‘Revealed: the secret of human behaviour,’ read the banner headline in the British Sunday newspaper the Observer on 11 February 2001. ‘Environment, not genes, key to our acts.’ The source of the story was Craig Venter, the self-made man of genes who had built a private company to read the full sequence of the human genome (his own) in competition with an international consortium funded by taxes and charities. That sequence – a string of three billion letters composed in a four-letter alphabet containing the complete recipe for building and running a human body – was to be published later in the week. The first analysis had revealed that there were just 30,000 genes in the human genome, not the 100,000 that many had been estimating up until a few months before.

Details had already been circulated to journalists under embargo. But Venter spilt the story at an open meeting in Lyons on 9 February. Robin McKie of the Observer was in the audience and recognised at once that the 30,000 figure was now public. He went up to Venter and asked him if he realised that this broke the embargo; he did. Not for the first time in the increasingly bitter rivalry over the genome project, Venter’s version of the story would hit the headlines before that of his rivals. ‘We simply do not have enough genes for this idea of biological determinism to be right,’ Venter said to McKie. The wonderful diversity of the human species is not hard-wired in our genetic code. Our environments are critical.’²

Seeing the Observer’s first edition, other newspapers followed suit. ‘Genome discovery shocks scientists: genetic blueprint contains far fewer genes than thought – DNA’s importance downplayed,’ proclaimed the San Francisco Chronicle later that Sunday.³ The scientific journals promptly lifted the embargo and the story was in newspapers around the world. ‘Analysis of human genome discovers far fewer genes,’ intoned the New York Times.⁴ Not only had McKie scooped the story; Venter had set the theme.

This was the making of a new myth. In truth, the number of human genes changed nothing. Venter’s remarks concealed two massive non sequiturs: first, that fewer genes implied more environmental influences, and second, that 30,000 genes were ‘too few’ to explain human nature where 100,000 would have been enough. As Sir John Sulston, one the leaders of the Human Genome Project, put it to me a few weeks later, just 33 genes, each coming in just two varieties (such as on or off), would be enough to make every human being in the world unique. There are more than ten billion ways of flipping a coin 33 times. So 30,000 does not look such a small number after all. Two multiplied by itself 30,000 times produces a number larger than the total number of particles in the known universe. Besides, if fewer genes meant more free will, that made fruit flies freer than people, bacteria freer still, and viruses the John Stuart Mills of biology.

Fortunately, there was no need for such sophisticated calculations to reassure the population. People were not seen weeping in the street at the humiliating news that our genome had less than twice as many genes as a worm’s. Nothing had been hung on the number 100,000, which was just a bad guess. But it was fitting after a century of increasingly repetitive argument over environment versus heredity that the publication of the human genome should be broken on the Procrustean bed of nature-versus-nurture. It was, with the possible exception of the Irish question, the intellectual argument that had changed least in the century just ended. It had divided fascists from communists as neatly as their politics. It had continued unabashed through the discovery of chromosomes, DNA and Prozac. It was fated to be just as bitterly debated in 2003 as it was in 1953, the year of the discovery of the structure of the gene, or in 1900, the year modern genetics began. Even the human genome, at its birth, was being claimed for nurture-versus-nature.

For more than fifty years sane voices have called for an end to the debate. Nature-versus-nurture has been declared everything from dead and finished to futile and wrong – a false dichotomy. Everybody with an ounce of common sense knows that human beings are a product of a transaction between the two. Yet nobody could stop the argument. Immediately after calling the debate futile or dead, the protagonist would charge into the battle himself and start accusing others of overemphasising one or other extreme. The two sides of this argument are the nativists, who I will sometimes call geneticists, hereditarians or naturians; and the empiricists, who I will sometimes call environmentalists or nurturists.

