A study has found that the number of genetic mutations in children is directly linked to the age of the father, adding to evidence that the increase in some mental health problems and learning disabilities may be due to men having children later in life.
According to Dr Kari Stefansson, of Decode Genetics, the results show it is the age of men, rather than women, which is likely to have an effect on the health of the child.
"Society has been very focused on the age of the mother. But apart from [Down's syndrome] it seems that disorders such as schizophrenia and autism are influenced by the age of the father and not the mother."
From the study of 78 parents and their children, published in the journal Nature, Decode found a direct correlation between the number of mutations or slight alterations to the DNA of the child and the age of their father.
The results indicate that a father aged 20 passes, on average, about 25 mutations, while a 40-year-old father passes on about 65. The study suggests that for every year a man delays fatherhood, they risk passing 2 more mutations on to their child.
While previous studies have suggested this link this is the first time that the results have been quantified and they show that 97% of all mutations passed on to children are from older fathers.
Dr Stefansson added: "No other factor is involved which for those of us working in the field is very surprising.
"The average age of fathers has been steeply rising [in industrialised countries] since 1970. Over the same period there has been an increase in autism and it is very likely that part of that rise is accounted for by the increasing age of the father," he said.
The researchers did clarify that "nearly all children born to older fathers will be healthy but across the population the number of children born with disorders is likely to increase" based on this study.
Older fathers and therefore genetic mutations have been linked with neurological conditions because the brain depends on more genes for its development and regulation.
It also suggested the key reason that men rather than women drive the mutation rate is that women are born with all their eggs whereas men produce new sperm throughout their adult life and it is during sperm production that genetic errors creep in.
A spokesman for the National Autistic Society called for more research into this: "While there is evidence to suggest that genetic factors may play a role in some forms of autism, there are many 'younger' fathers who have children with the condition.
"Far more investigation needs to be done into the connection between genetics and autism before we can draw any reliable conclusions."