You could have poor eyesight because you didn't spend enough time outdoors as a child. That's the conclusion of a series of studies on myopia published in Optometry and Vision Science originally presented last year at the 12th International Myopia conference.

Myopia or commonly known as short-sightedness is getting increasingly common; around the world there are 1.6 billion people with myopia and this is expected to rise to 2.5 billion by 2020 according to the Institute of Eye Research (IER) Annual Report 2006-7.

The global jump in myopia cases is thought to be result of more and more children growing up in environments where they don't see objects far away, and the eye doesn't learn to focus on distant objects as it develops.

It's most common in apartment-dwelling societies where children watch TV and play computer games rather than playing outdoors. In Asian cities like Tokyo, Singapore and Hong Kong, between 30 and 50 per cent of 12-year-old children have some degree of short-sightedness. In the USA, 20 per cent of kids this age are myopic.

The problem isn't as bad in Australia - only about 2 per cent of four-year-olds and 15 per cent of 12-year-olds are myopic. But by the age of 20 this rises to about 30 per cent.

What causes myopia is a bit of a mystery, it's thought to be partly an inherited genetic trait (with several different possible genes involved). Hence, children with short-sighted parents are more likely to be short-sighted themselves.

But an individual's environment also determines whether a person will develop myopia, though we don't really understood how. One theory is that too much indoor close up viewing causes the eyeballs to fail to develop correctly - the length, or axis, of the eyeball is too short to be able to focus correctly on more distant objects.

Another theory is that it might be due to a lack of light, which retards the development of the eye in kids who spend too much time indoors.

Whatever the developmental failure is, it tends to affect the eyes in other ways too. Myopia doubles the risk of glaucoma and retinal detachment, two common causes of vision loss in older age groups.

Preventing myopia

Myopia can easily be corrected in children and adults with spectacles, contact lenses or laser surgery.

But it's much better to prevent it altogether so there are fewer treatment costs, better school performance and less chance of eye disease later on in life . This means using the eyes the ways they were intended - visualising not just objects close up but at longer distances - will allow the eye to develop the way it should in childhood.

According to one study, the Sydney Myopia Study, the risk of developing myopia is great - about 60 per cent- in children who get five hours or less per week of outdoor activity. But if they do more 14 hours or more a week - two hours or more a day - the risk drops to 20 per cent.

It's being in an outdoors environment that seems to be important, not the type of activity. Studies show that both active and passive outdoor activities have the same protective effect.