Children born to older fathers face a greater chance of developing bipolar disorder, according to one of the largest studies linking mental illness with advanced paternal age.
Previous research has connected schizophrenia and autism with older dads, and a Danish study published last year added bipolar disorder to the list. The new study led by researchers at Sweden's Karolinska Institute strengthens the evidence.
The leading theory is that older men's sperm may be more likely to develop mutations. Even so, the odds of a person becoming bipolar are so low that the study's authors said it shouldn't dissuade older men from becoming fathers.
Researchers analyzed Swedish national registry data from more than 80,000 people, including 13,428 with bipolar disorder who were born between 1932 and 1991.
The risks started increasing around age 40 but were strongest among those 55 and older. Children born to these dads were 37 percent more likely to develop bipolar disorder than those born to men in their 20s.
They also faced more than double the risk of developing bipolar disorder before age 20. Scientists call that early onset disease, and while they have long known that bipolar disorder tends to run in families, early onset disease has been thought to be most strongly linked with genetics.
The age of the mothers didn't appear to be much of a factor.
The study, released Monday, appears in September's Archives of General Psychiatry.
While the findings don't explain what might cause some older men to have bipolar children, it "reinforces the notion that there's a strong biological component to this," said Dr. Harold Pincus, vice chair of psychiatry at Columbia University.
Bipolar disorder causes dramatic mood swings, from deep depression to manic highs. It affects more than 5 million Americans.
Lifetime risks for it have been estimated at roughly 1 percent to 4 percent. The study results suggest that having an older father might increase that slightly. The findings aren't definitive, but even if the link proves to be real, Pincus noted that still means most people with older fathers won't ever get bipolar disorder.
Factors involving mothers, including age and health, have long been thought to be most closely linked with birth defects and other abnormalities. But the new study adds to mounting evidence that paternal factors also play an important role, said New York University researcher Susan Harlap.
Sperm are produced throughout a man's lifetime, and scientists believe that as men age there is a greater chance for mutations that could contribute to disorders in their children.
Advanced paternal age also has been linked with birth defects, and some sperm banks have age limits for donors because of that.
While important for scientists, the study results shouldn't discourage older men from fathering children, said Emma Frans, the lead author.
She said the results suggest that similar mechanisms might contribute to risks for bipolar disorder, schizophrenia and autism. Each of these disorders is thought to have many causes including biologic and outside factors.