Previous studies have suggested that genes play a key role in determining who is at risk of developing the potentially socially debilitating illness.

But two studies carried out in the UK and the US suggest that environmental factors such as experiences in the womb, may also be involved.

However, the National Autistic Society has warned that more research is needed.

In the first study, David Greenberg, a geneticist at Columbia University in New York, examined a database of families in which at least two siblings had autism.

He began to have doubts about the genetic basis of autism when he identified a significant number of both identical and fraternal twins in the database.

Of the 166 sibling pairs they identified, 17 were identical twins and 12 were fraternal twins.

The figure for identical twins was 12 times higher than the general population.

But the figure for fraternal twins was also high, at four times the rate for the general population.

Unlike identical twins, fraternal twins do not share the exact same genetic blueprint.

A second study, by Christopher Gillberg of St George's Hospital Medical School in London, also identified a high proportion of twins with autism.

He studied 79 sibling pairs and identified nine pairs of identical twins - 14 times the level in the general population.

Environmental factors

The findings, published in New Scientist, raise the prospect that the experiences of twins in the womb are important factors.

The authors suggested that the battle between twins for resources, such as food, could be a factor.

They added that high rates among identical twins could be due to a fiercer competition for these resources.

Identical twins can be more closely associated in the womb, lacking one or two membranes that separate fraternal twins.

Dr Greenberg speculated that not having enough of these resources could increase the risk of developing autism.

But he added: "The lesson here is that even in the age of the Human Genome Project, you can't jump to the conclusion that everything is genetics."

More research

A spokesman for the National Autistic Society said: "This study adds to other work suggesting environmental exposures in the womb might play a significant role in the development of the disorder.

"However, as the recent Medical Research Council Review of autism reported, no specific prenatal exposures have been established as contributory as yet."

He added: "Since the condition is not wholly genetic, environmental factors also need to be explained.

"Only 10% of the cases of autism can be ascribed to a particular medical condition. This leaves an awful lot of explanation still to be done."