A rise in the number of older dads may at least partially explain increasing autism rates, a new study suggests.

Researchers reported Wednesday that fathers pass down more genetic mutations to their children the older they are at the time of conception. While some mutations are harmless, others have been linked to autism and other disorders, so possessing a greater volume of the altered genes puts a child at higher risk.

In looking at the genomes of 78 families whose children had a diagnosis of autism or schizophrenia, researchers found an average increase of two mutations for each year added to a father’s age, according to the study published in the journal Nature. At the same time, the age of the mother did not appear to influence the number of gene mutations a child inherited.

“Our results all point to the possibility that as a man ages, the number of hereditary mutations in his sperm increases, and the chance that a child would carry a deleterious mutation that could lead to diseases such as autism and schizophrenia increases proportionally,” said Kari Stefansson, CEO of deCODE Genetics in Reykjavik, Iceland who led the study. “It is of interest here that conventional wisdom has been to blame developmental disorders of children on the age of mothers, whereas the only problems that come with advancing age of mothers is a risk of Down syndrome and other rare chromosomal abnormalities. It is the age of fathers that appears to be the real culprit.”

While paternal age has been linked to autism and schizophrenia previously, this study is the first to explain why.

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