Children born to fathers who are over 30 have greater chances of developing bipolar disorder also known as manic depression, said researchers. Sweden's Karolinska Institute led one of the largest studies to try to find a link between mental illness and paternal age. Previous studies had said that there was a link between schizophrenia and autism in children born to older fathers and last year a Danish study added bipolar disorder in list as well.

Emma Frans, an epidemiologist at the Karolinska Institute in Sweden, who led the study, said the risk starts increasing by 40 years and as the father’s age rises so does the risk with it going up to 37 % by the time a man is 55 years old as compared to a 20 year old. Frans and colleagues wrote, "Advanced paternal age is a risk factor for bipolar disorder in the offspring."

Said to affect more than 5 million Americans, bipolar disorder is marked by extreme mood changes, as well as extreme changes in energy and the ability to function. The moods can swing from ups and elation to irritability and periods of extreme sadness and hopelessness sometimes lasting for months. The disorder has been known to run in families but the early onset of the disease has been linked to genetics. It is considered early onset is when the disorder is developed before the age of 20 years. The mother’s age does not seem to play any role in the disease though.

The reason seems to be that older men’s sperm may be more likely to develop mutations leading to bipolar disorder. Although sperm are produced throughout a man’s life with age the mutation chances increase along with chances of birth defects which is why most sperm banks have age limits set for donors. The study authors added that as the odds of the child becoming bipolar are low therefore it should not dissuade older men from fathering children.

Dr. Harold Pincus, vice chair of psychiatry at Columbia University said the findings don’t tell us why older men have greater risks of their children developing bipolar but it “reinforces the notion that there's a strong biological component to this."

The study, released Monday, appears in the Archives of General Psychiatry says though having an older father increases the risk of bipolar, but as the findings aren't definitive that still means most people with older fathers won't ever get bipolar disorder. Lifetime risks for it have been estimated at roughly 1 % to 4%.

New York University researcher Susan Harlap said a mother’s age and health have always thought to play an important role in birth defects and other abnormalities but this new study points to paternal factors also playing an important role.  

"Despite the robust evidence supporting the association between paternal age and severe mental disorders, the association between advanced paternal age and bipolar disorder has not been investigated," the team added.