots of reading, you're likely to become short-sighted, particularly if you do it at a young age - that's probably why there's an epidemic of myopia in the Far East.
"But people who develop it young, and who become severely short-sighted, probably do have a genetic susceptibility."
He added: "We now hope to start working out the mechanics of how people become short-sighted."
Until more is known, he said parents should limit the time their children spend on activities such as close reading and computer games.
Dr Hammond said parents should be aware of the research, particularly if they are both myopic.
He added: "I recommend as an ophthalmologist that you take a few simple steps to ward off the development of myopia.
"Try moving the television further away from the sofa, don't let your children play computer games or surf the net for hours on end - and ensure that they get out of the house to enjoy the summer sun and play with friends."
The research is published in the American Journal of Human Genetics.