Researchers have found that men with higher intelligence tend to produce better quality sperm, suggesting a closer-than-expected relationship between intelligence and evolutionary fitness.

The researchers at the Institute of Psychiatry, King’s College London, and their collaborators set out to test the hypothesis that brainier men had better sperm as a new way to study the possible associations between intelligence, health, fertility, and overall fitness.

They found a small positive relationship: brighter men had better sperm.

The research is published online in the December issue of Intelligence, the main international journal for intelligence research.

In the last few years researchers have discovered that general intelligence is a surprisingly good predictor of many aspects of physical health. Scientists assumed that the relationship between intelligence and health exists because of lifestyle factors: brighter people may quit smoking earlier, exercise more, and have less dangerous jobs.

But some evolutionary-minded researchers including lead researcher Rosalind Arden and co-authors Linda Gottfredson, Geoffrey Miller and Arand Pierce believe that there may also be another reason for the intelligence-health link.

Intelligence may indicate underlying, genetically-based biological fitness. The reason is that genes influencing intelligence might also contribute to hundreds of other characteristics.

Arden noted, “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.”

The researchers analysed data from 4,462 former US soldiers who had served during the Vietnam War. The veterans took several intelligence tests and underwent a detailed medical exam; of these, 425 men also provided semen samples.

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