Do genes influence intelligence? Dr Stuart Derbyshire talked to top researcher Professor Robert Plomin about his search for genes which might influence variations in IQ.

James Heartfield for one is unconvinced, see below. Other views are invited

The sense we were born with?

David King, editor of GenEthics News, considers Plomin's research to be a 'very dangerous' step on the road towards 'downright eugenics'. King has organised a Campaign for Real Intelligence to try to prevent Plomin receiving further funding from the Medical Research Council. Mainstream clinical geneticists and ethicists are also concerned about Plomin's work on IQ. Two years ago Peter Harper, a leading British clinical geneticist, asked 'whether the research itself on genetic markers and IQ can be considered ethical' and purposively distanced himself from Plomin's work. The Nuffield Council on Bioethics has set up a working party to consider the ethical, social and legal implications of research into the genetics of mental disorders. The wider implications are clear: if the working party decides that aspects of genetic research on mental disorders are ethically questionable, which they almost certainly will, then research into the normal variation in IQ will be considered even more problematic, as it cannot be presented as a medical investigation of a diseased state.

Significantly, unlike in the 1970s, critics of Plomin's work no longer dispute the scientific data. Instead they say that the research is 'unethical'. This is a dramatic shift in attitudes towards scientific research. Rather than opposing specific applications of knowledge, the argument is being put that it is wrong to do the work to gain the knowledge in the first place. At base the argument says we should be afraid to find the truth. Critics of contemporary genes and behaviour research often claim that the work carries with it anti-humanist assumptions - that we are governed by our genes - and indeed some of it does. They also, quite rightly in some cases, point to possible harmful applications. But the call for an 'ethical' limit on research is equally problematic, for it suggests that we can neither handle the truth nor foster useful applications of the knowledge while preventing un-toward ones.

Rather than let this kind of ethical angst guide attitudes to research into genes and IQ, two questions should be asked. What does science tell us about the real relationships at work? And what constructive, beneficial use could be made of any findings?

The starting point for investigation is to grasp that all knowledge is cultural and social, not natural or genetic, in origin. Without the accumulated knowledge of society, and the socialisation process undergone by each member of society after birth, there would be no such thing as purposive thinking and intelligence. Connected to this, average intelligence is rising generation by generation (a process hidden by the fact that the average IQ is always set at 100). In addition, social and educational differences among children do have an impact on their educational abilities. A study in 1989 demonstrated that children born of lower class parents but adopted into middle class homes scored an average of 12 IQ points higher than those adopted into lower class homes (C Capron and M Duyme, 'Assessment of effects of socio-economic status on IQ in a full cross-fostering study', Nature, 1989, 340: 552-554).

And yet, for all that we know intelligence is not 'natural', there is now a wealth of evidence to show that those researching into genetic influences on intelligence are on to something. While intelligence itself is not a simple product of the genes, at any moment in time the evidence is that genetic factors do impact upon individual variations within the normal range of intelligence.

Over the past 20 years a lot of research has been published on the degree of similarity between family members for general cognitive ability, usually summarised under the rubric of 'IQ'. The most powerful evidence comes from the study of identical or 'monozygotic' (MZ) twins separated at an early age. The degree of similarity between the adult twins is attributed to their shared genetic inheritance, and the degree of difference is attributed to their different environment and/or upbringing. This design is not perfect. Any similarity in the twins' upbringing incidental to their genetic inheritance, which results in similar performance, can be misattributed to genetic influence. This problem is particularly acute for MZ twins because they are always the same sex and age and always look very similar.

But these problems in experiment design can be overcome. One way is to compare the MZ twins with a group of fraternal or 'dizygotic' (DZ) twins. DZ twins only share an average of 50 per cent of their genetic inheritance and should, on average, only be half as alike as the MZ twins. Any deviation from this pattern is indicative of effects which are non-genetic in origin. So a simple way to assess genetic influence over any given trait is to subtract the measured similarity in DZ twins reared apart from the measured similarity in MZ twins reared apart and multiply by two. More powerful and sophisticated methods can further refine the measurements: additional data can be taken from parents; other siblings and adopted siblings can be used as controls to eliminate the effects of upbringing.

The table summarises the results of four studies examining the similarity between twins on a variety of measures of cognitive ability. Combined, the studies involved more than 10 000 twin pairs. The analysis outlined above suggests that 48 per cent of the observed variability in cognitive performance is associated with genetic factors.

