Children who are fathered by older men tend to perform worse in intelligence tests than the children of younger dads, Australian and U.S. scientists report in study they say is the first of its kind.
But in an interesting contrast, children with older mothers tended to have higher scores in the same tests, the researchers found.
Lead scientist John McGrath, from the Queensland Brain Institute in Brisbane, says he found his own results "quite startling," as researchers had thought that the age of the father was less a factor in their children's outcomes compared to the age of the mother.
He notes the study has implications for men in Western societies who have delayed fatherhood until their 40s or older.
McGrath's examined data collected on more than 33,000 children in the United States between 1959 and 1965 fathered by men aged from 15 to 65, as part of the U.S. Collaborative Perinatal Project.
They found the children of older dads performed less well in intelligence tests conducted at age eight months, four years and seven years.
The older the father was when he had the child, the worse his children tended to do in the tests.
Regardless of their mothers' ages, children whose fathers were 50 years old had lower scores on all tests than those whose fathers were 20. The only exceptions were results of tests that assessed physical co-ordination.
The review found that kids born to 20-year-old men scored, on average, three points higher in the tests than children whose fathers were 50 years old when they were born.
In contrast, the older the mother, the higher the scores of the child in the tests, which measured reading, spelling, arithmetic ability, sensory discrimination and hand-eye co-ordination.
"Previous researchers have suggested that the children of older mothers may perform better because they experience a more nurturing home environment; if this is the case, this study suggests that children of older fathers do not necessarily experience the same benefit," the researchers wrote in a statement.
The research is published in the journal, PLoS Medicine.
The researchers said the lower scores from the children of older men may have to do with mutation in the father's sperm.
"Unlike a woman's eggs which are formed when she herself is in the womb, a man's sperm accumulates over his lifetime, which previous studies have suggested can mean increased incidence of mutations in the sperm at an older age," they wrote.
Women's eggs undergo 22 cell divisions in the womb, while male sperm cells have divided 150 times by age 20 and 840 times by age 50. This increased the chance of "copy error mutations" that could lead to health problems, the researchers said.
Recent evidence has demonstrated a link between older fathers and specific health problems in their children, including birth deformities and neuropsychiatric conditions such as autism and schizophrenia.
The researchers say that given the trend in the developing world towards older maternal and paternal ages, policy-makers may want to consider promoting awareness of the risks to children that may be associated with delayed fatherhood.
