Robert Plomin believes we may soon be able to detect the genes responsible for a significant proportion of intelligence. Lucy Hodges reports. For more than a century scientists have argued about the extent to which nature or nurture influences intelligence. The battle has gone back and forth: in some periods nature has held sway; in others nurture. But today the pendulum is coming to rest somewhere between the two, according to Robert Plomin, deputy director of a new research centre at the Institute of Psychiatry in south London.

Recruited from America where he had a "distinguished" professor title at Pennsylvania State University, Plomin, 47, is better equipped than some to speak on the subject. The author or co-author of 12 books, he is conducting a large twin study of the interplay of heredity and environment in mild mental handicap. And he has jumped straight into the fray because he is fed up with the amount of ignorance and misunderstanding out there.

Research on twins and adopted children going back 75 years shows that genetic influence on individual differences in intelligence is significant and substantial, he says. It accounts for 50 per cent of individual differences in intelligence (for intelligence read performance in IQ tests), the other half being explained by environmental or other factors. That conclusion is widely accepted by social and behavioural scientists, but not, it seems, by educationists. Why? Plomin, a professor in behavioural genetics, is not sure, though it may be because the research suggests inequalities among people. The mass media has not helped, he says. He does not really want to speculate.

The argument may also be because of the murky history of his subject, and the fact that it is associated with eugenics and with Sir Cyril Burt, whose work has been discredited in some people's eyes. Recently another controversial work, The Bell Curve, by Richard Herrnstein and Charles Murray, provoked a furore in the United States when it came out. It rehearsed the evidence linking genes and intelligence and concluded that the government should spend less money on the poor (for "poor" read "black") because there was nothing you could do about them. "When you're supping with people like Murray and Herrnstein you have to be aware that your work can be taken in ways you might not like and used for their political agenda," says Steve Jones, professor of genetics at University College, London.

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Even before he came to Britain, Plomin and his work were criticised by Peter Harper, professor of medicine at the University of Wales college of medicine. In a letter to the journal Behaviour Genetics Harper expressed concern about a particularly far-reaching aspect of Plomin's research - the possibility of identifying specific genes linked to differences in intelligence. In his letter Harper says his concerns include "whether the research itself on specific genetic markers and IQ can be considered ethical" , "whether misuse of any results is possible in the future" and "the potential for misinterpretation".

"The powerful impact of identifying a specific gene involved in the normal variation of intelligence, as opposed to a more general involvement of unidentified genes, should not be underestimated," wrote Harper, who urged that the widest possible professional and public consultation be embarked upon as soon as possible.

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Plomin, who did not think Harper's letter worth replying to, says he has no political agenda for his research. (His politics are not relevant anyway, he says. When pressed, he says he is a member of the Labour party.) Murray and Herrnstein's conclusions in The Bell Curve do not necessarily follow from the evidence, he says. The statistics could just as well be used to advocate more funding for the poor. He conducts research on individuals, not groups. Genetic research describes what is, not what ought to be, he emphasises. "It does not predict what could be, nor does it prescribe what should be," he says in an article to be published in the journal Intelligence. "Evidence for genetic influence does not imply that differences among individuals are immutable or irremediable - novel, environmental factors could make a difference. No research tells us what should be because this is a matter of goals and values."

It is precisely because of fears about the implications of heredity mattering so much that these ideas have aroused such hostility. Will people with inferior IQs end up on the scrapheap because no one can be bothered with them? What will become of those with mental handicaps? Will the super-bright be bred into a master race?

Some fellow academics from outside the world of behavioural genetics are worried, too. For example, Steven Rose, professor of biology at the Open University, says the research on twins and adopted children does not provide evidence that is acceptable to other scientists. That is because the methodology, notably the method for calculating heritability, is dubious, says Rose. It depends on the assumption that there is not much interaction between genes and the environment. In twin studies, however, the genetic and environmental factors interact in such an intricate and complicated way that it is not possible to separate out how much is due to nature and how much to nurture. "It's the heritability estimates which lie at the root of the fallacy in the way these calculations are done," he adds.

Behavioural geneticists do not agree. Peter McGuffin, professor of psychological medicine at the University of Wales College of Medicine, says Rose has got it wrong. He is a good biologist who delights in attacking behavioural geneticists, says McGuffin. The research done in this field, and by Plomin, is thoroughly respectable, adds McGuffin. "Plomin's work is not only respectable but is a very important part of biology. The next frontier is to discover the molecular genetic basis of behavioural traits."

Behavioural geneticists conduct their research on twins and adopted children precisely because they say it enables them to disentangle the nature/nurture conundrum. For example, it enables them to answer questions such as whether sisters who are adopted and brought up separately are similar, even though they do not share the same family environment; whether sisters living together are more similar than adopted family members who are not related; and whether identical twins are more similar than fraternal twins. The answer to all the questions is yes, they say. Which shows the importance of genes.

The tide began to turn towards more emphasis on genetic influences during the 1960s. A famous study of schizophrenia in 1966 showed that children adopted at birth away from their schizophrenic mothers faced the same risk of developing schizophrenia as the children who stayed with such mothers. That strongly suggested the illness ran in families for reasons of nature rather than nurture.

A similar finding has been made for autism. Until Sir Michael Rutter, (director of the Institute of Psychiatry's new research centre and the man who hired Plomin), conducted his first twin study of autism, people had thought the condition was caused by environmental factors. His results were a turning point. They showed that autism depends largely on genes. But not all behaviour patterns can be put down to heredity. Alcoholism, for example, has been found from twin studies to be only modestly influenced by genes, and then only in men. "The lesson is that genetic influence needs to be assessed, not simply assumed," says Plomin.

