The tabula of human nature was never rasa.
W. D. Hamilton
Nobody doubts that genes can shape anatomy. The idea that they also shape behaviour takes a lot more swallowing. Yet I hope to persuade you that on chromosome 7 there lies a gene that plays an important part in equipping human beings with an instinct, and an instinct, moreover, that lies at the heart of all human culture.
Instinct is a word applied to animals: the salmon seeking the stream of its birth; the digger wasp repeating the behaviour of its long-dead parents; the swallow migrating south for the winter — these are instincts. Human beings do not have to rely on instinct; they learn instead; they are creative, cultural, conscious creatures. Everything they do is the product of free will, giant brains and brainwashing parents.
So goes the conventional wisdom that has dominated psychology and all other social sciences in the twentieth century. To think otherwise, to believe in innate human behaviour, is to fall into the trap of determinism, and to condemn individual people to a heartless fate written in their genes before they were born. No matter that the social sciences set about reinventing much more alarming forms of determinism to take the place of the genetic form: the parental determinism of Freud; the socio-economic determinism of Marx; die political determinism of Lenin; the peer-pressure cultural determinism of Franz Boas and Margaret Mead; the stimulus—response determinism of John Watson and B. F. Skinner; the linguistic determinism of Edward Sapir and Benjamin Whorf. In one of the great diversions of all time, for nearly a century social scientists managed to persuade thinkers of many kinds that biological causality was determinism while environmental causality preserved free will; and that animals had instincts, but human beings did not.
Between 1950 and 1990 the edifice of environmental determinism came tumbling down. Freudian theory fell the moment lithium first cured a manic depressive, where twenty years of psychoanalysis had failed. (In 1995a woman sued her former therapist on the grounds that three weeks on Prozac had achieved more than three years of therapy.) Marxism fell when the Berlin wall was built, though it took until the wall came down before some people realised that subservience to an all-powerful state could not be made enjoyable however much propaganda accompanied it. Cultural determinism fell when Margaret Mead’s conclusions (that adolescent behaviour was infinitely malleable by culture) were discovered by Derek Freeman to be based on a combination of wishful prejudice, poor data collection and adolescent prank-playing by her informants. Behaviourism fell with a famous 1950s experiment in Wisconsin in which orphan baby monkeys became emotionally attached to cloth models of their mothers even when fed only from wire models, thus refusing to obey the theory that we mammals can be conditioned to prefer the feel of anything that gives us food — a preference for soft mothers is probably innate.1
In linguistics, the first crack in the edifice was a book by Noam Chomsky, Syntactic structures, which argued that human language, the most blatantly cultural of all our behaviours, owes as much to instinct as it does to culture. Chomsky resurrected an old view of language, which had been described by Darwin as an ‘instinctive tendency to acquire an art’. The early psychologist William James, brother of the novelist Henry, was a fervent protagonist of the view that human behaviour showed evidence of more separate instincts than animals, not fewer. But his ideas had been ignored for most of the twentieth century. Chomsky brought them back to life.
By studying the way human beings speak, Chomsky concluded that there were underlying similarities to all languages that bore witness to a universal human grammar. We all know how to use it, though we are rarely conscious of that ability. This must mean that part of the human brain comes equipped by its genes with a specialised ability to learn language. Plainly, the vocabulary could not be innate, or we would all speak one, unvarying language. But perhaps a child, as it acquired the vocabulary of its native society, slotted those words into a set of innate mental rules. Chomsky’s evidence for this notion was linguistic: he found regularities in the way we spoke that were never taught by parents and could not be inferred from the examples of everyday speech without great difficulty. For example, in English, to make a sentence into a question we bring the main verb to the front of the statement. But how do we know which verb to bring? Consider the sentence, ‘A unicorn that is eating a flower is in the garden.’ You can turn that sentence into a question by moving the second ‘is’ to the front: ‘Is a unicorn that is eating a flower in the garden?’ But you make no sense if you move the first ‘is’: ‘Is a unicorn that eating a flower is in the garden?’ The difference is that the first ‘is’ is part of a noun phrase, buried in the mental image conjured by not just any unicorn, but any unicorn that is eating a flower. Yet four-year-olds can comfortably use this rule, never having been taught about noun phrases. They just seem to know the rule. And they know it without ever having used or heard the phrase ‘a unicorn that is eating a flower’ before. That is the beauty of language — almost every statement we make is a novel combination of words.
