A team from the Institute of Psychiatry analysed data from 425 former US soldiers who served during the Vietnam war era.

They found that those who performed better on intelligence tests tended to have more - and more mobile - sperm.

The study, which appears in the journal Intelligence, appears to support the idea that genes underlying intelligence may have other biological effects too. Therefore, if tiny mutations impair intelligence, they might also harm other characteristics, such as sperm quality.

Conversely, people with robust genes might be blessed with a biological "fitness factor" making them fit, healthy and smart.

Previously, scientists tended to assume that lifestyle factors were more likely to underlie any relationship between intelligence and health. For instance, brighter people may be less likely to smoke, and more likely to take exercise, both of which are known to impact on mental performance.

But the researchers found that independently of age and lifestyle, intelligence was correlated with all three measures of sperm quality - numbers, concentration, and ability to move.

Lead researcher Dr Rosalind Arden said: "This does not mean that men who prefer Play-Doh to Plato always have poor sperm: the relationship we found was marginal. But our results do support the theoretically important 'fitness factor' idea.

Dr Allan Pacey is an expert in fertility at the University of Sheffield. He said: "The improvement in semen quality with intelligence observed in this paper is small and therefore it is unlikely to have a big impact on the ability of men of different intelligences to conceive."