sequences called introns (rather like one of those irritating magazine articles interrupted by forty-eight advertisements). There are repetitive stretches in the middle of the gene that are inclined to vary in length, perhaps affecting the difference between one person's intelligence and another. Since it seems to be a gene vaguely connected with insulin-like proteins and the burning of sugar, it is perhaps relevant that another study has found that people with high I Q s are more
'efficient' at using glucose in their brains. While learning to play the computer game called Tetris, high-I Q people show a greater fall in their glucose consumption as they get more practised than do low-IQ people. But this is to clutch at straws. Plomin's gene, if it proves real at all, will be one of many that can influence intelligence in many different ways.10
The chief value of Plomin's discovery lies in the fact that, while people may still dismiss the studies of twins and adoptees as too indirect to prove the existence of genetic influences on intelligence, 8 8 G E N O M E
they cannot argue with a direct study of a gene that co-varies with intelligence. One form of the gene is about twice as common in the superintelligent Iowan children as in the rest of the population, a result extremely unlikely to be accidental. But its effect must be small: this version of the gene can only add four points to your I Q , on average. It is emphatically not a 'genius gene'. Plomin hints at up to ten more 'intelligence genes' to come from his Iowa brainboxes. Yet the return of heritable IQ to scientific respectability is greeted with dismay in many quarters. It raises the spectre of eugenic abuse that so disfigured science in the 1920s and 1930s. As Stephen Jay Gould, a severe critic of excessive hereditarianism, has put it: 'A partially inherited low IQ might be subject to extensive improvement through proper education. And it might not. The mere fact of its heritability permits no conclusion.' Indeed. But that is exactly the trouble. It is by no means inevitable that people will react to genetic evidence with fatalism. The discovery of genetic mutations behind conditions like dyslexia has not led teachers to abandon such conditions as incurable - quite the reverse; it has encouraged them to single out dyslexic children for special teaching.11
Indeed, the most famous pioneer of intelligence testing, the Frenchman Alfred Binet, argued fervently that its purpose was not to reward gifted children but to give special attention to less gifted ones. Plomin cites himself as a perfect example of the system at work. As the only one of thirty-two cousins from a large family in Chicago to go to college, he credits his fortune to good results on an intelligence test, which persuaded his parents to send him to a more academic school. America's fondness for such tests is in remarkable contrast to Britain's horror of them. The short-lived and notorious eleven-plus exam, predicated on probably-faked data produced by Cyril Burt, was Britain's only mandatory intelligence test. Whereas in Britain the eleven-plus is remembered as a disastrous device that condemned perfectly intelligent children to second-rate schools, in meritocratic America similar tests are the passports to academic success for the gifted but impoverished.
Perhaps the heritability of IQ implies something entirely different, I N T E L L I G E N C E 8 9
something that once and for all proves that Galton's attempt to discriminate between nature and nurture is misconceived. Consider this apparently fatuous fact. People with high I Q s , on average, have more symmetrical ears than people with low I Q s . Their whole bodies seem to be more symmetrical: foot breadth, ankle breadth, finger length, wrist breadth and elbow breadth each correlates with I Q .
In the early 1990s there was revived an old interest in bodily symmetry, because of what it can reveal about the body's development during early life. Some asymmetries in the body are consistent: the heart is on the left side of the chest, for example, in most people.
But other, smaller asymmetries can go randomly in either direction.
In some people the left ear is larger than the right; in others, vice versa. The magnitude of this so-called fluctuating asymmetry is a sensitive measure of how much stress the body was under when developing, stress from infections, toxins or poor nutrition. The fact that people with high I Q s have more symmetrical bodies suggests that they were subject to fewer developmental stresses in the womb or in childhood. Or rather, that they were more resistant to such stresses. And the resistance may well be heritable. So the heritability of IQ might not be caused by direct 'genes for intelligence' at all, but by indirect genes for resistance to toxins or infections — genes in other words that work by interacting with the environment. You inherit not your IQ but your ability to develop a high IQ under certain environmental circumstances. How does one parcel that one into nature and nurture? It is frankly impossible.12
Support for this idea comes from the so-called Flynn effect. A New Zealand-based political scientist, James Flynn, noticed in the 1980s that IQ is increasing in all countries all the time, at an average rate of about three IQ points per decade. Quite why is hard to determine. It might be for the same reason that height is increasing: improved childhood nutrition. When two Guatemalan villages were given ad-lib protein supplements for several years, the IQ of children, measured ten years later, had risen markedly: a Flynn effect in miniature. But IQ scores are still rising just as rapidly in well-nourished western countries. Nor can school have much to do with 9 0 G E N O M E
it, because interruptions to schooling have demonstrably temporary effects on IQ and because the tests that show the most rapid rises are the ones that have least to do with what is taught in school. It is the ones that test abstract reasoning ability that show the steepest improvements. One scientist, Ulric Neisser, believes that the cause of the Flynn effect is the intense modern saturation of everyday life with sophisticated visual images — cartoons, advertisements, films, posters, graphics and other optical displays — often at the expense of written messages. Children experience a much richer visual environment than once they did, which helps develop their skills in visual puzzles of the kind that dominate IQ tests.13
But this environmental effect is, at first sight, hard to square with the twin studies suggesting such a high heritability for I Q . As Flynn himself notes, an increase of fifteen IQ points in five decades implies either that the world was full of dunces in 1950 or that it is full of geniuses today. Since we are not experiencing a cultural renaissance, he concludes that IQ measures nothing innate. But if Neisser is right, then the modern world is an environment that encourages the development of one form of intelligence - facility with visual symbols. This is a blow to 'g', but it does not negate the idea that these different kinds of intelligence are at least partly heritable. After two million years of culture, in which our ancestors passed on learnt local traditions, human brains may have acquired (through natural selection) the ability to find and specialise in those particular skills that the local culture teaches, and that the individual excels in. The environment that a child experiences is as much a consequence of the child's genes as it is of external factors: the child seeks out and creates his or her own environment. If she is of a mechanical bent, she practises mechanical skills; if a bookworm, she seeks out books. The genes may create an appetite, not an aptitude.
After all, the high heritability of short-sightedness is accounted for not just by the heritability of eye shape, but by the heritability of literate habits. The heritability of intelligence may therefore be about the genetics of nurture, just as much as the genetics of nature. What a richly satisfying end to the century of argument inaugurated by Galton.
C H R O M O S O M E 7
I n s t i n c t
The tabula of human nature was never rasa.