Chomsky’s conjecture has been brilliantly vindicated in the succeeding decades by lines of evidence from many different disciplines. All converge upon the conclusion that to learn a human language requires, in the words of the psycho-linguist Steven Pinker, a human language instinct. Pinker (who has been called the first linguist capable of writing readable prose) persuasively gathered the strands of evidence for the innateness of language skills. There is first the universality of language. All human people speak languages of comparable grammatical complexity, even those isolated in the highlands of New Guinea since the Stone Age. All people are as consistent and careful in following implicit grammatical rules, even those without education and who speak what are patronisingly thought to be ‘slang’ dialects. The rules of inner-city black Ebonics are just as rational as the rules of the Queen’s English. To prefer one to another is mere prejudice. For example, to use double negatives (‘Don’t nobody do this to me . . .’) is considered proper in French, but slang in English. The rule is just as consistently followed in each.
Second, if these rules were learnt by imitation like the vocabulary, then why would four-year-olds who have been happily using the word ‘went’ for a year or so, suddenly start saying ‘goed’? The truth is that although we must teach our children to read and write -skills for which there is no specialised instinct — they learn to speak by themselves at a much younger age with the least of help from us. No parent uses the word ‘goed’, yet most children do at some time. No parent explains that the word ‘cup’ refers to all cup-like objects, not this one particular cup, nor just its handle, nor the material from which it is made, nor the action of pointing to a cup, nor the abstract concept of cupness, nor the size or temperature of cups. A computer that was required to learn language would have to be laboriously equipped with a program that ignored all these foolish options - with an instinct, in other words. Children come preprogrammed, innately constrained to make only certain kinds of guess.
But the most startling evidence for a language instinct comes from a series of natural experiments in which children imposed grammatical rules upon languages that lacked them. In the most famous case, studied by Derek Bickerton, a group of foreign labourers brought together on Hawaii in the nineteenth century developed a pidgin language — a mixture of words and phrases whereby they could communicate with each other. Like most such pidgins, the language lacked consistent grammatical rules and remained both laboriously complex in the way it had to express things and relatively simple in what it could express. But all that changed when for the first time a generation of children learnt the language in their youth. The pidgin acquired rules of inflection, word order and grammar that made it a far more efficient and effective language — a creole. In short, as Bickerton concluded, pidgins become Creoles only after they are learnt by a generation of children, who bring instinct to bear on their transformation.
Bickerton’s hypothesis has received remarkable support from the study of sign language. In one case, in Nicaragua, special schools for the deaf, established for the first time in the 1980s, led to the invention, de novo, of a whole new language. The schools taught lip-reading with little success, but in the playground the children brought together the various hand signs they used at home and established a crude pidgin language. Within a few years, as younger children learnt this pidgin, it was transformed into a true sign language with all the complexity, economy, efficiency and grammar of a spoken language. Once again, it was children who made the language, a fact that seems to suggest that the language instinct is one that is switched off as the child reaches adulthood. This accounts for our difficulty in learning new languages, or even new accents, as adults. We no longer have the instinct. (It also explains why it is so much harder, even for a child, to learn French in a classroom than on holiday in France: the instinct works on speech that it hears, not rules that it memorises.) A sensitive period during which something can be learnt, and outside which it cannot, is a feature of many animals’ instincts. For instance, a chaffinch will only learn the true song of its species if exposed to examples between certain ages. That the same is true of human beings was proved in a brutal way by the true story of Genie, a girl discovered in a Los Angeles apartment aged thirteen. She had been kept in a single sparsely furnished room all her life and deprived of almost all human contact. She had learnt two words, ‘Stopit’ and ‘Nomore’. After her release from this hell she rapidly acquired a larger vocabulary, but she never learnt to handle grammar — she had passed the sensitive period when the instinct is expressed.
