NEW YORK (Bloomberg) — Advancing age increases a man’s risk of having an autistic child as much as six-fold, according to a study in the Archives of General Psychiatry.The research studied more than 132,000 Israeli births in the 1980s. For every 1,000 children born, fathers aged 40 or older average 3.3 with autism or a related disorder compared with 0.6 born to fathers aged 29 and younger, the study said.

More research is needed to examine why a man’s age appears to be a risk factor, the report said, adding that it’s possible autism may be related to spontaneous mutations or alterations in sperm-produced cells. Older age in mothers isn’t associated with an increased risk of autism, the study said.

A total of 110 children in the study, or one in every 1,200, were diagnosed with an autism spectrum disorder, which includes autism and related disabilities that impair communication and social interaction. Men aged 30 to 39 years old average 0.9 children with autism or a related disorder for every 1,000 born.

As many as 1 in 166 US children may have an autism spectrum disorder, according to the Centers for Disease Control and Prevention’s website. Autism’s prevalence has risen in the past decade, in part because of new classification standards and better diagnostic tools.