Corbis

FEW scientific fields are as fraught with risk as that of research into human intelligence. The two questions that arise over and over again are “is it a result of nature or nurture?” and “to the extent it is nature, does race make a difference?”

Making stupid comments about the second question can be a career-killing move, as James Watson, a co-discoverer of the structure of DNA, recently found. He suggested that he was “inherently gloomy about the prospect of Africa” because “all our social policies are based on the fact that their intelligence is the same as ours [presumably he meant white people]—whereas all the testing says not really”. Such remarks are not merely offensive, they are scientifically weird. If the term race has any useful scientific meaning, then Africa, the continent where modern humanity began, is the most racially diverse place on the planet. The resulting hoo-ha caused Dr Watson to be eased out of the chancellorship of Cold Spring Harbor Laboratory, near New York, where he had worked for almost 40 years.

Fortunately, the study of links between intelligence and genetics has some wiser practitioners than Dr Watson. One of them, Terrie Moffitt, of King's College, London, has just supervised a project investigating the first perennial question—the relative importance of nature and nurture. The result, published this week in the Proceedings of the National Academy of Sciences, neatly illustrates how complex the subject really is. It also shows how the hoary old thesis and antithesis of genetics and upbringing combine in a most intriguing synthesis.

Suck it and see

Dr Moffitt's team (the actual work was led by her colleague, Avshalom Caspi) looked at the effect on intelligence of breastfeeding, but in a genetic context. Several studies in the past have shown that breastfed children are more intelligent, by about six IQ points, than those given baby formulas—an open-and-shut case, it might appear, of nurture trumping nature.

Dr Caspi and Dr Moffitt, however, were not so sure. They suspected the involvement of a gene called FADS2. This regulates the metabolism of a group of molecules called long-chain polyunsaturated fatty acids. These are important for the growth of nerve cells and are abundant in human milk but generally absent from formulas. FADS2 comes in two varieties, known as C and G, and the researchers wondered if these two varieties interacted differently with breast milk.

To find out, they drew on data from two groups of people, one in New Zealand and one in Britain. Each of these groups is an annual cohort (in other words, their members were all born in the same twelve-month period) established for just this sort of long-term medical investigation. Data have been collected on the members of each for years (the New Zealanders were born between April 1972 and March 1973; the Brits in 1994 and 1995). Indeed, Dr Moffitt has already used the New Zealand group to show how a violent family upbringing and different versions of another neurologically important gene interact to produce more and less violent people.

What Dr Caspi and Dr Moffitt found was that the increase in intelligence associated with breastfeeding only happened to people who had inherited at least one copy of the C version of FADS2. (Most genes are present as two copies, one inherited from the mother and one from the father.) The effect did not depend on the social classes or IQs of the parents, nor on the birthweight of the child in question (low birthweight has been linked to lower IQ). And the difference in IQ was preserved into adulthood.

Only about 10% of the population is double-G, but what is curious about this result is that the G version of the gene has survived at all. If intelligence is valuable, the C version might be expected to have become universal. Indeed, this is the nub of the nurturists' argument. Natural selection should have pushed intelligence genes as far as they will go, so all variation should be environmental. That it is not suggests there is some unknown countervailing advantage—at least in reproductive terms—to being less than averagely bright.

It is a nice irony, given the traditional association of the naturist position with eugenic arguments, that if variation in intelligence really is caused by underlying genetic variation, then the dull are as evolutionarily fit as the clever. But that is the logical conclusion.