Children born to fathers who are 45 years old or older are more likely to develop serious mental illness than children born to fathers in their early twenties, a large-scale study comparing siblings suggests.
The researchers posit accumulation of genetic mutation in older men's sperm as a possible reason for the increased risk, which is greater than that suggested by previous studies. Children born to older fathers were 25 times more likely to develop bipolar disorder, for example.
Dr Brian D'Onofrio, the study's first author, from Indiana University, USA, said: 'The findings in this study are more informative than many previous studies'. The research was performed in collaboration with two Swedish research centres. It drew on the medical records of more than 2.6 million people (about 90 percent of the population) born in Sweden between 1973 and 2001, to more than 1.4 million fathers.
'First, we had the largest sample size for a study on paternal age. Second, we predicted numerous psychiatric and academic problems that are associated with significant impairment. Finally, we were able to estimate the association between paternal age at childbearing and these problems', said Dr D'Onofrio.
The researchers could 'control for many factors that other studies could not', he added.
Compared across all the people in the study, paternal age made little difference to mental health. But when the researchers compared the health of siblings born several years apart, clearer findings emerged. Bipolar disorder showed the strongest association with advancing paternal age, followed by ADHD, autism and psychotic disorders. Children born to older men also had lower scores at school and completed fewer levels of education overall, but these trends were less pronounced.
The researchers suggest the increased incidence of psychiatric problems is due to a higher rate of novel mutations in sperm cells of older fathers. Unlike women, who are born with all their egg cells, men produce sperm throughout their lives. In older men, the sperm-producing mechanism may work less well than in young men, leading to more genetic changes in the resulting sperm, some of which may cause problems.
Opinions of other experts, not involved in the study, vary. Speaking to The Guardian, Professor Jennifer Roff, of the City University of New York, said she was unconvinced by the link to mutations in sperm. 'I'm not saying that there is no possible genetic role for paternal age', she said, 'I simply think that this could be confounded with other environmental factors like birth order'.
But Professor Patrick Sullivan, at the University of North Carolina, commented positively on the research. He told The New York Times: 'This is the best paper I’ve seen on this topic, and it suggests several lines of inquiry into mental illness. But the last thing people should do is read this and say, "Oh no, I had a kid at 43, the kid’s doomed". The vast majority of kids born to older dads will be just fine'.
The study was published in the journal JAMA Psychiatry.