Yet even bad ideas take a lot of killing, and the notion that language is a form of culture that can shape the brain, rather than vice versa, has been an inordinate time a-dying. Even though die canonical case histories, like the lack of a concept of time in the Hopi language and hence in Hopi thought, have been exposed as simple frauds, the notion that language is a cause rather than consequence of the human brain’s wiring survives in many social sciences. It would be absurd to argue that only Germans can understand the concept of taking pleasure at another’s misfortune; and that the rest of us, not having a word for Schadenfreude, find the concept entirely foreign.2
Further evidence for the language instinct comes from many sources, not least from detailed studies of the ways in which children develop language in their second year of life. Irrespective of how much they are spoken to directly, or coached in the use of words, children develop language skills in a predictable order and pattern. And the tendency to develop language late has been demonstrated by twin studies to be highly heritable. Yet for many people the most persuasive evidence for the language instinct comes from the hard sciences: neurology and genetics. It is hard to argue with stroke victims and real genes. The same part of the brain is consistently used for language processing (in most people, on the left side of the brain), even the deaf who ‘speak’ with their hands, though sign language also uses part of the right hemisphere.3
If a particular one of these parts of the brain is damaged, the effect is known as Broca’s aphasia, an inability to use or understand all but the simplest grammar, even though the ability to understand sense remains unaffected. For instance, a Broca’s aphasic can easily answer questions such as ‘Do you use a hammer for cutting?’ but has great difficulty with: ‘The lion was killed by the tiger. Which one is dead?’ The second question requires sensitivity to the grammar encoded in word order, which is known by just this one part of the brain. Damage to another area, Wernicke’s area, has almost the opposite effect — people with such damage produce a rich but senseless stream of words. It appears as if Broca’s area generates speech and Wernicke’s area instructs Broca’s area what speech to generate. This is not the whole story, for there are other areas active in language processing, notably the insula (which may be the region that malfunctions in dyslexia).4
There are two genetic conditions that affect linguistic ability. One is Williams syndrome, caused by a change in a gene on chromosome 11, in which affected children are very low in general intelligence, but have a vivid, rich and loquacious addiction to using language. They chatter on, using long words, long sentences and elaborate syntax. If asked to refer to an animal, they are as likely to choose something bizarre like an aardvark as a cat or a dog. They have a heightened ability to learn language but at the expense of sense: they are severely mentally retarded. Their existence seems to undermine the notion, which most of us have at one time or another considered, that reason is a form of silent language.
The other genetic condition has the opposite effect: it lowers linguistic ability without apparently affecting intelligence, or at least not consistently. Known as specific language impairment (SLI), this condition is at the centre of a fierce scientific fight. It is a battleground between the new science of evolutionary psychology and the old social sciences, between genetic explanations of behaviour and environmental ones. And the gene is here on chromosome 7.
That the gene exists is not at issue. Careful analysis of twin studies unambiguously points to a strong heritability for specific language impairment. The condition is not associated with neurological damage during birth, is not associated with linguistically impoverished upbringings, and is not caused by general mental retardation. According to some tests — and depending on how it is defined — the heritability approaches one hundred per cent. That is, identical twins are roughly twice as likely to share the condition as fraternal twins.5
That the gene in question is on chromosome 7 is also not in much doubt. In 1997 a team of Oxford-based scientists pinned down a genetic marker on the long arm of chromosome 7, one form of which co-occurs with the condition of SLI. The evidence, though based only on one large English family, was strong and unambiguous.6
So why the battleground? The argument rages about what SLI is. To some it is merely a general problem with the brain that affects many aspects of language-producing ability, including principally the ability to articulate words in the mouth and to hear sounds correctly in the ear. The difficulty the subjects experience with language follow from these sensory problems, according to this theory. To others, this is highly misleading. The sensory and voice problems exist, to be sure, in many victims of the condition, but so does something altogether more intriguing: a genuine problem understanding and using grammar that is quite independent of any sensory deficits. The only thing both sides can agree upon is that it is thoroughly disgraceful, simplistic and sensationalist of the media to portray this gene, as they have done, as a ‘grammar gene’.
The story centres on a large English family known as the Ks. There are three generations. A woman with the condition married an unaffected man and had four daughters and one son: all save one daughter were affected and they in turn had between them twenty-four children, ten of whom have the condition. This family has got to know the psychologists well; rival teams besiege them with a battery of tests. It is their blood that led the Oxford team to the gene on chromosome 7. The Oxford team, working with the Institute of Child Health in London, belongs to the ‘broad’ school of SLI, which argues that the grammar-deficient skills of the K family members stem from their problems with speech and hearing. Their principal opponent and the leading advocate of the ‘grammar theory’ is a Canadian linguist named Myrna Gopnik.