Let me at once play my cards face up. I believe human behaviour has to be explained by both nature and nurture. I am not backing one side over the other. But that does not mean I am taking a ‘middle of the road’ compromise. As Jim Hightower, a Texan politician, once said: ‘There ain’t nothing in the middle of the road but a yellow line and a dead armadillo.’ I intend to make the case that the genome has indeed changed everything, not by closing the argument or winning the battle for one side or the other, but by enriching it from both ends till they meet in the middle. The discovery of how genes actually influence human behaviour, and how human behaviour influences genes, is about to recast the debate entirely. No longer is it nature-versus-nurture, but nature-via-nurture. Genes are designed to take their cues from nurture. To appreciate what has happened, you will have to abandon cherished notions and open your mind. You will have to enter a world where your genes are not puppet masters pulling the strings of your behaviour, but are puppets at the mercy of your behaviour; a world where instinct is not the opposite of learning, where environmental influences are sometimes less reversible than genetic ones, and where nature is designed for nurture. These cheap and seemingly empty phrases are coming to life for the first time in science. I intend to tell bizarre stories from the deepest recesses of the genome to show how the human brain is built for nurture. My argument in a nutshell is this: the more we lift the lid on the genome, the more vulnerable to experience genes appear to be.

I imagine a photograph taken in the year 1903. It is of a group of men gathered at some international meeting, in a fashionable spot like Baden-Baden or Biarritz, perhaps. ‘Men’ is not quite the right word, for though there are no women, there is one little boy, one baby and one ghost; the rest are middle-aged or elderly men, mostly rich and all white. There are 12 of them and, as befits the age, there is a great deal of facial hair. There are two Americans, two Austrians, two Britons, two Germans, one Dutchman, one Frenchman, one Russian and one Swiss.

It is, alas, an imaginary photograph, for most of them never met each other. But, like the famous group photograph of physicists at Solvay in 1927 – the one that includes Einstein and Bohr and Marie Curie and Planck and Schrödinger and Heisenberg and Dirac – my picture would capture that moment of ferment when a scientific endeavour throws up a host of new ideas.⁵ My 12 men were the ones who put together the chief theories of human nature that came to dominate the twentieth century.

The ghost that hovers overhead is Charles Darwin, dead for 21 years by the time of the photograph, and with the longest beard of all. Darwin’s idea is to seek the character of man in the behaviour of the ape and to demonstrate that there are universal features of human behaviour, like smiling. The elderly gent sitting bolt-upright on the far left is his cousin, Francis Galton, 81 years old but going strong; his whiskers hang down the sides of his face like white mice. Galton is the fervent champion of heredity. Next to him sits the American William James, 61, with a square, untidy beard. He is a champion of instinct and maintains that human beings have more impulses than other animals, not fewer. On Galton’s right is a botanist, out of place in a group concerned with human nature, and frowning unhappily behind his straggly beard. He is Hugo De Vries, 55, the Dutchman who discovered the laws of heredity only to realise he had been beaten to them more than 30 years before by a Moravian monk named Gregor Mendel. Beside him is the Russian, Ivan Pavlov, 54, his beard full and grey. He is a champion of empiricism, believing that the key to the human mind lies in the conditioned reflex. At his feet, uniquely clean-shaven, sits John Broadus Watson, who will turn Pavlov’s ideas into ‘behaviourism’ and famously claim to be able to alter personality at will merely by training. To Pavlov’s right stands the plump, bespectacled, moustachioed German, Emil Kraepelin, and the neatly bearded Viennese, Sigmund Freud, both 47 and both in the throes of influencing generations of psychiatrists away from ‘biological’ explanations and towards two very different notions of personal history. Beside him is the pioneer of sociology, the Frenchman Emile Durkheim, 45 and especially bushy in beard, busy insisting on the reality of social facts as more than the sum of their parts. His soulmate in this is standing next to him: a German-American (he had emigrated in 1885), the dashing Franz Boas, 45, with his drooping moustaches and duelling scar and his growing insistence that culture shapes human nature, not the other way round. The little boy in the front is the Swiss Jean Piaget, whose theories of imitation and learning will come to fruition, beardless, in mid-century. The baby in the pram is the Austrian Konrad Lorenz, who will revive the study of instinct and describe the vital concept of imprinting in the 1930s, while growing a fine white goatee.