Reviewing the evidence from adoption studies in the journal Science, Plomin took a cautious view: 'The error surrounding [an estimate of 50 per cent heritability] may be as high as 20 per cent, so we can only say with confidence that the heritability of IQ scores is between 30 and 70 per cent. Nonetheless, even if the heritability of IQ scores is at the bottom of this range, it is a remarkable finding. To account for 30 per cent of the variance of anything as complex as IQ scores is a remarkable achievement.' (Science, 1990, 248: 183-188). Plomin told me that, when he began this work eight years ago, the evidence for a genetic influence over intelligence was 'better than the evidence for anything else, better than the evidence of heritability for height or schizophrenia'.

The quantitative evidence for a genetic effect on variation in intelligence was enough to convince Plomin to set about trying to find the genes concerned. Plomin's approach is to take a sample of subjects representing the top and bottom five per cent of the IQ distribution and then to examine the portion of the subjects' DNA most likely to be involved in neural functioning. Differences between the two groups then become candidate genes for further investigation.

So this is the scientific story so far: family studies indicate that genes play a role in variation in intelligence, but we do not as yet know which genes, or how many, are involved. Finding these genes will immediately raise the question of what society or individuals should do with the knowledge. My view is that, far from being afraid to find the truth in this area, society should be prepared to act upon it, even though economic, educational and other forms of inequality are the biggest barrier to most people realising their potential. For if genes do account for about 50 per cent of the variability in IQ scores, and if we believe that general cognitive ability is useful in life, then manipulating genes or their effects to increase IQ should be investigated.

Plomin agrees that in principle this is possible, but points to practical difficulties and emphasises his aim of 'preventing IQ from being lowered rather than being heightened'. He does not like the idea of prenatal selection 'allowing yuppie parents to select their kids' and advises any would-be parents: 'if you want bright kids marry a bright person.' Plomin has the more modest aim of investigating how genes may interact with environment to impact upon IQ. He does not rule out the possibility of a pharmacological intervention, but thinks it is unlikely before we understand how genes work at the biochemical level, and this is 'still not understood even for Huntington's disease' where a single gene causes all the problems. 'To understand intelligence', Plomin told me, 'we will need to detect many genes, each of which accounts for less than one per cent of the variability'. Current techniques can only detect gene effects of five per cent or more.

Plomin's caveats are well made. But he is being a little naive. It seems clear that, in time, it will be possible to use the current research to mount a pharmacological intervention if we choose to. Unless scientists like Plomin are prepared to face up to this, and defend it if they think it is in principle a good idea, they will find it hard to win support for their work.

Plomin is rightly excited about the work: 'I could have chosen to study height but it is not interesting. In terms of societal importance you couldn't get better than intelligence, [and] it's really happening, there is a real change of tide.' He would like to believe that the ethical angst is a storm in a teacup produced by 'incredibly condescending' reporters who see themselves as 'protecting the public' against genetic determinism. He told me that he is trying to organise an epidemiological study to determine what people really think about genes and behaviour research. I hope it works out as I am sure the report will be interesting, but I doubt it will reveal everything in the garden to be as rosy as Plomin seems to expect. There is general unease today with all things scientific, and there is particular hostility towards aspects of genetic research.

'Sometimes', Plomin told me, 'I want to put my head down and do my work'. This is understandable. Plomin's work has the potential for greatly benefiting humanity, yet he is denounced as irresponsible for looking at the issue. Who would not get fed up and want to get away from the fuss? But the quiet life is not an option: scientists cannot afford to retreat in the face of professional or public hostility. Only through open dialogue and a public defence of this kind of work will any of us be able to deal with our critics. This is more than an exercise in assuaging public concern - it is a struggle for integrity and survival.

The table shows the results of four studies examining the inheritance of 'IQ' as measured by a battery of cognitive tests.

Zero represents no relationship between the two twins' scores; 1 represents exact concordance between the scores

James Heartfield doesn't believe in IQ

A fool's errand

In each case, the object of enquiry could not yield to scientific enquiry because it is not properly a natural or technical entity that is being investigated. What presents itself as if it were a natural or technical object is in fact a human need, only fetishistically misunderstood to be a real thing. The philosopher's stone expressed a desire to turn iron into gold, at an early stage of the development of the market, when gold was considered to be valuable in itself, while iron was only a base metal. The real answer to the problem was not to be found in chemistry, but in trade. Likewise the yankee quest for the perpetual motion machine expressed a desire to be rid of dependence upon labour to make goods for sale - a fool's errand if ever there was one. These are problems that can only really be resolved by social science because, though they present themselves as natural-scientific objects, they are in fact confusions generated within society.

