Children born to older dads are at a higher risk of psychiatric disorders, autism and academic problems, a study of more than two million people in Sweden has found.
An Indiana University study in collaboration with medical researchers from Karolinska Institute in Stockholm examined an immense data set - 2.6 million people - accounting for everyone born in Sweden from 1973 until 2001.
Researchers found that compared to a child born to a 24-year-old father, a child born to a 45-year-old father is 25 times more likely to have bipolar disorder and 13 times more likely to have Attention deficit hyperactivity disorder (ADHD).
The child is 3.5 times more likely to have autism, two times more likely to have a psychotic disorder and 2.5 times more likely to have suicidal behaviour or a substance abuse problem.
For most of these problems, the likelihood of the disorder increased steadily with advancing paternal age, suggesting there is no particular paternal age at childbearing that suddenly becomes problematic, researchers said.
They also documented a compelling association between advancing paternal age at childbearing and educational problems in their children such as failing grades, low educational attainment and low IQ scores.
"We were shocked by the findings," said Brian D'Onofrio, lead author and associate professor in the Department of Psychological and Brain Sciences in the College of Arts and Sciences at IU Bloomington.
"The specific associations with paternal age were much, much larger than in previous studies.
"In fact, we found that advancing paternal age was associated with greater risk for several problems, such as ADHD, suicide attempts and substance use problems, whereas traditional research designs suggested advancing paternal age may have diminished the rate at which these problems occur," D'Onofrio said.
By comparing siblings, which accounts for all factors that make children living in the same house similar, researchers found that associations with advancing paternal age were greater than estimates in the general population.
The working hypothesis for D'Onofrio and his colleagues who study this phenomenon is that unlike women, who are born with all their eggs, men continue to produce new sperm throughout their lives.
As men age, they are also exposed to numerous environmental toxins, which have been shown to cause mutations in the DNA found in sperm. Molecular genetic studies have shown that sperm of older men have more genetic mutations.
"While the findings do not indicate that every child born