One of the most interesting discoveries of the past decade is that genetic influence on intelligence becomes increasingly important throughout the lifespan. That is not what you would expect. We would expect, says Plomin, "that environmental factors would become more important with advancing years, as accidents and illnesses accumulate. However, it is possible that genetic dispositions nudge us towards environments that accentuate our genetic propensities, thus leading to increased heritability throughout the lifespan." In other words, children endowed with genes that make reading easy find themselves reading more and more books, spending more time in libraries, lecture theatres and chatting to fellow eggheads. And so the effect multiplies.

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New research is showing a strong genetic influence in other areas, such as hyperactivity, reading ability and whether children are slow to learn to talk. Scientists are looking at genetic links between disorders and finding that genetic influences on anxiety and depression overlap considerably. Perhaps the most important boost to the influence of heredity on behaviour has been in molecular genetics where one sees research moving beyond the nature/nurture questions. Researchers in that area have made amazing advances in cloning and sequencing DNA and in identifying specific genes responsible for certain disorders. They found that the second most important cause of mental handicap after Down's syndrome was associated with a gene on the X chromosome called fragile X. Similarly late-onset Alzheimer's disease has been linked to another gene.

In the next year or two we will see reports of genes being identified that are associated with differences in individual intelligence, according to Plomin. Such research is extremely difficult because with intelligence one is talking about multiple genes (as well as multiple environmental factors). Says Plomin: "The challenge is to use the thousands of new DNA markers to identify not the gene for intelligence but some of the many genes, each of which makes a small contribution to the variance in the population."

No one should be frightened by that, he says. "It's not the end of the world," he adds. "If we can start coming to grips with it now, we will save ourselves a lot of concern later on."

None of which means environmental factors are unimportant. Plomin has found, after all, that non-genetic factors account for half of an individual's performance in cognitive tests, a figure with which many scientists would agree. Nurture is important too. In fact, the genetic research - on twins and adopted children - that has revealed the importance of genetics also gives the best possible evidence for environmental influence, he says. For example, the risk that an identical twin of a schizophrenic will also have the disease is about 45 per cent. Which means that as often as not when one identical twin has schizophrenia, the other does not. Why is that? Genes do not come into it, says Plomin. One can only assume environmental factors are at work.

In recent years genetic research has yielded two of the most far-reaching discoveries about the environment, he says, the first being that the environmental influences that matter most are those that make children in the same family different from one another, not similar. Plomin has called this phenomenon the non-shared environment, because it refers to experiences children have which they do not share with their siblings. This finding has broken new ground, he says.

Scientists had assumed that the key environmental influences on a child's development were things like the parents' personality, the parents' marriage, their education, the neighbourhood and the school. Why, then, do children in the same family often turn out so differently? Plomin's example is the Attenborough brothers, one an award-winning director and actor, the other a lover of plants and furry animals. His answer seems to be that children have differing experiences. They see in ways that are specific to them, and not general to the whole family. "The question of sibling differences is a key that holds the potential to unlock the secrets of environmental influence on the development of all children, not just siblings," he says.

The second discovery he calls the nature of nurture. Much of what people label environmental factors, for example, whether parents are warm and loving, whether children have books in the home, how much parents read to children, are actually closely entwined with genetics. That is because being affectionate, having books on shelves and reading aloud are not easy to separate from you as a person, from attributes you probably inherited from your own parents. Similarly, things that are seen as quite separate from you as a person - like divorce, losing your job, and having arguments with your spouse -- have been assumed to be environmental, things that happen to you willy-nilly. But we need to examine the links between such "life events" and the people involved, says Plomin. Such events are not independent of the kinds of people we are. We have something to do with them. We often have large elements of choice in whether we divorce, lose our jobs or have arguments.

"People are not passive recipients of environmental effects," he says. "They actively select, modify, and create their environments and reconstruct their experiences in memory." Musical children, for example, seek out music; athletic children make a beeline for soccer pitches; bookish children stay at home and read. Parents can nudge them along, but the children are writing their own scripts because of the way they are.

Plomin is married to Judy Dunn, who is British and a former Cambridge fellow specialising in social development, the environmental side of human behaviour. The pair have been hired as the first two Medical Research Council research professors in Britain and will be probing what makes us tick through an ambitious Pounds 1.5 million study of all twins (21,000 pairs) born in Britain in the three years, 1994-1996. The research is being funded by the MRC and will focus on language development and behavioural problems. No one has ever studied mild mental handicap from a genetic perspective. Plomin's study will be the first to do so.

The aim is to bring together genetic and environmental research. In fact that is the purpose of the new research centre that opened in autumn 1994 at the Institute of Psychiatry. Called the Research Centre on Social, Genetic and Developmental Psychiatry, it is concentrating on the interplay between nature and nurture in the development of behavioural disorders. Genetic and environmental research needs to be integrated, Plomin thinks, to illuminate the duet between nature and nurture during development. Thus his study of twins will examine how much genes and the environment play a part in delays in the children's behavioural problems or in delays in learning to talk.

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He is hoping to undertake a further study to examine the DNA of the children involved. The aim is to genotype a very large sample of 10,000 children to look at the links between the molecular structure of genes and any difficulties the children may be having with language or behaviour. Plomin and his colleagues have applied to the MRC for Pounds 1.8 million to undertake the second strand of the research. All of which should enable us to better understand ourselves and our children, and to disentangle further the relative parts played by nature and nurture in how we behave.