Sperm can carry genetic errors
Older fathers are more likely to have children with bipolar disorder, research suggests.
The risk goes up when men are older than 29 before they start their family, and is highest if they are over 55.
Increasing paternal age has already been linked with schizophrenia and autism, but not bipolar disorder, formerly known as manic depression.
The Swedish study, in Archives of Psychiatry, suggests the risk may, in part, be explained by ageing sperm.
Unlike women who are born with all their eggs, men make new sperm throughout their adult life.
The process of making sperm involves copying DNA, and this is prone to error, particularly as men age, say the Karolinska Institute researchers.
People with bipolar disorder fluctuate between intense depression and mania, interspersed by periods of relative calm
The exact causes of bipolar disorder aren't known, but it appears to run in families
About 1 in every 100 people develop bipolar disorder in their lifetime
Lead researcher Emma Frans explained: "Women are born with their full supply of eggs. Therefore, DNA copy errors should not increase in number with maternal age."
Consistent with this notion, they found smaller effects of increased maternal age on the risk of bipolar disorder in the families they studied.
For the study, they identified 13,428 patients in Swedish registers with a diagnosis of bipolar disorder.
For each one they randomly selected from the registers five controls who were the same sex and age but did not have bipolar disorder.
After controlling for other factors like age of the individual's mother, number of siblings and family history of mental health problems, they found a clear link between risk of bipolar disorder and father's age.
The older an individual's father, the more likely he or she was to have bipolar disorder.
The children of 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.
Marjorie Wallace, chief executive of the mental health charity SANE, said: "Manic depression can cause great distress and a heightened risk of suicide and SANE welcomes this contribution to our understanding of this condition.
"There are, however, a complex range of factors that make people more or less prone to illnesses such as this, so a great deal of work still needs to be done if we are to develop better treatments and raise hope for the future."
Neil Tinning, patron of MDF The Bi-Polar Organisation, said: "From the point of view of a bipolar sufferer, this study demonstrates we need more investment, more education and more representative research into this debilitating and life threatening illness."
Bipolar disorder affects half a million people in the UK. Other than a family history of psychotic disorders, few risk factors for the condition have been identified.