CHILDREN with older fathers seem to perform worse in intelligence tests, according to a study due out this week.

They tended to obtain significantly lower scores in a variety of cognitive tests than those born to younger fathers, researchers have found.

The results could be controversial. Until recent years it had been thought that it was a mother’s age that had most impact on the health and abilities of children. The father’s age, by contrast, was thought to be much less important.

The research, led by John McGrath, of the Queensland Brain Institute at the University of Queensland in Australia, suggests such ideas need rethinking.

“The offspring of older fathers show subtle impairments on tests of neurocognitive ability during infancy and childhood,” he said. “In light of the trends to delay fatherhood, the clinical implications and the mechanisms underlying these findings warrant closer scrutiny.”

Other research has shown linkage between advanced paternal age (men over 35) and an increased risk of neurodevelopmental disorders, such as autism and schizophrenia, as well as dyslexia. Such findings prompted James Watson, co-discoverer of the structure of DNA, to speak of his concern. His son Rufus suffers from schizophrenia and as more is uncovered about its causes Watson has publicly questioned if he is to blame. “I worry that I was 42 with Rufus,” he says. “I read that the frequency of schizophrenia goes up with the age of both parents.”

The tests, designed to measure the ability to think and reason, also generated a second startling finding — that children with older mothers gain higher intelligence scores.

McGrath analysed data on 33,437 Americans born between 1959 and 1965. All were tested at eight months, four years and seven. The data set, despite its age, remains one of the best resources. McGrath also used advanced statistical techniques to remove environmental influences.

For McGrath one of the key questions is the underlying biological mechanisms. One idea is that as men age the cells that produce sperm suffer increasing numbers of mutations, which are passed on to an offspring.

Why, though, would children born to older mothers tend to have higher intelligence? McGrath suggests this is because women’s eggs are formed when they are still in the womb and so their DNA is protected from mutation until they are used.