A study published in JAMA Psychiatry looked at nearly 6,000 autism cases and 31,000 control cases and found that those who grandfathers who sired children at age 50 or older were more likely to have autism.
Grandfathers who had a daughter when they were 50 or older were 1.79 times more likely to have a grandchild with autism and those who had a son when they were 50 or older were 1.67 times more likely to have a grandchild with autism.
Whether the grandfather was on the child's mother's or father's side didn't seem to make a difference, Forbes reported.
The study found that the child's autism status was independent of their parents' ages when they were born.
Researchers said the cause for the link may be that new mutations taking place over the lifetime in a father's germline might pass to offspring. If that's possible for the father, then it's possible for the grandfather to add to those autism-related gene changes thus making it more likely for autism to be found in the offspring, the researchers suggested.
"Although confidence intervals overlapped, the grandpaternal age risk estimates were lower than the risk estimates for paternal age which is consistent with the hypothesis of genetically mediated causes since the strength of a genetically mediated effect should reflect the genetic relatedness. On average, you inherit 50 percent of your genes from your parent and only 25 percent from your grandparent so the overall mutational load would thus be higher if your father is old as compared to if your grandfather is old. Similarly, if the de novo mutation hypothesis is true the risk for autism would be greater in offspring of old fathers than in grandchildren of older men," the study said.