MELBOURNE, March 10 – Children fathered by older men are likely to be less intelligent than the offspring of younger dads, Australian and US scientists have found in a report published Tuesday.
The surprise result contrasted sharply with earlier studies showing that older mothers produced children more likely to record above average intelligence scores, the researchers concluded.
Lead scientist John McGrath, from the Brisbane-based Queensland Brain Institute, said the result was a world first and had implications for men in Western societies who have delayed fatherhood until their 40s or older.
"The results were quite startling as it was thought that the age of the father was less of a concern compared to the age of the mother," McGrath said.
"Now we are getting more evidence of the age of the father being just as important — the older a dad is, the worse his children tend to do in intelligence tests."
Researchers examined data collected on 33,000 children in the United States between 1959 and 1965 fathered by men aged from 15 to 65.
They found that the children of older dads performed less well in intelligence tests conducted at age eight months, four years and seven years.
The researchers said previous studies had linked older fathers to an increased risk of health problems in their children, including schizophrenia, autism, dyslexia, epilepsy and dwarfism.
However, the study published in the US journal Public Library of Science Medicine is the first to suggest there is also a link with general intelligence.
The trend towards children of older fathers being less intelligent remained statistically significant even when factors such as socio-economic status and parental mental health were taken into account.
"Frankly, we were surprised to come up with such a clear-cut finding," McGrath said.
He said the research team could not draw conclusions on how having an older father impacted on children later in life because the subjects in their study were only examined until they were aged seven.
"It is feasible that the offspring of older fathers ‘catch up’ during later childhood," the researcher said.
"How the subtle neurocognitive features associated with (older fathers) translate into later educational and mental health outcomes across the lifespan remains to be determined."
One theory about the difference in results for older fathers and mothers lies in differences between the male and female reproductive systems.
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, researchers said.
"We are concerned that older men accumulate more mutations in the developing sperm cells," McGrath said.
"These mistakes then pile up and increase the risks of problems in the children, and it is possible that these mistakes will carry on into the next generation."