Abraham Reichenberg, PhD, of the Mount Sinai School of Medicine, New York, and Institute of Psychiatry, King's College London, and colleagues studied data from more than 130,000 individuals born during the 1980s in Israel. Their investigation is based on biographical information on the subjects who were being assessed for eligibility to serve in the Israeli military at age 17.

They found that those whose fathers were over 40 when they were born were 5.75 times as likely to have autism or related disorders (known collectively as "autism spectrum disorders") than those whose fathers were under 30. They were more than one and a half times as likely to have those conditions as individuals whose fathers were 30 to 39. A total of 110 individuals were identified with some type of autism spectrum disorder.

Although the study is not the first to link parental age with autism, the findings add support to the theory that genetics contributes to the mental disorder.

Autism is characterized by social abnormalities such as avoidance of physical or eye contact, communicating with gestures rather than words, and engaging in repetitive patterns of behavior such as head-banging. More common in boys than in girls, autism is usually diagnosed in the first few years of life.

Autism spectrum disorders have become increasingly common, currently affecting 50 in every 10,000 children -- up from five in 10,000 twenty years ago. The increase may be due in part to an increase in recognition of autism and better diagnostic procedures, the authors note, but it could also mean that autism is occurring in more people than it used to.

There are several possible genetic mechanisms for the paternal age effect, according to the researchers, including spontaneous mutations in sperm-producing cells or alterations in genetic imprinting.

Earlier research by some of the same authors found an association between older age at fatherhood and both lower intelligence scores and schizophrenia. Studies conducted by other investigators have shown that sperm mutate more often in older men, possibly increasing the risk for brain abnormalities in their children.

The mothers' age at childbirth seems to have little or no effect on autism in the child.

"Although further work is necessary to confirm this interpretation, we believe that our study provides the first convincing evidence that advanced paternal age is a risk factor for autism spectrum disorder," the authors conclude.