Autism causes communication problems

A Bristol University team will also examine the possible impact of problems with birth, diet, infections and exposure to toxins.

They will also test the theory that other conditions, such as coeliac disease, may increase the risk.

More than 500,000 people in the UK are estimated to be affected by autism spectrum disorders.

While it is true that this disorder does sometimes run in families, it isn't purely genetic Professor Jean Golding

The condition affects the way people communicate and relate to people around them and sufferers have problems with everyday social interaction.

They have a limited ability to develop friendships and find it hard to understand other people's emotional feelings.

The incidence of the condition appears to have risen sharply over the last 30 years. However, nobody knows why this is, and it is possible that more cases are simply being diagnosed than in the past.

Some scientists have suggested the MMR jab may be linked to autism.

However, no research has ever proved a link, and the overwhelming majority of experts believe the vaccine is safe.

The new research will be based on data generated from 14,000 children already taking part in the Children of the 90s study - a long-term project to examine the role of environment and genes on children's health.

Lead researcher Professor Jean Golding said: "Because of the number of children we'll be looking at, and the quality and type of data available, our study should help find the answers to a number of currently unanswered questions about the environmental risks for developing autism spectrum disorders.

Several theories

Rising rate of autistic spectrum disorders in UK 1966: one in 2,222 1979: one in 492 1993: one in 141 2004: one in in 110 Source: National Autistic Society

"One theory suggests that the causes of autistic behaviours arise very early in pregnancy - even in the first few weeks.

"While it is true that this disorder does sometimes run in families, it isn't purely genetic.

"One possibility is that something happens in the womb, which interacts with a gene - and the result is a child with an autistic spectrum disorder.

"For instance, is there evidence that the mothers of children with particular autistic traits were exposed to infections more often in pregnancy than mothers of unaffected children?

"A number of possible causes have been suggested - and we shall be testing various hypotheses concerning the cause and origin of each trait, whether it is genetic or environmental."

The project is one of four studies into autism funded by the Medical Research Council.

These include using imaging techniques to examine differences in the brain anatomy of adults with autism.

Stuart Notholt, of the National Autistic Society, said: "Autism affects millions of people worldwide yet there are many unanswered questions surrounding the condition.

"Today's announcement is a very positive step forward, however until the outcomes of these new research studies are seen, the significance of their contribution to understanding the development of autism will remain undetermined."