The older a man is when he fathers a child, the more likely it is that he will pass on damaging mutations, such as those that lead to developmental problems like autism. While that had been suspected, scientists in Iceland have quantified the risk. They found that the average child born to a 20-year-old father inherited 25 random mutations, but a child of a 40-year-old had 65 mutations.

The age of the mother had little effect on the risk of autism, although older mothers are more likely to have a child with Down syndrome.

The finding may help explain the increase in the diagnosis of autism in the United States, as more Americans – men and women – wait to have children.

The study, by deCODE Genetics, was published online by the journal Nature.