Older fathers are more likely to have children with bipolar disorder, a mental illness also known as manic depression, research suggests. The paternal risk goes up when men are older than 50. It may rise to 37 percent by the time a man is 55 years, lead researcher Emma Frans said. Children fathered by men 55 years and older were 1.37 times more likely to go on to be diagnosed with bipolar disorder than those of men aged 20 to 24. Children born to fathers older than 30 had an 11 percent higher risk of developing bipolar disorder compared to younger fathers.

The study follows the line of other recent studies that showed that men over 40 were almost a third less likely to conceive with their partner compared to those under 35.

The psychological condition bipolar disorder, formerly known as Manic-depressive Illness, is characterized by episodes of significant disturbance in mood and the individuals affected are cycling between manic and depressive episodes. Bipolar disorder also carries a high risk of suicide. It’s still unclear what causes the disease but genetic factors are definitely playing a major role, as the disease was found to run in families. Other than a family history of psychotic disorders, few risk factors for the condition have been identified. People diagnosed with bipolar disorder receive medication such as mood stabilizers, anti-psychotic and antidepressants.

The findings published in the Archives of General Psychiatry suggest that increased paternal age is linked to a higher risk of psychological conditions such as bipolar disorder, autism and schizophrenia.

The Swedish study says the risk may be explained by ageing sperm. The quality of the sperm changes as a man ages and this factor could increase the risk of genetic mutations that may lead to psychological conditions.

“Women are born with their full supply of eggs. Therefore, DNA copy errors should not increase in number with maternal age,” Emma Frans, an epidemiologist at the Karolinska Institute in Sweden, explained. But men continue to produce sperm that replicates throughout a man’s life, increasing the potential that DNA will be copied incorrectly, the researchers explained.

13,428 people with a diagnosis of bipolar disease were included in the study. The researchers took into account a family’s history of psychotic disorders and socioeconomic status in their analyses. For each patient, researchers also randomly selected five people of the same age and sex without the condition.

The research is the first to link paternal age and bipolar disorder.

Separately, the study found a smaller increased risk of bipolar disorder associated with the age of the mother, if she was older than 35.

A study published last year found that in 1994, 25 per 100,000 people aged 19 and under were diagnosed with Bipolar Disorder in the United States. The number grew to 800,000 from 20,000 between 1994 and 2003. In the U.S., 5.7 million adults suffer from the disorder, according to the US National Institute of Mental Health. About 1 in 100 people develop bipolar disorder in their lifetime.