pair bought identical sets of champagne glasses as presents for each other at the same time at different ends of the country. Twins that were born different, by contrast, grew more different as they grew older. ‘They were never alike either in body or mind, and their dissimilarity increases daily,’ said one of his respondents. ‘The external influences have been identical; they have never been separated.’ Galton sounded almost embarrassed by the strength of his conclusion: ‘There is no escape from the conclusion that nature prevails enormously over nurture…My fear is, that my evidence may seem to prove too much, and be discredited on that account, as it appears contrary to all experience that nurture should go for so little.8
SPLITTING PAIRS
With hindsight one can pick all sorts of holes in Galton’s first twin study. It was anecdotal, small, and the argument was circular: twins that appeared identical behaved identically. He had not distinguished identicals from fraternals genetically. Yet the study was remarkably persuasive. By the end of his life Galton had seen his hereditarian beliefs move from scepticism to orthodoxy. ‘Nature limits the powers of the mind as definitely as those of the body,’ said The Nation in 1892, ‘On these points, among thinkers everywhere, [Galton’s] opinions have prevailed.’9 The old empiricism of John Locke, David Hume and John Stuart Mill, whereby the mind was seen as a blank sheet of paper on which experience would write its script, had been replaced by a sort of neo-Calvinist notion of inherited individual destiny.
There are two ways to look at this development. You can damn Galton for being seduced by his ‘convenient jingle’ into presenting a false dichotomy. You can see him as one of the evil spirits of the twentieth century, cursing the three generations that followed to swinging like a pendulum between ridiculous extremes of environmental and genetic determinism. You can note with horror that from the beginning, Galton’s motives were eugenic. On the very first page of Hereditary Genius in 1869 he was already extolling the virtues of ‘judicious marriage’, lamenting the ‘degradation of human nature’ by the propagation of the unfit and invoking the ‘duty’ of the authorities to exercise power to change human nature by progressive breeding. These suggestions would grow into the pseudo-science of eugenics. With hindsight, therefore, you can blame him for an idea that would cause misery and cruelty to millions in the century to come, not just in Nazi Germany but in some of the most tolerant countries of the world.10
All this would be true, though it is a little harsh to expect that none of it would have happened without Galton, let alone that he should have foreseen where his ideas would lead. Even the convenient jingle would have soon occurred to somebody else. A more charitable reading of history would see Galton as a man far ahead of his time who hit upon a remarkable truth: that many aspects of our behaviour start within us in some way, that we are not putty in the hands of society or victims of our surroundings. You could even – though this might be stretching it – assert that this notion was vital in keeping alive the flame of liberty in the environmentalist despotisms of the twentieth century: those of Lenin, Mao and their imitators. Galton’s insights into heredity were remarkable, considering he knew nothing about genes. He would have had to wait more than a century to see that the study of twins did in the end prove much of what he had suspected. To the extent that they can be teased apart, nature prevails over one kind of (shared) nurture when it comes to defining differences in personality, intelligence and health between people within the same society. Note the caveats.
This is a recent development. Twenty years ago, the picture was very different. By the 1970s the whole notion of studying twins to learn about heredity was in eclipse. Two of the largest studies of twins since Galton were in disgrace. In Auschwitz, Josef Mengele was notoriously fascinated by twins. He sought them out among new arrivals at the concentration camp, and segregated them into special quarters for study. Ironically, this ‘favouritism’ led to a higher survival rate among twins than singletons – most of the small children who survived Auschwitz were twins. In exchange for submitting to procedures that were often brutal and sometimes fatal, they were at least better fed. All the same, few survived.11
In Britain meanwhile, the educational psychologist Cyril Burt was slowly accumulating a set of identical twins reared apart, which enabled him to calculate the heredity of intelligence. In 1966, when he published the full set of results, he claimed to have found 53 pairs of such twins. This was an extraordinarily large sample, and Burt’s conclusion that IQ was highly heritable influenced British education policy. But it later emerged that at least some of the data was almost certainly faked. The psychologist Leon Kamin noticed that the correlation had remained exactly the same, to the third decimal place, even while the data set had expanded over several decades. The Sunday Times simultaneously asserted that two of Burt’s co-authors probably did not exist (one has since reappeared, however).12
With a history like this, it was little wonder that twin research was a tainted subject in the 1970s. Yet today the study of twins is reborn as the principal method of a scientific discipline known as behaviour genetics that has flowered especially in the United States, Holland, Denmark, Sweden and Australia. It is sophisticated, argumentative, mathematical and expensive – everything that a thoroughly modern science should be. But at its core lies Galton’s insight: that human twinning provides a beautiful natural experiment for discerning the contributions of nature and nurture.
In this respect, fortune has been generous to human beings. The ability to produce identical twins seems to be rare in the animal kingdom. It is unknown in mice, for example, which produce litters of non-identical litter-mates. Human beings occasionally produce litters, too. Among white people, about one birth in every 125 consists of two non-identical, fraternal or ‘dizygotic’ twins – derived from two zygotes or fertilised eggs. The rate is higher among Africans and lower among Asians. But one birth in every 250 consists of identical (or monozygotic) twins, derived from a single fertilised egg. Without a genetic test, identical twins cannot be reliably distinguished from fraternal twins, though there are telltale signs. Their ears tend to be identical.13
Behaviour genetics is a simple matter of measuring how similar are identical twins, how different are fraternals, and how both identicals and fraternals turn out if separately adopted into different families. The result is an estimate of ‘heritability’ for any trait. Heritability is a slippery concept, much misunderstood. For a start, it is a population average, meaningless for any individual person: you cannot say that Hermia has more heritable intelligence than Helena. When somebody says that the heritability of height is 90 per cent, he does not and cannot mean that 90 per cent of my inches came from my genes and 10 per cent from my food. He means that the variation in height in a particular sample is attributable 90 per cent to genes and 10 per cent to environment. There is no variability in height for the individual and therefore no heritability.
Moreover, heritability can only measure variation, not absolutes. Most people are born with ten fingers. Those with fewer have usually lost some through accidents – through the effects of the environment. The heritability for finger number is therefore close to zero. Yet it would be absurd to argue that environment is the cause of us having ten fingers. We grow ten fingers because we are genetically programmed to grow ten fingers. It is the variation in finger number that is environmentally determined; the fact that we have ten fingers is genetic. Paradoxically, therefore, the least heritable features of human nature may be the most genetically determined.14
So, too, with intelligence. It cannot be right to say that Hermia’s intelligence is caused by her genes: it is obvious that you cannot become intelligent without food, parental care, teaching or books. Yet in a sample of people who have
