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Men who wait until after the age of 40 to father children are far more likely to have offspring with autism than those who have kids before the age of 30, according to a new study.

The older dads are six times more likely to have autistic children than the younger ones, researchers found.

The mother's age does not appear to have an influence on the likelihood of autism.

"We believe that our study provides the first convincing evidence that advanced paternal age is a risk factor for autism spectrum disorder," said Abraham Reichenberg, a researcher in psychiatry at the Mount Sinai School of Medicine in New York.

While the biological mechanism is not entirely clear, he said, the phenomenon could be due to genetic mutations in sperm-producing cells "associated with advancing age" or changes in genetic imprinting in older men.

(A child has two copies of every gene, a maternal one and a paternal one, and it is believed that the gene's "memory" is more likely to falter in older fathers.)

It is also possible that the increase in autism among offspring of older men may be due to socio-cultural factors, Dr. Reichenberg said.

Autism and related conditions, known under the catch-all term autism spectrum disorders, have become increasingly common in recent years.

The soaring numbers are due to a combination of factors, including greater awareness, changes in diagnostic criteria and, perhaps, an increase in children born with the disorder.

The Autism Society of Canada estimates the incidence rate in this country is one in every 286 births. The condition is about four times more likely in boys than girls.

It is not clear what causes autism spectrum disorders, but there are many theories, including exposure to environmental toxins, diet, a malfunctioning immune system and now paternal age. (There are those who also believe that autism can be caused by a mercury derivative that used to be found in childhood vaccines, but that theory has repeatedly been debunked.)

The new study, published in today's edition of the Archives of General Psychiatry, is based on medical records collected as part of the testing done by the draft board in Israel.

Researchers had access to the medical records of almost 320,000 would-be soldiers who were born in the 1980s. They had detailed information on the age of both parents for 132,000 of them.

A total of 208 of the recruits had a diagnosis of autism spectrum disorder, including 110 for whom there was complete information -- an incidence rate of 8.3 per 10,000.

Dr. Reichenberg and his team found a close association between the father's age and the likelihood of a child having autism.

Of the 110 cases, 34 were in the 15-29 paternal age group, 62 in the 30-39 group, 13 in the 40-49 bracket, and one case with a father over 50. (Only 5 per cent of the recruits were born of fathers over the age of 40.)

Currently, there are no medical treatments for autism. Rather, children are treated with intense cognitive therapies, but this approach is expensive and controversial.