Girls appear to inherit a genetic predisposition from their fathers that protects them against autism, whereas boys are more vulnerable because they inherit the factor only from their mothers, whose protective genes are more likely to be switched off.
The research is part of a wider programme of work trying to explain why boys are much more likely than girls to suffer from a range of serious developmental problems involving social behaviour and language, such as autism and attention-deficit disorder.
"Even among normal children, girls generally outperform boys in those very same skills," said Professor David Skuse, of the Institute of Child Health in London.
"We have evidence for a genetic mechanism that could explain female superiority over males in some aspects of social intelligence.
"Boys are far more likely than girls to be socially maladjusted. Social skills are probably distributed as a `bell curve' in the general population. We have found the curve is shifted to the left in boys, so there are relatively more boys than girls with poor skills."
Severe autism affects about four children in every 10,000, although, if milder autistic symptoms are included, the rate climbs to about one in every 1,000 children.
"The vulnerability of boys to autism and other neuro- developmental disorders, all of which are associated with the impairment of social skills, has never been satisfactorily explained," Professor Skuse said. "We propose that there is a lower threshold of susceptibility in boys than girls."
Although boys are more affected than girls, there is no evidence that the genes that predispose to autism are carried on the X-chromosome, as they are with sex-linked disorders such as haemophilia, which usually affect boys but not girls.
Boys have only one X- chromosome, which they inherit from their mothers, whereas girls have two, one from each parent.
"The threshold hypothesis we are suggesting is that normal girls who carry their father's X-chromosome also have a protective factor on that chromosome that helps to prevent them from developing autism," Professor Skuse said.
"We believe it is an imprinted gene, which is switched off when inherited from one parent and switched on when inherited from the other.
"In this particular instance we are suggesting the gene is always switched on when it transmitted by a father and always switched off when it is transmitted by a mother."
One possible explanation for why boys are more likely to be autistic is that it offered some evolutionary benefit in the past, Professor Skuse said. "It may be that to be slightly less socially responsive may have been an advantage to males."
A workshop at the Festival of Science, in Cardiff, demonstrating how a total eclipse of the Sun will occur on 11 August next year Tom Pilston