I am not going to claim that these were necessarily the greatest students of human nature, nor that they were all equally brilliant. There are many, both dead and unborn, who would otherwise deserve inclusion in the photograph. David Hume and Immanuel Kant ought to be there, but they died too long ago (only Darwin manages to cheat death for the occasion); so should the modern theorists George Williams, William Hamilton and Noam Chomsky, but they were unborn. So should Jane Goodall, who discovered individuality in apes. So perhaps should some of the more perceptive novelists and playwrights.

But I am going to claim something rather surprising about these 12 men. They were right. Not right all the time, not even wholly right, and I do not mean morally right. They nearly all went too far in trumpeting their own ideas and criticising each other’s. One or two of them deliberately or accidentally give birth to grotesque perversions of ‘scientific’ policy that will haunt their reputations forever. But they were right in the sense that they all contributed an original idea with a germ of truth in it; they all placed a brick in the wall.

Human nature is indeed a combination of Darwin’s universals, Galton’s heredity, James’s instincts, De Vries’s genes, Pavlov’s reflexes, Watson’s associations, Kraepelin’s history, Freud’s formative experience, Boas’s culture, Durkheim’s division of labour, Piaget’s development and Lorenz’s imprinting. You can find all these things going on in the human mind. No account of human nature would be complete without them all.

But – and here is where I begin to tread new ground – it is entirely misleading to place these phenomena on a spectrum from nature to nurture, from genetic to environmental. Instead, to understand each and every one of them, you need to understand genes. It is genes that allow the human mind to learn, to remember, to imitate, to imprint, to absorb culture and to express instincts. Genes are not puppet masters, nor blueprints. Nor are they just the carriers of heredity. They are active during life; they switch each other on and off; they respond to the environment. They may direct the construction of the body and brain in the womb, but then they set about dismantling and rebuilding what they have made almost at once – in response to experience. They are both cause and consequence of our actions. Somehow the adherents of the ‘nurture’ side of the argument have scared themselves silly at the power and inevitability of genes, and missed the greatest lesson of all: the genes are on their side.

CHAPTER ONE The paragon of animals

Is man no more than this? Consider him well: Thou owest the worm no silk, the beast no hide, the sheep no wool, the cat no perfume:—Ha! here’s three of us are sophisticated!—Thou art the thing itself: unaccommodated man is no more but such a poor, bare, forked animal as thou art.

King Lear1

Similarity is the shadow of difference. Two things are similar by virtue of their difference from another; or different by virtue of one’s similarity to a third. So it is with individuals. A short man is different from a tall man, but two men seem similar if contrasted with a woman. So it is with species. A man and a woman may be very different, but by comparison with a chimpanzee, it is their similarities that strike the eye – the hairless skin, the upright stance, the prominent nose. A chimpanzee, in turn, is similar to a human being when contrasted with a dog: the face, the hands, the 32 teeth, and so on. And a dog is like a person to the extent that both are unlike a fish. Difference is the shadow of similarity.

Consider, then, the feelings of a naïve young man, as he stepped ashore in Tierra del Fuego on 18 December 1832 for his first encounter with what we would now call hunter-gatherers, or what he would call ‘man in a state of nature’. Better still, let him tell us the story:

It was without exception the most curious & interesting spectacle I ever beheld. I would not have believed how entire the difference between savage & civilized man is. It is much greater than between a wild & domesticated animal, in as much as in man there is greater power of improvement…I believe if the world was searched, no lower grade of man could be found.²

The effect on Charles Darwin was all the more shocking, because these were not the first Fuegian natives he had seen. He had shared a ship with three who had been transported to Britain, dressed in frocks and coats and taken to meet the king. To Darwin they were just as human as any other person. Yet here were their relatives, suddenly seeming so much less human. They reminded him of…well, of animals. A month later, on finding the camp site of a single Fuegian limpet hunter in an even more remote spot, he wrote in his diary: ‘We found the place where he had slept – it positively afforded no more protection than the form of a hare. How very little are the habits of such a being superior to those of an animal.’³ Suddenly, he is writing not just about difference (between civilised and savage man), but about similarity – the affinity between such a man and an animal. The Fuegian is so different from the Cambridge graduate that he begins to seem similar to an animal.

