Having a son at 50 or older increased a man's chance of having an autistic grandchild 1.67 times.

Older fathers are more likely than younger men to have grandchildren with autism, research has shown.

A new study suggests genetic risk factors for the condition accumulate over generations.

Scientists analysed family and psychiatric records on almost 6,000 individuals with autism born in Sweden from 1932.

Their data were compared with that from 31,000 unaffected members of the population.

The study found that autism risk in a grandchild increased the older the grandfather was when his son or daughter was born.

Men who had a daughter at the age of 50 were 1.79 times more likely to have a grandchild with autism than those aged 20 to 24.

Having a son at 50 or older increased a man's chance of having an autistic grandchild 1.67 times.

The findings are published today in the journal JAMA Psychiatry.

Co-author Avi Reichenberg, from King’s College London’s Institute of Psychiatry, said: “We tend to think in terms of the here and now when we talk about the effect of the environment on our genome. For the first time in psychiatry, we show that your father's and grandfather’s lifestyle choices can affect you.

“This doesn't mean that you shouldn't have children if your father was old when he had you, because whilst the risk is increased, it is still small. However, the findings are important in understanding the complex way in which autism develops.”

Lead researcher Emma Frans, from the Karolinska Institute in Stockholm, said: “We know from previous studies that older paternal age is a risk factor for autism. This study goes beyond that and suggests that older grandpaternal age is also a risk factor for autism, suggesting that risk factors for autism can build up through generations.”

The developmental disorder affects the ability to communicate and socialise, and varies from a minor problem to one that requires a lifetime of specialist support.

Autism is known to be caused by a combination of genetic and environmental factors. Previous studies have shown that fathers aged 50 and older are more than twice as likely to have a child diagnosed with autism than younger fathers.

Experts think the link with paternal age could be explained by genetic mutations in sperm cells, which build up over time.

The new research suggests that “silent” mutations that leave a son or daughter unaffected may increase the risk of autism in later generations.

A combination of mounting mutations and interactions with other risk factors could finally cause the disorder to emerge.