In 1990 Gopnik first suggested that the K family and others like them have a problem knowing the basic rules of English grammar. It is not that they cannot know the rules, but that they must learn them consciously and by heart, rather than instinctively internalise them. For example, if Gopnik shows somebody a cartoon of an imaginary creature and with it the words “This is a Wug’, then shows them a picture of two such creatures together with the words ‘These are . . .’, most people reply, quick as a flash, Wugs’. Those with SLI rarely do so, and if they do, it is after careful thought. The English plural rule, that you add an V to the end of most words, is one they seem not to know. This does not prevent those with SLI knowing the plural of most words, but they are stumped by novel words that they have not seen before, and they make the mistake of adding V to fictitious words that the rest of us would not, such as ‘saess’. Gopnik hypothesises that they store English plurals in their minds as separate lexical entries, in the same way that we all store singulars. They do not store the grammatical rule.7
The problem is not, of course, confined to plurals. The past tense, the passive voice, various word-order rules, suffixes, word-combination rules and all the laws of English we each so unconsciously know, give SLI people difficulty, too. When Gopnik first published these findings, after studying the English family, she was immediately and fiercely attacked. It was far more reasonable, said one critic, to conclude that the source of the variable performance problems lay in the language-processing system, rather than the underlying grammar. Grammatical forms like plural and past tense were particularly vulnerable, in English, in individuals with speech defects. It was misleading of Gopnik, said another pair of critics, to neglect to report that the K family has a severe congenital speech disorder, which impairs their words, phonemes, vocabulary and semantic ability as well as their syntax. They had difficulty understanding many other forms of syntactical structure such as reversible passives, post-modified subjects, relative clauses and embedded forms.8
These criticisms had a whiff of territoriality about them. The family was not Gopnik’s discovery: how dare she assert novel things about them? Moreover, there was some support for her idea in at least part of the criticism: that the disorder applied to all syntactical forms. And to argue that the grammatical difficulty must be caused by the mis-speaking problem, because mis-speaking goes with the grammatical difficulty, was circular. Gopnik was not one to give up. She broadened the study to Greek and Japanese people as well, using them for various ingenious experiments designed to show the same phenomena. For example, in Greek, the word ‘likos’ means wolf. The word ‘likanthropos’ means wolfman. The word ‘lik’, the root of wolf, never appears on its own. Yet most Greek speakers automatically know that they must drop the ‘-os’ to find the root if they wish to combine it with another word that begins with a vowel, like ‘-anthropos’, or. drop only the Y, to make ‘liko-’ if they wish to combine it with a word that begins with a consonant. It sounds a complicated rule, but even to English speakers it is immediately familiar: as Gopnik points out, we use it all the time in new English words like ‘technophobia’.
Greek people with SLI cannot manage the rule. They can learn a word like ‘likophobia’ or ‘likanthropos’, but they are very bad at recognising that such words have complex structures, built up from different roots and suffixes. As a result, to compensate, they effectively need a larger vocabulary than other people. ‘You have to think of them’, says Gopnik, ‘as people without a native language.’ They learn their own tongue in the same laborious way that we, as adults, learn a foreign language, consciously imbibing the rules and words.9
Gopnik acknowledges that some SLI people have low IQ on non-verbal tests, but on the other hand some have above-average IQ. In one pair of fraternal twins, the SLI one had higher nonverbal IQ than the unaffected twin. Gopnik also acknowledges that most SLI people have problems speaking and hearing as well, but she contends that by no means all do and that the coincidence is irrelevant. For instance, people with SLI have no trouble learning the difference between ‘ball’ and ‘bell’, yet they frequently say ‘fall’ when they mean ‘fell’ — a grammatical, not a vocabulary difference. Likewise, they have no difficulty discerning the difference between rhyming words, like ‘nose’ and ‘rose’. Gopnik was furious when one of her opponents described the K family members’ speech as ‘unintelligible’ to outsiders. Having spent many hours with them, talking, eating pizza and attending family celebrations, she says they are perfectly comprehensible. To prove the irrelevance of speaking INSTINCT IOI and hearing difficulties, she has devised written tests, too. For example, consider the following pair of sentences: ‘He was very happy last week when he was first.’ ‘He was very happy last week when he is first.’ Most people immediately recognise that the first is grammatical and the second is not. SLI people think they are both acceptable statements. It is hard to conceive how this could be due to a hearing or speaking difficulty.10
None the less, the speaking-and-hearing theorists have not given up. They have recently shown that SLI people have problems with ‘sound masking’, whereby they fail to notice a pure tone when it is masked by preceding or following noise, unless the tone is forty-five decibels more intense than is detectable to other people. In other words, SLI people have more trouble picking out the subtler sounds of speech from the stream of louder sounds, so they might, for example, miss the ‘-ed’ on the end of a word.
But instead of supporting the view that this explains the entire range of SLI symptoms, including the difficulty with grammatical rules, this lends credence to a much more interesting, evolutionary explanation: that the speech and hearing parts of the brain are next door to the grammar parts and both are damaged by SLI. SLI results from damage to the brain caused in the third trimester of pregnancy by an unusual version of a gene on chromosome 7. Magnetic-resonance imaging confirms the existence of die brain lesion and the rough location. It occurs, not surprisingly, in one of the two areas devoted to speech and language processing, the areas known as Broca’s and Wernicke’s areas.