Six years after his encounter with the Fuegian natives, in the spring of 1838, Darwin visited London Zoo and there for the first time saw a great ape. It was an orang-utan named Jenny, and it was the second ape to be brought to the zoo. Its predecessor, Tommy, a chimpanzee, was exhibited at the zoo for a few months in 1835 before he died of tuberculosis. Jenny was acquired by the zoo in 1837, and like Tommy she caused a small sensation in London society. She seemed such a human animal, or was it such a beastly person? Apes posed uncomfortable questions about the distinction between people and animals, between reason and instinct. Jenny featured on the cover of the Penny Magazine of the Society for the Diffusion of Useful Knowledge, where the editorial reassured readers that ‘extraordinary as the Orang may be compared with its fellows of the brute creation, still in nothing does it trench upon the moral or mental provinces of man’. Queen Victoria, who saw a different orang-utan at the zoo in 1842, begged to differ. She described it as ‘frightful and painfully and disagreeably human’.⁴

After his first encounter with Jenny in 1838, Darwin returned to the zoo twice more a few months later. He came armed with a mouth organ, some peppermint and a sprig of verbena. Jenny seemed to appreciate all three. She seemed ‘astonished beyond measure’ at her reflection in a mirror. He wrote in his notebook: ‘Let man visit Ouran-outang in domestication…see its intelligence…and then let him boast of his proud pre-eminence…Man in his arrogance thinks himself a great work, worthy the interposition of a deity. More humble and I believe true to consider him created from animals.’ He was applying to animals what he had been taught to apply to geology: the uniformitarian principle that the forces shaping the landscape today are the same as those that shaped the distant past. Later that September, while reading Malthus’s essay on population, he had his sudden insight into what we now know as natural selection.

Jenny had played her part. When she took the mouth organ from him and placed it to her lips, she had helped him realise how high above the brute some animals could rise, just as the Fuegians had made him realise how low beneath civilisation some humans could sink. Was there a gap at all?

He was not the first person to think this way. Indeed, a Scottish judge, Lord Monboddo, had speculated in the 1790s that orang-utans could speak – if educated. Jean-Jacques Rousseau was only one of several Enlightenment philosophers who wondered if apes were not continuous with ‘savages’. But it was Darwin who changed the way human beings think of their own nature. Within his lifetime, he saw educated opinion come to accept that human bodies were those of just another ape modified by descent from a common ancestor.

But Darwin had less success in persuading his fellow human beings that the same argument could apply to the mind. His consistent view, from his earliest notebooks after reading David Hume’s Treatise on Human Nature to his last book, about earthworms, was that there was similarity, rather than difference, between human and animal behaviour. He tried the same mirror test on his children that he had tried on Jenny. He continually speculated on the animal parallels and evolutionary origins of human emotions, gestures, motives and habits. As he stated plainly, the mind needed evolution as much as the body did.

But in this he was deserted by many of his supporters, the psychologist William James being a notable exception. Alfred Russel Wallace, for example, the co-discoverer of the principle of natural selection, argued that the human mind was too complex to be the product of natural selection. It must instead be a supernatural creation. Wallace’s reasoning was both attractive and logical. It was based on similarity and difference again. Wallace was remarkable for his time in being mostly devoid of racial prejudice. He had lived among natives of South America and South-East Asia and he thought of them as equals, morally if not always intellectually. This led him to the belief that all races of humanity had similar mental abilities, which puzzled him because it implied that in most ‘primitive’ societies, the great part of human intelligence went unused. What was the point of being able to read or do long division if you were going to spend all your life in a tropical jungle? Ergo, said Wallace, ‘some higher intelligence directed the process by which the human race was developed’.⁵

We now know that Wallace’s assumption was entirely right, where Darwin’s was wrong. The gap between the ‘lowest’ human and the ‘highest’ ape is enormous. Genealogically, we all descend from a very recent common ancestor who lived just 150,000 years ago, whereas the last common ancestor with a chimpanzee lived at least five million years ago. Genetically, the differences between a human being and a chimpanzee are at least ten times as numerous as those between the two most dissimilar human beings. But Wallace’s deduction from this assumption, that therefore the human mind required a different kind of explanation from the animal mind, is not warranted. Just because two animals are different does not mean they cannot also be similar.

