Genome: The Autobiography of a Species in 23 Chapters. Matt Ridley

general IQ remains surprisingly constant at different ages: between six and eighteen, your intelligence increases rapidly, of course, but your IQ relative to your peers changes very little. Indeed, the speed with which an infant habituates to a new stimulus correlates quite strongly with later IQ, as if it were almost possible to predict the adult IQ of a baby when only a few months old, assuming certain things about its education. IQ scores correlate strongly with school test results. High-IQ children seem to absorb more of the kind of things that are taught in school.⁴

      Not that this justifies fatalism about education: the enormous inter-school and international differences in average achievement at mathematics or other subjects shows how much can still be achieved by teaching. ‘Intelligence genes’ cannot work in a vacuum; they need environmental stimulation to develop.

      So let us accept the plainly foolish definition of intelligence as the thing that is measured by the average of several intelligence tests – ‘g’ – and see where it gets us. The fact that IQ tests were so crude and bad in the past and are still far from perfect at pinning down something truly objective makes it more remarkable, not less, that they are so consistent. If a correlation between IQ and certain genes shows through what Mark Philpott has called ‘the fog of imperfect tests’,⁵ that makes it all the more likely that there is a strongly heritable element to intelligence. Besides, modern tests have been vastly improved in their objectivity and their insensitivity to cultural background or specific knowledge.

      In the heyday of eugenic IQ testing in the 1920s, there was no evidence for heritability of IQ. It was just an assumption of the practitioners. Today, that is no longer the case. The heritability of IQ (whatever IQ is) is a hypothesis that has been tested on two sets of people: twins and adoptees. The results, however you look at them, are startling. No study of the causes of intelligence has failed to find a substantial heritability.

      There was a fashion in the 1960s for separating twins at birth, especially when putting them up for adoption. In many cases this was done with no particular thought, but in others it was deliberately done with concealed scientific motives: to test and (it was hoped) demonstrate the prevailing orthodoxy – that upbringing and environment shaped personality and genes did not. The most famous case was that of two New York girls named Beth and Amy, separated at birth by an inquisitive Freudian psychologist. Amy was placed in the family of a poor, overweight, insecure and unloving mother; sure enough, Amy grew up neurotic and introverted, just as Freudian theory would predict. But so – down to the last details – did Beth, whose adoptive mother was rich, relaxed, loving and cheerful. The differences between Amy’s and Beth’s personalities were almost undetectable when they rediscovered each other twenty years later. Far from demonstrating the power of upbringing to shape our minds, the study proved the very opposite: the power of instinct.⁶

      Started by environmental determinists, the study of twins reared apart was later taken up by those on the other side of the argument, in particular Thomas Bouchard of the University of Minnesota. Beginning in 1979, he collected pairs of separated twins from all over the world and reunited them while testing their personalities and IQs. Other studies, meanwhile, concentrated on comparing the IQs of adopted people with those of their adoptive parents and their biological parents or their siblings. Put all such studies together, totting up the IQ tests of tens of thousands of individuals, and the table looks like this. In each case the number is a percentage correlation, one hundred per cent correlation being perfect identity and zero per cent being random difference.

      

The same person tested twice	87
Identical twins reared together	86
Identical twins reared apart	76
Fraternal twins reared together	55
Biological siblings	47
Parents and children living together	40
Parents and children living apart	31
Adopted children living together	0
Unrelated people living apart	0

      Not surprisingly, the highest correlation is between identical twins reared together. Sharing the same genes, the same womb and the same family, they are indistinguishable from the same person taking the test twice. Fraternal twins, who share a womb but are genetically no more similar than two siblings, are much less similar, but they are more similar than ordinary brothers, implying that things experienced in the womb or early family life can matter a little. But the astonishing result is the correlation between the scores of adopted children reared together: zero. Being in the same family has no discernible effect on IQ at all.⁷