Maybe his baby will be very intelligent. The baby father ... Alfie Patten with little Maisie

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 yesterday.

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 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 US 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 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.

McGrath 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."