Nature via Nurture: Genes, experience and what makes us human. Matt Ridley
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 all these advantages, the variation between who does well in exams and who does not could indeed be down to genes. In that sense, variation in intelligence can be genetic.
Through accident of geography, class or money, most schools have pupils from similar backgrounds. By definition, they give them similar teaching. Having therefore minimised the differences in environmental influences, they have unconsciously maximised the role of heredity: it is inevitable that the difference between the high-scoring and the low-scoring pupils must be down to their genes, for that is just about all that is left to vary. Again, heritability is a measure of what is varying, not what is determining.
Likewise, in a true meritocracy, where all have equal opportunity and equal training, the best athletes will be the ones with the best genes. Heritability of athletic ability will approach 100 per cent. In the opposite kind of society, where only the privileged few get sufficient food and the chance to train, background and opportunity will determine who wins the races. Heritability will be zero. Paradoxically, therefore, the more equal we make society, the higher heritability will be, and the more genes will matter.
COINCIDENCE
I’ve laboured the caveats deliberately before even mentioning the results of modern twin studies. The story of those studies begins in 1979, when there appeared in a Minneapolis newspaper an account of a pair of identical twin men from western Ohio reunited at the age of 40. Jim Springer and Jim Lewis had been reared apart in adopted families since they were a few weeks old. Intrigued, the psychologist Thomas Bouchard asked to meet them to record their similarities and differences. Within a month of their re-encounter, Bouchard and his colleagues examined the Jim twins for a day and were astonished by the similarities. Though they had different hairstyles, their faces and voices were almost indistinguishable. Their medical histories were very similar: high blood pressure, haemorrhoids, migraines, ‘lazy eye’, chain-smoked Salem cigarettes, bitten nails, weight gain at the same age. As expected, their bodies showed remarkable similarity. But so did their minds. Both followed stock-car racing and disliked baseball. Both had carpentry workshops. Both had built a white seat around a tree trunk in the garden. They went to the same Florida beach on vacation. Some of the coincidences were, well, coincidences. Both had dogs named Toy. Both had wives named Betty. Both had divorced women named Linda. Both had named their first children James Alan (though one spelled it James Allen).
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