Israeli researchers looked at birth data of more than 130,000 people born in the 1980s, where the paternal and maternal ages had been recorded. Of these 110 – or just over 1 in 1000 – had an "autism spectrum disorder". ASD can include autism, Asperger syndrome and Rett syndrome.

“Men who were over 40 at the time of the child’s birth were 5.75 times as likely to have a child with ASD compared with men under 30,” says Abraham Reichenberg at Mount Sinai School of Medicine in New York, US. “The risk was slightly higher for men in their 30s – they were 1.6 times as likely to produce a child with ASD.”

Older maternal age was not found to be a risk factor for ASD. “There might be a maternal age effect, but there were so few women in our study that gave birth after the age of 40 that it was not statistically significant,” explains Reichenberg, who carried out the study with colleagues.

As people age, their cells’ DNA repair mechanisms lose some functionality. As a result, more of the randomly occurring genetic mutations occurring in sperm-producing cells may be passed from father to offspring, suggests Reichenberg, who now works at the Institute of Psychiatry at Kings College London, UK.

There may also be a change in genetic imprinting – the way a gene is expressed – as we age, he adds. “The ‘silencing’ device may not work as we age, so certain gene expressions that are usually silenced might be expressed in the offspring,” Reichenberg said.

Previous studies have shown a link between paternal age and the risk of other neurological conditions, including schizophrenia and Down’s syndrome.

Reichenberg is now researching the clinical implications of this study. “It might give us some information about the types of mutations that lead people to develop these disorders,” he says.