Previous studies suggested linkage between advanced paternal age (men over 35) and an increased risk of neurodev- elopmental disorders, such as autism and schizophrenia, as well as dyslexia.

Now, an international team has carried out the study and found that children with older fathers perform worse in intelligence tests as compared to those born to younger men, 'The Sunday Times' reported.

According to researchers, one idea is that as men age the cells that produce sperm suffer increasing numbers of mutations, which are passed on to an offspring.

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

What's interesting is the study found that kids with older mothers gain higher IQ scores. "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," he said.

McGrath of University of Queensland, and colleagues have based their findings on an analysis of data on 33,437 US- born citizens between 1959 and 1965. All were tested at eight months, four years and seven.