René Descartes had decreed firmly in the seventeenth century that people were rational and animals were automata. ‘They act not from knowledge but from the disposition of their organs…Brutes not only have a smaller degree of reason than men, but are wholly lacking in it.’⁶ Darwin dented this Cartesian distinction for a while. Freed at last from the need to think of the human mind as a divine creation, some of Darwin’s contemporaries, the ‘instinctivists’, began to think of humans as automata driven by instinct; others, the ‘mentalists’, began to credit the animal brain with reason and thought.

The mentalist anthropomorphism reached its apogee in the work of the Victorian psychologist, George Romanes, who eulogised the intelligence of pets, such as dogs that could lift latches and cats that seemed to understand their masters. Romanes believed that the only explanation for their behaviour was conscious choice. He went on to argue that each species of animal had a mind just like the human one, only frozen at a stage equivalent to a child of a certain age. Therefore, a chimpanzee had the mind of a young teenager, while a dog was equivalent to a younger child, and so on.⁷

Ignorance of wild animals sustained this notion. So little was known about the behaviour of apes that it was easy to go on thinking of them as primitive versions of people, rather than sophisticated animals that were brilliantly good at being apes. Especially with the discovery of the seemingly fierce gorilla in 1847, encounters between human beings and wild apes were exclusively brief and violent. When apes were brought to zoos, they had little opportunity to show their repertoire of wild habits, and their keepers seemed to evince more interest in their ability to ‘ape’ human customs than in what came naturally to them. For instance, from the very first arrival of chimpanzees in Europe, there seems to have been an obsession with serving them tea. The great French naturalist, Georges Leclerc, Comte de Buffon, was one of the first ‘scientists’ to see a captive chimp in about 1790. What did he find worth remarking? That he watched it ‘take a cup and saucer and lay them on the table, put in sugar, pour out its tea, leave it to cool without drinking’.⁸ Thomas Bewick, a few years later, reported breathlessly that an ape ‘shewn in London some years ago was taught to sit at table, make use of a spoon or fork in eating its victuals’.⁹ And when Tommy and Jenny reached London Zoo in the 1830s, they were quickly taught to eat and drink at table for the benefit of a paying audience. The tradition of the chimpanzee tea party was born. By the 1920s it was a daily ritual at London Zoo, the chimps trained both to ape human customs and to break them: ‘there was the ever present danger that their table manners would become too polished.’¹⁰ The chimpanzee tea parties at zoos ran for 50 years. In 1956, the Brooke Bond company made the first of many hugely successful television commercials for its tea using a chimps’ tea party, and Tetley finally dropped its chimps’ tea party advertisements only in 2002. By 1960, human beings still knew more about chimps’ ability to learn tea-table manners than they did about how the animals behaved in the wild. No wonder apes were viewed as ridiculous apprentice people.

In psychology, mentalism was soon ridiculed and demolished. The early twentieth-century psychologist Edward Thorndike demonstrated that Romanes’s dogs invariably learned their clever tricks by accident. They did not understand how a door latch worked; they simply repeated any action that accidentally enabled them to open the door. In reaction to the credulity of mentalism, psychologists began to make the opposite assumption: that animal behaviour was unconscious, automatic and reflex. The assumption was soon a creed. The radical behaviourists who brushed aside the mentalists in the same decade as the Bolsheviks brushed aside the Mensheviks asserted brusquely that animals did not think, reflect or reason; they just responded to stimuli. It became heresy even to talk about animals having mental states, let alone to attribute human understanding to them. Soon, under Burrhus Skinner, the behaviourists would apply the same logic to human beings. After all, people do not just anthropomorphise animals, accuse toasters of perversity and thunderstorms of fury. They also anthropomorphise other people, crediting them with too much reason and too little habit. Try reasoning with a nicotine addict.

But since nobody took Skinner all that seriously on the subject of people, the behaviourists had unwittingly restored the distinction between the human and the animal mind to exactly where Descartes had placed it. Sociologists and anthropologists, with their emphasis on the peculiarly human attribute called culture, had outlawed all talk of human instinct. By the middle of the twentieth century, it was heresy to speak of animal minds, and heresy to speak of human instincts. Difference, not similarity, was all.