The question has fuelled some of history's fiercest scientific and political feuds. Now we have an answer
The monster Caliban, according to his master, Prospero, was “a devil, a pure devil, on whose nature nurture can never stick”. Yet only a few decades before Shakespeare wrote The Tempest, St Ignatius Loyola had founded the Jesuit order, with its famous maxim: “Give me the child until he is 7, and I will show you the man.”
This ancient debate over the relative contributions of inheritance and experience to the human condition has never been more charged than in the genetic age. On one side stood those who sought and saw genetic explanations for human psychology; on the other, those who believed it to be moulded by culture. There was little common ground. Sarah Blaffer Hrdy, an evolutionary psychologist, has even joked that perhaps we are genetically programmed to set nature against nurture.
Since the middle of the last century the nurture camp has been dominant. Just as molecular biology began to unravel the secrets of DNA, genetics and evolution were relegated to psychological bit-players by a new orthodoxy, which held that biology has forged a human mind of almost limitless malleability. It was the doctrine of the blank slate.
The idea, usually traced to the 17th-century philosopher John Locke, grew popular in the Enlightenment, fitting the mood of challenge to the supposedly innate authority of monarchy and aristocracy. It was a statement of individual freedom, which became strongly associated with the political Left. Though many early socialists were enthusiasts for eugenics, later generations grew suspicious of genetics, particularly after it was abused to justify oppression of disadvantaged racial and social groups, most brutally in Nazi Germany. Liberal opinion turned against the concept of a biological human nature, which was increasingly seen as a tool with which male and bourgeois elites could rationalise hegemony.
The movement was driven by the social sciences. From psychology came Sigmund Freud's notion that attitudes and mental health are explained by childhood experience. The behaviourism of B.F.Skinner added the claim that human beings could be conditioned by training, much as Ivan Pavlov's celebrated dogs salivated at the sound of a bell.
From anthropology came the research of Franz Boas and Margaret Mead, whose comparative studies of different societies suggested that traditions could steer human behaviour in a multitude of directions. Mead's purported discovery of free love among Samoan women was influential because - though founded on poor data - it challenged prevailing sexual mores. Karl Marx's political and economic theories saw human nature as something to be reshaped and directed to facilitate revolution. And postmodernism contributed the mantra that even knowledge and truth are socially constructed and relative.
What emerged was a new model of behaviour, in which human nature is anything but fixed or shared, but can be moulded into many configurations by culture. If genetic influences are allowed at all, they are wholly secondary to those of the environment. To its supporters, this became axiomatic to a fair society: if anything can be learnt, and anybody can do the learning, then people can be taught to value equality. Social justice and morality became intertwined with the concept that little in life is laid down, or even much affected, by inherited genes.
Though well-intentioned, and in some respects an important antidote to pseudoscientific genetic determinism, this view was dangerously inflexible. Any evidence that genetics might be seriously influential after all would threaten the very foundations of liberty and equality - so it would have to be resisted, as would research that might provide it.
The result was that scientists who investigated effects on human behaviour found their positions caricatured and their politics demonised as reactionary, even fascist. E.O.Wilson, the great evolutionary theorist and conservationist, is no man of the Right. Yet when he dared in the 1970s to suggest that human nature, like that of other animals, has a biological basis that might fruitfully be studied, his lectures were picketed and students doused him with water. The left-wing biologists Steven Rose, Leon Kamin and Richard Lewontin responded with a book entitled Not in Our Genes, which accused Wilson, Richard Dawkins and other sociobiologists of a crude determinism designed to legitimise the status quo. “Its adherents claim, first, that the details of present and past social arrangements are the inevitable manifestations of the specific action of genes,” they wrote.
Such attacks were misconceived. First, as Steven Pinker has pointed out, they set up a straw man. It is simply impossible to find serious biologists who believe that behaviour and social structure are “the inevitable manifestations of the specific action of genes”. Those who reject cultural determinism make a much more modest proposal - that genes, as well as the environment, make a contribution. As Dawkins wrote in a review of Not in Our Genes: “Reductionism, in the ‘sum of the parts' sense, is obviously daft, and is nowhere to be found in the writings of real biologists.”
What is more, cultural determinism can be just as inimical to freedom as its genetic counterpart. It implies that instead of being prisoners of our genes, we are prisoners of our parents, teachers and societies. Those who grow up in poverty will be forever disadvantaged, while those who come from privilege will retain it. Autism can be blamed on “refrigerator mothers”, and adults' relationship problems on their overprotective families. As a world view it is quite as bleak as one based on inheritance.
It has also become scientifically unsustainable. As research reveals more about inheritance, it has become abundantly clear that humans are not blank slates. Neither, however, are our personalities and behaviour forged by genes alone. The great controversy, indeed, is giving way to consensus, as improved understanding of how genes actually work shows the difficulty of separating nature and nurture.
Much of the critical evidence has emerged through the study of twins. Identical twins share all their DNA, while fraternal twins share only half - they are no more closely related on a genetic level than are ordinary siblings. Both kinds of twins, however, share a womb, a family and a cultural environment. Comparisons between the two types can thus tease out the extent to which inheritance is important.
Across a wide range of traits, including IQ, personality indicators such as extroversion and neuroticism, and even homosexuality, religiosity and political conservatism, identical twins are more similar to one another than are fraternal pairs. This indicates that genes must affect these aspects of personality.
The concordance between identical twins, however, is rarely 100per cent - their IQ scores, for example, tend to be around 70 per cent similar, compared with around 50per cent for non-identical pairs. By definition, inheritance therefore cannot be the only factor involved: if it were, identical twins would always turn out the same. For most human qualities, neither the extreme-nurture nor the extreme-nature hypothesis can be correct.
Even more striking evidence has come from a recent series of studies led by Avshalom Caspi and Terrie Moffitt. These scientists have been following up a cohort of children born in 1972-73 in Dunedin, New Zealand, recording details of their life experiences and testing their DNA. The results have demolished the nature- nurture dichotomy.
First, Moffitt and Caspi studied a gene called MAOA, which has two variants or alleles. Boys with one allele are more likely to behave antisocially and get into trouble with the law - but only if they were also maltreated as children. When raised in well-adjusted families, those with the “risky” allele are fine. It is not a gene “for” criminality, and no determinism - genetic or environmental - is involved. A genetic variant must be activated by an environmental influence to do any potential harm.
The serotonin transporter gene, 5HTT, also has two alleles, and is known to be involved in mood. Moffitt and Caspi found that people with one allele were 2.5 times more likely to develop clinical depression than those with the other - but, again, only under particular circumstances. The risk applies only to people who also experience stressful life events such as unemployment, divorce or bereavement. When their environments are happy, their genotypes made no difference.
These results show the sterility of the old nature-nurture debate. Nature works through nurture, and nurture through nature, to shape our personalities, aptitudes, health and behaviour. The question should not be which is the dominant influence, but how they fit together.
From 50 Genetics Ideas You Really Need to Know by Mark Henderson to be published by Quercus at £9.99 on April 2. To buy it for £9.49 inc p&p call 0845 2712134 or visit timesonline.co.uk/booksfirst