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Intelligence predicts sperm quality in new study

DECEMBER 05, 2008

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

Researchers in the MRC SGDP Centre 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: the likelihood of surviving and reproducing under evolutionarily-typical conditions. The research is published online in the December issue of Intelligence, the main international journal for intelligence research.

Commenting on the study, lead researcher Rosalind Arden said: “we were interested in testing the idea that if most of our genes act on many characteristics (not one gene – one trait), there might be a weak, but discoverable, relationship right across all of our characteristics – from nose to toes.  This set of weak relationships would give rise to a ‘fitness factor’ in evolutionary terms”

Scientists regularly use intelligence tests that measure general intelligence about as accurately as bathroom scales measure weight. 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 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.  If so, tiny mutations that impair intelligence may also harm other characteristics.  We all carry thousands of mildly harmful mutations, but some people carry more than others.  More mutations may cause lower intelligence and worse health.

The lead researcher for this latest study, Rosalind Arden, wanted to test this idea.  She said “As an initial proof-of-concept, we took two characteristics that seemed, on the surface, unlikely to be associated with each other– intelligence and sperm quality – and tested whether there was a statistical relationship between them.  We found a small positive relationship: brighter men had better sperm.  This association wasn’t caused by habits like avoiding smoking or drinking – the big hitters of health”.

Research Study
The researchers analyzed data from former US soldiers who had served during the Vietnam War.  4,462 veterans took several intelligence tests and underwent a detailed medical exam; of these, 425 men also provided semen samples. The researchers examined the relationships between intelligence, semen quality (sperm motility, sperm concentration, sperm count), age, and the main lifestyle factors known to predict health: obesity (body mass index), and use of alcohol, tobacco, marijuana, and hard drugs. They found that, independently of age and lifestyle, intelligence was correlated with all three measures of semen quality. The correlations were small but highly statistically significant – about the same size as the correlation between male height and intelligence.  Arden concluded “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. We look forward to seeing if the results can be replicated in other data sets, with other measures of intelligence and other measures of physical health that are also strongly related to evolutionary fitness”.

A full copy of the paper can be accessed online from the journal Intelligence:
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