Older paternal age has been linked to autism in a man’s offspring , but now researchers have found that the older a man is when his child is born, the greater the risk for autism in his grandchild.

Writing online in JAMA Psychiatry, researchers used Swedish government data on parental and grandparental ages of 5,936 children with autism, comparing them with more than 30,000 children without autism. They found that compared with men who had a child when they were 20 to 24, those who became fathers when they were 50 or older were about 73 percent more likely to have a grandchild with autism. The connection held even after controlling for other factors, including the age of the grandchild’s parents.

The mechanism is unknown, and there may be various genetic and environmental factors involved. But it appears that a mutation may pass from a man to his child and then get activated, or turned on, not in the child but in the grandchild.

A study co-author, Avi Reichenberg, a researcher at the Seaver Autism Center at the Icahn School of Medicine at Mount Sinai in New York, said that the risk for autism is small in any case. “The majority of children and grandchildren of older dads and granddads develop normally,” he said. “But our findings are helping us understand how autism may develop.”