Children born to older fathers start life with more genetic mutations than the offspring of younger fathers, scientists report.
From about age 20, every year that a man waits to procreate increases the number of mutations their progeny will carry by two mutations.
The study, which analysed the mechanism responsible for this "ticking", also adds weight to previous research that has found children with older fathers have a higher incidence of schizophrenia and autism.
A group of Icelandic scientists sequenced the entire genomes of 78 families made up of two or three generations and compared the number of new mutations that arose in each offspring with the age of their parents. The results are published in the journal Nature today.
The scientists found that a 20-year-old father transmits, on average, 25 mutations to his child, while a 40-year-old father transmits about 65. By contrast, the number of new mutations transmitted by the mother was roughly 15, regardless of her age, wrote an American evolutionary geneticist, Alexey Kondrashov, in an analysis of the study.
John McGrath, a psychiatric epidemiologist from the Queensland brain institute at the University of Queensland, said while many of these mutations would have no impact on an individual's health, their accumulative effect could create a "mutational timebomb".
"If we, as a society, continue to delay parenthood then this may skew the incidence of a range of disorders," he said.
Scientists suspect fathers pass on more mutations because of the way male germ cells, or sperm, are produced.
Unlike female reproductive cells, sperm cells continue to divide during a male's life, splitting more than 800 times by the time he reaches 50, said Professor McGrath, who was not involved in the research.
"Every time there is a cell division there's a chance for a mutation," he said.