Matt Ridley

This is positively bizarre. It flies in the face of common sense: surely our intelligence is influenced by the books and conversations found in our childhood homes? Yes, but that is not the question.

After all, heredity could conceivably account for the fact that both parents and children from the same home like intellectual pursuits.

No studies have been done - except for twin and adoption studies

- that discriminate between the hereditary and parental-home explanation. The twin and adoption studies are unambiguous at present in favouring the hereditary explanation for the coincidence of parents' and children's I Q s . It remains possible that the twin and adoption studies are misleading because they come from too narrow a range of families. These are mostly white, middle-class families, and very few poor or black families are included in the samples.

Perhaps it is no surprise that the range of books and conversations found in all middle-class, American, white families is roughly the same. When a study of trans-racial adoptees was done, a small correlation was found between the children's IQ and that of their adoptive parents (nineteen per cent).

But it is still a small effect. The conclusion that all these studies converge upon is that about half of your IQ was inherited, and less than a fifth was due to the environment you shared with your siblings - the family. The rest came from the womb, the school and outside influences such as peer groups. But even this is misleading.

Not only does your IQ change with age, but so does its heritability.

As you grow up and accumulate experiences, the influence of your genes increases. What? Surely, it falls off? No: the heritability of childhood IQ is about forty-five per cent, whereas in late adolescence it rises to seventy-five per cent. As you grow up, you gradually express I N T E L L I G E N C E 8 5

your own innate intelligence and leave behind the influences stamped on you by others. You select the environments that suit your innate tendencies, rather than adjusting your innate tendencies to the environments you find yourself in. This proves two vital things: that genetic influences are not frozen at conception and that environmental influences are not inexorably cumulative. Heritability does not mean immutability.

Francis Galton, right at the start of this long debate, used an analogy that may be fairly apt. 'Many a person has amused himself, he wrote, 'with throwing bits of stick into a tiny brook and watching their progress; how they are arrested, first by one chance obstacle, then by another; and again, how their onward course is facilitated by a combination of circumstances. He might ascribe much importance to each of these events, and think how largely the destiny of the stick had been governed by a series of trifling accidents.

Nevertheless, all the sticks succeed in passing down the current, and in the long run, they travel at nearly the same rate.' So the evidence suggests that intensively exposing children to better tuition has a dramatic effect on their IQ scores, but only temporarily. By the end of elementary school, children who have been in Head Start programmes are no further ahead than children who have not.

If you accept the criticism that these studies mildly exaggerate heritability because they are of families from a single social class, then it follows that heritability will be greater in an egalitarian society than an unequal one. Indeed, the definition of the perfect meritoc-racy, ironically, is a society in which people's achievements depend on their genes because their environments are equal. We are fast approaching such a state with respect to height: in the past, poor nutrition resulted in many children not reaching their 'genetic' height as adults. Today, with generally better childhood nutrition, more of the differences in height between individuals are due to genes: the heritability of height is, therefore, I suspect, rising. The same cannot yet be said of intelligence with certainty, because environmental variables - such as school quality, family habits, or wealth — may be growing more unequal in some societies, rather than more equal. But 8 6 G E N O M E

it is none the less a paradox: in egalitarian societies, genes matter more.

These heritability estimates apply to the differences between individuals, not those between groups. IQ heritability does seem to be about the same in different populations or races, which might not have been the case. But it is logically false to conclude that because the difference between the IQ of one person and another is approximately fifty per cent heritable, that the difference between the average IQ s of blacks and whites or between whites and Asians is due to genes. Indeed, the implication is not only logically false, it so far looks empirically wrong, too. Thus does a large pillar of support for part of the thesis of the recent book The bell curve9

crumble. There are differences between the average IQ scores of blacks and whites, but there is no evidence that these differences are themselves heritable. Indeed, the evidence from cases of cross-racial adoption suggests that the average I Q s of blacks reared by and among whites is no different from that of whites.

If IQ is fifty per cent heritable individually, then some genes must influence it. But it is impossible to tell how many. The only thing one can say with certainty is that some of the genes that influence it are variable, that is to say they exist in different versions in different people. Heritability and determinism are very different things. It is entirely possible that the most important genes affecting intelligence are actually non-varying, in which case there would be no heritability for differences caused by those genes, because there would be no such differences For instance, I have five fingers on each hand and so do most people. The reason is that I inherited a genetic recipe that specified five fingers. Yet if I went around the world looking for people with four fingers, about ninety-five per cent of the people I found, possibly more, would be people who had lost fingers in accidents. I would find that having four fingers is something with very low heritability: it is nearly always caused by the environment. But that does not imply that genes had nothing to do with determining finger number. A gene can determine a feature of our bodies that is the same in different people just as surely as it can determine features that are different in different I N T E L L I G E N C E 8 7

people. Robert Plomin's gene-fishing expeditions for IQ genes will only find genes that come in different varieties, not genes that show no variation. They might therefore miss some important genes.

Plomin's first gene, the IGF2R gene on the long arm of chromosome 6, is at first sight an unlikely candidate for an 'intelligence gene'. Its main claim to fame before Plomin linked it with intelligence was that it was associated with liver cancer. It might have been called a 'liver-cancer gene', thus neatly demonstrating the foolishness of identifying genes by the diseases they cause. At some point we may have to decide if its cancer-suppressing function is its main task and its ability to influence intelligence a side-effect, or vice versa. In fact, they could both be side-effects. The function of the protein it encodes is mystifyingly dull: 'the intracellular trafficking of phosphorylated lysosomal enzymes from the Golgi complex and the cell surface to the lysosomes'. It is a molecular delivery van.

Not a word about speeding up brain waves.

IGF2R is an enormous gene, with 7,473 letters in all, but the sense-containing message is spread out over a 98,000-letter stretch of the genome, interrupted forty-eight times by nonsense