Today scientists look to find the natural components of intelligence in the genes. The principle, 'freedom of enquiry' demands that they should have the utmost liberty to proceed. But equally social scientists have an absolute obligation to tell them that they are on a fool's errand. Indeed we have known that this is a fool's errand for 190 years, since the German philosopher GWF Hegel first exposed the pretensions of phrenology - the science of judging intelligence and character by the shape of the skull. Hegel wrote 'it must be regarded as a complete denial of Reason to pass off a bone as the actual existence of consciousness' (Phenomenology of Mind, 1807, 1977 OUP ed, p205). Hegel's confident dismissal of a natural basis to intelligence did not arise from any great knowledge of phrenology - still less of genetics. He did not need to understand either of those things, because he did understand the character of intelligence itself.

The quest to find intelligence hard-wired into the genes is just a confusion, as absurd as trying to find it in the bones of the skull. Such 'intelligence' as is genetically coded, like the walking motions found in babies, or the spider's knowledge of how to spin a web is, by definition, not intelligence at all, but mere instinctual repetition. Genetically inherited? Fathers pass on intelligence to their sons in between the covers of car-manuals, but not in between the sheets.

Alive perhaps to the absurdity that intelligence could be inherited, scientists take refuge in the bastard concept 'intelligence quotient' or IQ. If intelligence may not by definition be passed on through the genes, then perhaps the capacity for thinking might. Might IQ not be genetically inherited, if intelligence is not?

But 'intelligence quotient' is an unsustainable category. It is an oxymoron, where the noun 'quotient', a specifically limited amount, is at odds with its adjective 'intelligence'. In its very nature intelligence has no limitations, is not parcelled out in specific amounts. If intelligence were finite, it would not be intelligence, but mere rote learning. You can only describe a person's intelligence quantitatively at a given moment. Is he asleep, or in prison, or living in Norfolk? Then he has little need of his wits and will doubtless leave them undeveloped. But the idea that God - or nature - has rationed out the intelligence at birth is just a confusion about what intelligence is.

Intelligence is something that people participate in and contribute to, but it is not a natural property of individual human beings. The impact of head-start programmes of early learning in the United States shows that concentrated teaching can raise IQ scores. So too can listening to classical music immediately beforehand improve your score on an IQ test. Each generation the tested IQ scores rise by 15 per cent - the 'Flynn effect'. Whatever it is that is being tested, it is not a finite allotment of intelligence.

Though the capacity for intelligence is not finite, neither is intelligence a merely abstract potential, like strength. Rather, it has a real and determinate content. IQ tests have, over the years been designed to factor out cultural knowledge, under pressure from educational rights campaigners. Verbal reasoning is eschewed in favour of visual sequence tests with geometrical shapes, in the mistaken belief that these somehow capture native intelligence.

But for an intelligence test actually to measure an individual's participation in and mastery of the intelligence of his time the tests would be very different indeed. Back when IQ testing was the norm, the proper thing would have been to set hour-long essay questions on the institution of the monarchy or the theory of evolution, or ask a testee to solve a chess problem or play a piece of music. Doubtless that would have led to discrimination on class grounds. But it would at least have the virtue of testing people's mastery of the intelligence of their own time. The abstract questions in modern IQ tests measure knowledge of nothing at all. In their desire to abstract from cultural differences, the IQ test authors devised tests that abstracted from real understanding. There are no naturally intelligent people, living in the wilderness, waiting for their native intelligence to be woken from its slumbers, because there is no abstract reasoning without a real content.

Aware, perhaps of the unsustainability of the concept of an IQ that is strictly determined by the genes, today's fashion is to propose a mix of 'nature' and 'nurture'. But this is a category error. The influence of society is not comparable to the influence of nature on an individual, because even our individual physiognomy is a product of society, in that it is only at a given level of productivity that the current population could have come into being. Social and natural causes cannot be subtracted from each other any more than apples can be subtracted from pears. Surely we should reject a theory that is so dependent upon the deus ex machina of 'nurture' to match its theory to its findings.

Finally, how are we to understand the scientists' fascination with IQ. The answer is not to be found in any natural object, but in society itself. This model of IQ is drawn from property inheritance not from a natural process. For nearly a century, IQ has served the role of justifying social inequality. Challenged to explain the social inequalities that accompany capitalism, ideologues have asserted that we live in a merito-cracy, where income is distributed according to merit, and merit is defined as intelligence plus effort.

The contemporary elevation of intelligence as a virtue is largely a concession to the petty prejudices of the middle classes, whose artificially sustained privileges take some explaining. It is a mark of the self-loathing of our modern elites that they take refuge in the fantasy of IQ tests instead of more manly virtues like courage or perseverance - but then no-body would believe them if they said that those were the things that marked them out for success. Only nerds join Mensa.

What then of the evidence? None of it is to date convincing. And considering the perverse amount of effort put into discovering the natural basis of intelligence, that in itself is an indicator. Where such a discovery is announced we should look at it with all the scepticism owed the discovery of the philosopher's stone, the perpetual motion machine or the location of Noah's Ark.