Having an older grandfather or being a mother who was abused in her youth are linked to higher odds of having a child with autism, according to two studies published in a leading US psychiatry journal.
The analyses by two separate teams appeared in the Journal of the American Medical Association (JAMA), Psychiatry, adding to research that suggests hereditary and environmental influences on autism risk probably reach across multiple generations.
The study on grandfathers examined national records of nearly 6,000 people with autism and 31,000 healthy controls born in Sweden since 1932.
They found that men who were 50 or older when they had a daughter were 1.79 times more likely to have a grandchild with autism, compared to men who had children when they were age 20 to 24. Men who had a son when they were 50 or older were 1.67 times more likely to have a grandchild with autism.
The study was led by King's College London's Institute of Psychiatry, Karolinska Institutet in Sweden and the Queensland Brain Institute in Australia.
Lead author Emma Frans said scientists already know that paternal age is a risk factor for autism, which affects as many as one in 88 in the United States and about one in 100 in Britain.
"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," said Frans.
Co-author Avi Reichenberg, from King's Institute of Psychiatry, said that although the increased risk was "small", the findings are "important in understanding the complex way in which autism develops".
The second study was carried out by researchers at the Harvard School of Public Health. It examined records from 451 mothers of children with autism and more than 52,000 mothers of children without autism.
"Notably, women exposed to the highest level of physical and emotional abuse - one-quarter of the women in our study - were at 61.1 per cent elevated risk for having a child with autism compared with women not exposed to abuse," the study said.