The same genes may govern intelligence and sperm quality
Men of higher intelligence tend to produce better quality sperm, UK research suggests.
A team from the Institute of Psychiatry analysed data from 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.
The latest study tested the gene theory by taking two characteristics that seemed unlikely to be associated with each other - intelligence and sperm quality.
They found a small, but statistically significant link, and were able to show that this could not be explained by unhealthy habits, such as smoking or drinking alcohol.
The study was based on 425 men who undertook several intelligence tests and provided semen samples.
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.
"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."
Dr Allan Pacey is an expert in fertility at the University of Sheffield.
He said: "The fact that it's possible to detect a statistical relationship between intelligence and semen quality in adult men probably says more about the co-development of brain and testicles when the man was in his mother's womb, and therefore how well they both function in adult life, rather than suggesting that playing Sudoku can somehow stimulate more sperm to be produced.
"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."
The semen samples were collected in 1985 by the US Centers for Disease Control as part of a large-scale study into the health of US soldiers who served during the Vietnam Era. Some of the men in the sample had served in Vietnam, some had served in Germany, Korea and the USA.