A study by researchers at the University of Queensland, Australia, appearing in the journal PLoS Medicine, has found that children of older fathers perform less well in a range of cognitive tests than children born to younger fathers.

The research, led by John McGrath, studied data from intelligence tests taken by 33,437 children born between 1959 and 1965 in the US. The tests focused on memory, learning and concentration skills in the children at the ages of eight months, four years and seven years. McGrath stated, 'the offspring of older fathers show subtle impairments on tests of neurocognitive ability during infancy and childhood'.

It was previously thought that the mother's age had more effect on a child's abilities and intelligence, but the data found that children born to older mothers did well on the tests. The researchers have concluded that as men age the cells that produce sperm are subject to genetic mutations, whereas the mother's eggs are formed while still in the womb, and are therefore protected from mutation until they are used.

Other recent studies have shown a link between fathers over the age of 35 and health problems in children, such as birth deformities, cancer, and conditions such as autism and schizophrenia. James Watson, the co-discoverer of the structure of DNA, has publicly speculated whether his age when he became a father to his son Rufus had a causal impact on Rufus' diagnosis of schizophrenia. Watson has commented that 'I worry that I was 42 with Rufus. I read that the frequency of schizophrenia goes up with the age of both parents'.

In the developed world the age at which both men and women are having children is increasing; in 1993, 23 per cent of births in England and Wales were to men aged 35 to 54 years old, but this had increased to 40 per cent by 2003. While genetic factors are considered important, the research team also took socioeconomic factors into account, with the potential for older fathers to provide better access to education and healthcare.

Dr Allen Pacey, a fertility expert at the University of Sheffield, took a sobering view, stating that 'the author's observation that most neurocognitive outcomes is also reduced in the children of older fathers provides a further piece of evidence to remind us that nature intended us to have our children earlier in our lives than we currently are'.