A study of children born in Israel finds that, as a man's age increases, so does his risk of fathering a child with autism.
Until a few years ago, scientists who study childhood disorders tended to focus on risks related to a mother's age. Their research showed, for example, that older moms were more likely to have a child born with a problem such as Down syndrome.
Then a few scientists started asking whether a father's age mattered, too. Abraham Reichenberg, an assistant professor of psychiatry at the Mount Sinai School of Medicine in New York, says they soon found evidence that it did. A series of studies showed an association between paternal age and schizophrenia.
"It made us wonder if there was something in autism," says Reichenberg.
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.
The analysis of the information showed a clear trend.
"The older the age of the father at the time of birth, the higher the chances of the offspring to have autism," Reichenberg says. "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."
The mother's age didn't seem to matter.
There was another intriguing finding. Most studies show that autism is at least three times more common in boys than in girls. But that didn't hold true for the children of older fathers.
"The ratio of male to female was almost one to one," Reichenberg says, "which makes you think that some of the mechanisms might be a little bit different."
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 men are more likely to father children with autism. But there are at least two possibilities, suggests Craig Newschaffer, professor and chairman of epidemiology and biostatistics at the Drexel University School of Public Health in Philadelphia. One theory is that as men age, there is a greater chance of new mutations in their sperm. When combined with the egg, the sperm's mutations could have adverse health consequences – such as autism.
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.
It is tempting to ask whether the apparent rise in the number of children with autism is partly the result of a growing number of older fathers. But there's no ready answer. The study doesn't give a reliable indication of exactly how much the risk rises with age.
"If the magnitude of that effect truly were large," Newschaffer says, "I might be more inclined to say, perhaps the aging of dads in general could explain a piece of that trend."
The study also isn't clear enough to help parents decide whether it's too risky to have children based on the father's age, Newschaffer says. But he believes that this study and other studies of older fathers do offer a message: "Probably like females, males have a reproductive age." And that's how "we should start thinking about it."
Newschaffer's study appears in the September issue of the Archives of General Psychiatry.