Traditionally, research has focused on women's "biological clock." But in recent years, scientists have been looking more and more at how the father's age at conception might affect the baby, too.
A study published Wednesday hints that age really might matter — in terms of the child's mental health.
Researchers from Indiana University and the Karolinska Institute found that compared with children fathered by men who were 20-24 years old, kids born to dads who were 45 or older were three times as likely to have autism and 13 times as likely to have ADHD. Kids born to older dads were also more likely to go on to develop substance abuse problems and get lower grades in school. The findings appear in JAMA Psychiatry.
"Paternal age may have a stronger effect than we previously realized," says Brian D'Onofrio, the study's lead author and an associate professor of psychology at Indiana University.
To figure out how paternal age was related to children's psychiatric health, the researchers looked at millions of parents in Sweden who had children between 1973 and 2001. The researchers took into account the mother's age, as well as other demographic factors that might play a role in the child's cognitive development and mental health.
"There's a growing body of literature that suggests that advancing paternal age is associated with a host of problems," D'Onofrio tells Shots. Another study, published in JAMA Psychiatry last month, found that the children of older fathers seemed to be at greater risk for developing schizophrenia and autism.
D'Onofrio and his colleagues paid special attention to siblings and cousins, and found that even among kids in the same extended family, a dad's age when his child was born made a difference.
Most previous studies with similar findings had compared children born to unrelated families, D'Onofrio says, and that made the earlier results harder to untangle. Men who tend to become fathers at a young age can be very different from those who choose to have kids later in life, he says. For example, older dads (as a group) tend to be more educated and wealthier. By comparing kids within the same extended family, the researchers were able to account for such differences in their current work.
The results are in line with a growing body of research linking older fatherhood with various developmental problems in children.
"This study gives a bigger picture," says Avraham Reichenberg, a professor of psychiatry at the Mount Sinai School of Medicine in New York, who wasn't involved in D'Onofrio's study but researches similar issues. "The older age of dads is associated with not just one disorder or another, but with a large number of adverse developmental outcomes," he says.
However, the study looks only at how paternal age and children's mental health are associated — it's a correlation, Reichenberg cautions, not a proven causal link. Scientists haven't yet determined the mechanisms of the effect. But it doesn't seem to be simply a matter of overdiagnosis among the children of older parents, the scientists say. Other research has found that as men get older, their sperm cells are more likely to contain random mutations that might, theoretically, contribute to disorders like autism in their kids.
Other environmental and epigenetic factors could also be at play, Reichenberg says. It could be that kids with older dads tend to grow up in different environments than the kids of younger dads. The epigenetic argument, Reichenberg says, postulates that "an environmental factor can activate or deactivate something in the genome." Previous studies have looked at this phenomenon in animals.
Ultimately, men and women of all ages, he says, should remember that age is only one of many factors influencing the developing baby's health.
"The most important thing is [that] future mothers and fathers should still go ahead and have children, even if the father is older than 45 or 50," Reichenberg says. "Most of these children will be absolutely fine."
D'Onofrio agrees that men and women who aren't ready to have kids in their 20s shouldn't fret too much. "Delaying childbearing [also] enables people to become more financially secure, complete more education," he says. "And that has a positive effect on children."
It's routine for doctors to remind women older than 35 that there are risks associated with older motherhood, Reichenberg says. "So should be the case, maybe, for fathers."