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Older women have been disproportionately blamed for genetic disorders that affect their children, citing the increased risk for complications such as diabetes during pregnancy and the greater potential to have children with chromosomal disorders such as Down syndrome. But a new study in Nature is correcting the “overemphasis on the riskiness of women giving birth at older ages,” as the LA Times reports.

In the in the international study, which sequenced the entire genomes of 78 Icelandic parents, nearly all of the new mutations detected in children came from the father, and the older the father, the more mutations he passed on. For example, A man aged 29.7 at the time he fathered a child contributed 63 new mutations to his offspring, on average, but a man aged 46.2 contributed double the mutations — 126, specifically. According to the Times:

Many of the mutations would confer no effect either for good or ill on the children, scientists noted. But some would — and that is significant because in developed countries there has been a shift over the decades toward older men fathering children, said study senior author Dr. Kari Stefansson.

Stefansson, who is a human geneticist and neurologist at the University of Iceland and the company deCODE Genetics in Reykjavík, noted for example that the average age of Iceland’s fathers at the time of a child’s conception was 34.9 in 1900, falling to 27.9 in 1980, then rising back up again to 33 in 2011.

“Similar changes have taken place all over the Western world,” Stefansson said. “It’s very likely to have made meaningful contributions to increased diagnoses of autism in our society. What percentage is due to that and what percentage is due to increased focus on diagnosis, I cannot tell you.”

The basis of this story isn’t entirely new, as it was already known that fathers who have children later in life likely contribute strongly to the development of autism and schizophrenia in their children. But because of the emphasis on women and their biological clocks, it’s important to note that men have biological clocks of their own that could affect an unborn child’s health— not to mention the psychological shift of sharing the burden of blame when it comes to children with developmental issues born to older parents. Both mother and father need to be aware of the risk they present if they are contemplating having kid later in life.

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