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There is new research about the developmental disability autism. It shows that as a man's age increases, so does his risk of fathering a child with autism. The findings come from a study of children born in Israel.

NPR's Jon Hamilton reports.

JON HAMILTON: Until a few years ago, scientists tended to focus on risks related to a mother's age. They did studies showing that older moms were more likely to have a child born with problems like Down's Syndrome.

But then a few scientists started asking whether a father's age mattered, too. Abraham Reichenberg of the Mount Sinai School of Medicine in New York says they soon found evidence that it did.

Mr. ABRAHAM REICHENBERG (Mount Sinai School of Medicine): There were a series of studies that showed that there was an association between paternal age and schizophrenia. And it made us wonder if there was something in autism.

HAMILTON: Both conditions have a genetic component and appear to involve subtle changes to a child's developing brain. So Reichenberg led a team of scientists that studied more than 300,000 children born in Israel. One set of government records showed the age of each father when a child was born and usually the age of the mother. Another set of records showed whether a child was later diagnosed with autism. Reichenberg says an analysis of the information showed a clear trend.

Mr. REICHENBERG: The older the age of the father at the time of birth, the higher the chances of the offspring to have autism. In fathers who were 40 years or older, the risk for autism was almost six times higher than in the offspring of fathers who were younger than 30 years of age

HAMILTON: The mother's age didn't seem to matter. And there was something else. Most studies show that autism is at least three times more common in boys than in girls. But Reichenberg says that wasn't true for the children of older fathers.

Mr. REICHENBERG: The ratio of male to female was almost one to one, which makes you think that some of the mechanisms might be a little bit different.

HAMILTON: In other words, autism in children of older fathers may have a different cause than autism in children with younger dads. It's not clear why older fathers are more likely to have children with autism. But Craig Newschaffer of Drexel University School in Philadelphia says there are at least two possibilities. One is that as men age, their sperm becomes more susceptible to genetic errors.

Mr. CRAIG NEWSCHAFFER (Drexel University): There could be a greater chance of new mutations. And the genes in that sperm that then when combined with the egg, you know, go on to have adverse health consequences.

HAMILTON: Like autism or schizophrenia. Another possibility is that older men who carry genes associated with autism are, for some unknown reason, more likely to pass along the risk from those genes. Newschaffer says it's tempting to ask whether a growing number of older fathers might be partly responsible for the apparent rise in the number of children with autism. But he says it's hard to say. That's because the study doesn't give a reliable indication of exactly how much the risk rises with age.

Mr. NEWSCHAFFER: If the magnitude of that effect truly were large, I might be more inclined to say perhaps, well, aging of dads in general maybe that could be explaining a piece of that trend.

HAMILTON: Newschaffer says the study also isn't clear enough to help older parents decide whether it's too risky to have children. But he says this and other studies of older fathers do offer a message.

Mr. REICHENBERG: Probably like females, males have a reproductive age. And we should start thinking about it like that.

HAMILTON: Newschaffer's study appears in the September issue of the Archives of General Psychiatry.

Jon Hamilton, NPR News.

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