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
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