Illustration by Peter Schrank

THERE are few better ways of upsetting a certain sort of politically correct person than to suggest that intelligence (or, rather, the variation in intelligence between individuals) is under genetic control. That, however, is one implication of a paper about to be published in Intelligence by Rosalind Arden of King's College, London, and her colleagues. Another is that brainy people are intrinsically healthier than those less intellectually endowed. And the third, a consequence of the second, is that intelligence is sexy. The most surprising thing of all, though, is that these results have emerged from an unrelated study of the quality of men's sperm.

Ms Arden is one of a group of researchers looking into the connections between intelligence, genetics and health. General intelligence (the extent to which specific, measurable aspects of intelligence, such as linguistic facility, mathematical aptitude and spatial awareness, are correlated in a given individual) is measured by psychologists using a value called Spearman's g. Recently, it has been discovered that an individual's g value is correlated with many aspects of his health, up to and including his lifespan. One possible explanation for this is that intelligent people make better choices about how to conduct their lives. They may, for example, be less likely to smoke, more likely to eat healthy foods or to exercise, and so on.

Alternatively (or in addition) it may be that intelligence is one manifestation of an underlying, genetically based healthiness. That is a view held by many evolutionary biologists, and was propounded in its modern form by Geoffrey Miller of the University of New Mexico, who is one of Ms Arden's co-authors (and, as it happens, her husband). These biologists believe intelligence, as manifested in things like artistic and musical ability, is such a reliable indicator of underlying genetic fitness that it has been chosen by members of the opposite sex over the millennia. In the ensuing arms race to show off and get a mate it has been exaggerated in the way that a peacock's tail is. This process of sexual selection, Dr Miller and his followers believe, is the reason people have become so brainy.

Hitting the g spot

Ms Arden sought to test this idea in a way that excluded intelligent choice and got directly at any correlations between intelligence and health that operate at the physiological level. She chose sperm quality because it is both easily measured and about as far from intelligent choice as it is possible to imagine—and because the relevant data had already been collected.

Her retrospective “volunteers” were former American soldiers enrolled in what was known as the Vietnam Experience Study. In 1985 almost 4,500 veterans of that war volunteered for extensive medical and mental examinations. Some of them gave semen samples that were analysed for sperm concentration (ie, number of sperm per cubic centimetre), sperm count (ie, total number of sperm in the ejaculate) and sperm motility.

Ms Arden found 425 cases where samples had been collected and analysed from unvasectomised men who had managed to avoid spilling their seed during the collection process and had answered all the necessary questions for her to test her hypothesis, namely that their g values would correlate with all three measures of their sperm quality.

They did. Moreover, neither age nor any obvious confounding variable that might have been a consequence of intelligent decisions about health (obesity, smoking, drinking and drug use) had any effect on the result. Brainy men, it seems, do have better sperm.

By implication, therefore, they have fitter bodies over all, at least in the Darwinian sense of fitness, namely the ability to survive, to attract mates and to produce offspring. That is an important finding. Hitherto, biologists have tended to disaggregate the idea of fitness into a series of adaptations that are more or less independent of each other. This work adds to the idea of a general fitness factor, f, that is similar in concept to g—and of which g is one manifestation. To him that hath, in other words, shall be given. Unfortunately for the politically correct, Dr Miller's hypothesis looks stronger by the day.