There are two areas in the brains of monkeys that correspond precisely to these areas. The Broca-homologue is used for controlling the muscles of the monkey’s face, larynx, tongue and mouth. The Wernicke-homologue is used for recognising sound sequences and the calls of other monkeys. These are exactly die non-linguistic problems that many SLI people have: controlling facial muscles and hearing sounds distinctly. In other words, when ancestral human beings first evolved a language instinct, it grew in the region devoted to sound production and processing. That sound-production and processing module remained, with its connections to facial muscles and ears, but the language instinct module grew on top of it, with its innate capacity for imposing the rules of grammar on the vocabulary of sounds used by members of the species. Thus, although no other primate can learn grammatical language at all - and we are indebted to many diligent, sometimes gullible and certainly wishful trainers of chimpanzees and gorillas for thoroughly exhausting all possibilities to the contrary — language is intimately physically connected with sound production and processing. (Yet not too intimately: deaf people redirect the input and output of the language module to the eyes and hands respectively.) A genetic lesion in that part of the brain therefore affects grammatical ability, speech and hearing — all three modules.11
No better proof could be adduced for William James’s nineteenth-century conjecture that human beings evolved their complex behaviour by adding instincts to those of their ancestors, not by replacing instincts with learning. James’s theory was resurrected in the late 1980s by a group of scientists calling themselves evolutionary psychologists. Prominent among them were the anthropologist John Tooby, the psychologist Leda Cosmides and the psycho-linguist Steven Pinker. Their argument, in a nutshell, is this. The main goal of twentieth-century social science has been to trace the ways in which our behaviour is influenced by the social environment; instead, we could turn the problem on its head and trace the ways in which the social environment is the product of our innate social instincts. Thus the fact that all people smile at happiness and frown when worried, or that men from all cultures find youthful features sexually attractive in women, may be expressions of instinct, not culture. Or the universality of romantic love and religious belief might imply that these are influenced by instinct more than tradition. Culture, Tooby and Cosmides hypothesised, is the product of individual psychology more than vice versa. Moreover, it has been a gigantic mistake to oppose nature to nurture, because all learning depends on innate capacities to learn and innate constraints upon what is learnt. For instance, it is much easier to teach a monkey (and a man) to fear snakes than it is to teach it to fear flowers. But you still have to teach it. Fear of snakes is an instinct that has to be learnt.’12
The ‘evolutionary’ in evolutionary psychology refers not so much to an interest in descent with modification, nor to the process of natural selection itself - interesting though these are, they are inaccessible to modern study in the case of the human mind, because they happen too slowly - but to the third feature of the Darwinian paradigm: the concept of adaptation. Complex biological organs can be reverse-engineered to discern what they are ‘designed’ to do, in just the same way that sophisticated machines can be so studied. Steven Pinker is fond of pulling from his pocket a complicated thing designed for pitting olives to explain the process of reverse engineering. Leda Cosmides prefers a Swiss-army knife to make a similar point. In each case, the machines are meaningless except when described in terms of their particular function: what is this blade for? It would be meaningless to describe the working of a camera without reference to the fact that it is designed for the making of images. In the same way, it is meaningless to describe the human (or animal) eye without mentioning that it is specifically designed for approximately the same purpose.
Pinker and Cosmides both contend that the same applies to the human brain. Its modules, like the different blades of a Swiss-army knife, are most probably designed for particular functions. The alternative, that the brain is equipped with random complexity, from which its different functions fall out as fortunate by-products of the physics of complexity - an idea still favoured by Chomsky - defies all evidence. There is simply nothing to support the conjecture that the more detailed you make a network of microprocessors, the more functions they will acquire. Indeed, the ‘connectionist’ approach to neural networks, largely misled by the image of the brain as a general-purpose network of neurons and synapses, has tested the idea thoroughly and found it wanting. Pre-programmed design is required for the solving of pre-ordained problems.
