According to new research by Swedish scientists, older fathers are more likely to have children with bipolar disorder.
The researchers suggest the risk goes up when men are older than 29 before they start their family, and is highest if they are over 55 as their sperm is more likely to carry genetic errors.
Other research has already linked increased paternal age with schizophrenia and autism, but not with bipolar disorder.
The researchers say men make new sperm throughout their adult life unlike women who are born with all their eggs and the process of making sperm involves copying DNA, and this is subject to error, particularly as men age.
The researchers from the Karolinska Institute say as women are born with their full supply of eggs, DNA copy errors should not increase in number with maternal age.
Bipolar disorder appears to run in families, so the research team identified 13,428 patients in Swedish registers with a diagnosis of bipolar disorder and for each one, they randomly selected from the registers, five controls who were the same sex and born the same year but did not have bipolar disorder.
When factors such as age of the individual's mother, number of siblings, socioeconomic status and family history of mental health problems, were considered, a clear link between risk of bipolar disorder and father's age was seen and the older an individual's father, the more likely he or she was to have bipolar disorder.