Autism may be linked to hormone levels circulating in the developing foetus, research suggests.
About one in 1,000 people have autism
A team from Cambridge University found babies who produce high levels of the male hormone testosterone in the womb are more likely to show symptoms.
The finding suggests the condition may be genetic - and raises the possibility of a screening test.
It also lends weight to the theory that autism is basically an extreme form of the way men think and behave.
The research, led by psychologist Professor Simon Baron-Cohen, focused on 70 children whose mothers underwent an amniocentesis test while they were pregnant.
This enabled the researchers to measure levels of foetal testosterone in the samples.
When the children were four, their parents were asked to complete a checklist designed to record any signs of behavioural and social difficulties - which are associated with autism.
Speaking at the British Psychological Society annual meeting, Professor Baron-Cohen said: "Those who had a high level of testosterone also found it more difficult to fit into new social groups."
Children who had higher levels of foetal testosterone were also less curious than other children.
Although these children were not autistic, the pattern suggested a link between foetal testosterone and autism-like traits.
Professor Baron-Cohen added that earlier tests, carried out when the children were 12 months old, also showed that those exposed to higher testosterone levels were less willing to make eye contact - another key indicator of autism.
Previous research by the Cambridge team has shown that men are less empathic
and more systematic than women - differences that are greatly exaggerated in
people with autism.
Professor Baron-Cohen said: "What I am doing is testing this idea that
autism might be an extreme of the male brain.
"It's showing that the sexes are different. It's not about one being better than the other. You're going to find individuals who are not typical of either sex."
Stuart Notholt, director of policy and pulbic affairs for the National Autistic Society, told BBC News Online the research was a "significant contribution".
He said: "However, it is early days yet. We are still quite a long way off understanding what genetic underpinning - if, indeed that is the cause - there is for autism."
Mr Notholt said many people would be "very uneasy indeed" about the prospect of a pre-natal screening test for autism.
"It is a very varied condition. Many people with autism go on to live very full and happy lives."
Professor Jack Scarisbrick, chairman of the charity Life, said a screening test would inevitably lead to pressure being placed on parents to abort autistic babies.
"Testing and medical screening to provide information and prepare parents for the specific challenges a special-needs child brings is to be encouraged. But that will not happen in this case.
"What chance does an autistic child have of survival when children with cleft palates are aborted?"
The research is due to be published in the Journal of Child Psychology and Psychiatry.