There is a particular historical irony here. The concept of design in nature was once one of the strongest arguments advanced against evolution. Indeed, it was the argument from design that kept evolutionary ideas at bay throughout the first half of the nineteenth century. Its most able exponent, William Paley, famously observed that if you found a stone on the ground, you could conclude litde of interest about how it got there. But if you found a watch, you would be forced to conclude that somewhere there was a watchmaker. Thus the exquisite, functional design apparent in living creatures was manifest evidence for God. It was Darwin’s genius to use the argument from design just as explicitly but in the service of the opposite conclusion: to show that Paley was wrong. A ‘blind watchmaker’ (in Richard Dawkins’s phrase) called natural selection, acting step by step on the natural variation in the creature’s body, over many millions of years and many millions of individuals, could just as easily account for complex adaptation. So successfully has Darwin’s hypothesis been supported that complex adaptation is now considered the primary evidence that natural selection has been at work.13
The language instinct that we all possess is plainly one such complex adaptation, beautifully designed for clear and sophisticated communication between individuals. It is easy to conceive how it was advantageous for our ancestors on the plains of Africa to share detailed and precise information with each other at a level of sophistication unavailable to other species. ‘Go a short way up that valley and turn left by the tree in front of the pond and you will find the giraffe carcass we just killed. Avoid the brush on the right of the tree that is in fruit, because we saw a lion go in there.’ Two sentences pregnant with survival value to the recipient; two tickets for success in the natural-selection lottery, yet wholly incomprehensible without a capacity for understanding grammar, and lots of it.
The evidence that grammar is innate is overwhelming and diverse. The evidence that a gene somewhere on chromosome 7 usually plays a part in building that instinct in the developing foetus’s brain is good, though we have no idea how large a part that gene plays. Yet most social scientists remain fervently resistant to the idea of genes whose primary effect seems to be to achieve the development of grammar directly. As is clear in the case of the gene on chromosome 7, many social scientists prefer to argue, despite much evidence, that the gene’s effects on language are mere side-effects of its direct effect on the ability of the brain to understand speech. After a century in which the dominating paradigm has been that instincts are confined to ‘animals’ and are absent from human beings, this reluctance is not surprising. This whole paradigm collapses once you consider the Jamesian idea that some instincts cannot develop without learnt, outside inputs.
This chapter has followed the arguments of evolutionary psychology, the reverse-engineering of human behaviour to try to understand what particular problems it was selected to solve. Evolutionary psychology is a new and remarkably successful discipline that has brought sweeping new insights to the study of human behaviour in many fields. Behaviour genetics, which was the subject of the chapter on chromosome 6, aims at roughly the same goal. But the approach to the subject is so different that behaviour genetics and evolutionary psychology are embarked on a collision course. The problem is this: behaviour genetics seeks variation between individuals and seeks to link that variation to genes. Evolutionary psychology seeks common human behaviour — human universals, features found in every one of us — and seeks to understand how and why such behaviour must have become partly instinctive. It therefore assumes no individual differences exist, at least for important behaviours. This is because natural selection consumes variation: that is its job. If one version of a gene is much better than another, then the better version will soon be universal to the species and the worse version will soon be extinct. Therefore, evolutionary psychology concludes that if behaviour geneticists find a gene with common variation in it, then it may not be a very important gene, merely an auxiliary. Behaviour geneticists retort that every human gene yet investigated turns out to have variants, so there must be something wrong with the argument from evolutionary psychology.
In practice, it may gradually emerge that the disagreement between these two approaches is exaggerated. One studies the genetics of common, universal, species-specific features. The other studies the genetics of individual differences. Both are a sort of truth. All human beings have a language instinct, whereas all monkeys do not, but that instinct does not develop equally well in all people. Somebody with SLI is still far more capable of learning language than Washoe, Koko, Nim or any of the other trained chimpanzees and gorillas.
The conclusions of both behaviour genetics and evolutionary psychology remain distinctly unpalatable to many non-scientists, whose main objection is a superficially reasonable argument from incredulity. How can a gene, a stretch of DNA ‘letters’, cause a behaviour? What conceivable mechanism could link a recipe for a protein with an ability to learn the rule for making the past tense in English? I admit that this seems at first sight a mighty leap, requiring more faith than reason. But it need not be, because the genetics of behaviour is, at root, no different from the genetics of embryonic development. Suppose that each module of the brain grows its adult form by reference to a series of chemical gradients laid down in the developing embryo’s head — a sort of chemical road map for neurons. Those chemical gradients could themselves be the product of genetic mechanisms. Hard though it is to imagine genes and proteins that can tell exactly where they are in the embryo, there is no doubting they exist. As I shall reveal when discussing chromosome 12, such genes are one of the most exciting products of modern genetic research. The idea of genes for behaviour is no more strange than the idea of genes for development. Both are mind-boggling, but nature has never found human incomprehension a reason for changing her methods.