Children with older dads faced a higher risk of psychiatric problems, autism and attention disorders, found a massive study of more than two million people in Sweden, as reported on Wednesday.
Compared to people whose fathers were between the ages of 20 and 24 at the time of their birth, those with dads 45 or older were 25 times more likely to have bipolar disorder, said the findings in JAMA Psychiatry, a journal of the American Medical Association.
The study found people born to older dads also were 13 times more likely to have attention deficit hyperactivity disorder.
The research adds to a growing body of science on the negative effects of delayed procreation in men, an area that has traditionally focused on the risks to women and their offspring.
"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 Indiana University, Bloomington.
"The specific associations with paternal age were much, much larger than in previous studies," said D'Onofrio, who collaborated with researchers at Karolinska Institute in Stockholm.
The size of the data set -- 2.6 million people -- accounted for every person born in Sweden from 1973 until 2001.
Older paternal age was linked to a greater likelihood of schizophrenia, suicide attempts and substance abuse problems in offspring, as well as failing grades and lower IQ scores.
Children of dads aged 45 or older also were 3.5 times more likely to have autism and 2.5 times more likely to have suicidal behavior or a substance abuse problem than those born to dads aged 24 and under.
The likelihood of having one or more of the problems increased steadily with the age of the father, signaling that there was no age threshold which suddenly posed new risks.
And by comparing siblings, researchers found associations with advancing paternal age far higher than in the general population.
Scientists made allowances for the effects of higher income, common in older men and a factor which should negate some hardships of growing up, but found the strong associations remained.
- 'Informative' findings -
"The findings in this study are more informative than many previous studies," D'Onofrio said.
"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."
Women have long been counseled that their egg supply is constantly diminishing, while men are making new sperm well into old age.
Researchers now know that DNA mutations can happen each time sperm replicate. As men age, they are exposed to more environmental toxins that can lead to DNA changes in sperm.
Nevertheless, women and men over the past 40 years increasingly have delayed childbirth.
The average age of first-time mothers in the United States was 21.5 in 1970 and by 2011 it was 25.6, according to Centers for Disease Control and Prevention data.
Men tend to be about three years older on average, according to Indiana University researchers.