(British Medical Journal) -- Genes are likely to have a much bigger role than environmental factors in asthma, suggests research on twins in the Archives of Disease in Childhood. The UK, Australia and New Zealand have the highest rates of asthma in the world, with between 17 and 30 per cent of the population affected.

The research involved a survey of the parents of 4,910 four year old twins, all of whom had been born in England and Wales between 1994 and 1995. The parents were asked what treatment their children had been given for asthma and whether they were identical or non-identical twins.

Identical twins share all their genetic make-up, but non-identical twins share only half of it. Therefore, if identical twins are more similar in their rates of asthma, this suggests genetic factors at work.

The results showed that identical twins and their co-twins had more similar reported rates of asthma than non-identical twin pairs. Applying statistical techniques to estimate heritability, the authors found that genes accounted for by far the largest share of asthma prevalence at 68 per cent. Being exposed to the same environmental allergens accounted for just 13 per cent, while different environment al exposure accounted for 19 per cent.

The authors conclude that environmental factors such as family diet and air pollution are likely to be much less important than genetic make-up. But they stress that many genes are likely to be involved in the development of asthma.