In our series on Big Science Questions, Harriet Swain looks at controversies over intelligence and Robert Plomin discusses the crucial role of genetics.

For a quality traditionally associated with the mind, intelligence sparks strong emotions. What it is, how we identify it, how much of it is inherited and how much acquired are some of the most controversial questions facing scientists today. These questions have implications not only for how we see ourselves, but for how we rear our children. Parents lap up advice about altering diet or playing Mozart to a developing foetus in an effort to secure for their offspring a characteristic now seen as more of a key to future success than beauty or physical prowess.

The role of nurture in intelligence has been debated since Plato, who argued that learning was of little use to those of low mental ability. But the issue came to the fore after Charles Darwin published On the Origin of Species . In 1869, Francis Galton, a scientist and cousin of Darwin, published Hereditary Genius , in which he analysed the histories of famous families and found that there was a high probability of eminent people having eminent relatives. From this, he concluded that intellectual ability was inherited - rather underestimating the fact that intelligent parents were more likely to provide children with a stimulating environment. He believed that selective breeding would improve human cognitive powers.

Galton measured intelligence by testing reaction times to simple cognitive puzzles. Today's intelligence testing, which measures specific mental functions, originated in the early part of the 20th century with the French psychologist Alfred Binet. Schooling had just been made compulsory in France, and Binet was asked to develop ways of identifying children likely to fall behind without special educational help. He drew up a series of mental tasks associated with everyday life, such as counting coins or memorising lists, designed to measure mental functions, including memory, verbal ability and creativity. Children were judged by an overall score achieved across all tests. Binet also created the concept of mental age - the age at which a normal child should be able to accomplish a particular task.

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He insisted that his test was not designed to be used to measure adults or to differentiate between children of normal intellectual ability. It was supposed to be an indicator of future school success. But Binet helped to fuel the two great controversies in the study of intelligence. First, he established the disputed idea of a single measurable characteristic of "general intelligence". Second, he plunged directly into the debate about intelligence's heritability - the extent to which genes are responsible for differences between individuals.

His work was quickly appropriated. Lewis Terman, an American cognitive psychologist at Stanford University, developed what became known as the Stanford-Binet test. This was more an examination of "higher functioning" than a way of identifying backward students. It assigned to individuals a numerical intelligence quotient, or IQ, by dividing a person's mental age, identified by the test, by their chronological age and then multiplying by 100.

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The notion of a single general and measurable type of intelligence is usually referred to as "g". It was first identified by psychologist Charles Spearman at the beginning of the 20th century. Its supporters point out that IQ is a useful predictor of school success, that it is relatively consistent throughout life and that people who score highly in a test of one cognitive ability usually score highly in tests of another such ability. In the 1980s, Hans Eysenck, a German-born London University psychologist, claimed further evidence for g by showing a high correlation between IQ and reaction time. For example, people with high IQs asked to hit a button as soon as they saw a light go on, tended to hit it faster than those with lower IQs. Such a simple test appeared to eliminate the cultural, environmental and educational influences that critics of g say throw other tests of intelligence into question.

Those who believe in g think it is something substantially heritable. Galton was the first to use twin studies to measure how much differences between individuals' intelligence depended on the environment and how much on genetics and to conclude that genetic factors dominated. Since then, many more tests have been done on identical and non-identical twins and siblings brought up in different environments, with varying conclusions. More recently, some researchers such as Robert Plomin have been working to identify some of the specific genes responsible for the heritability of cognitive abilities and disabilities. Although Plomin stresses that this kind of work involves looking at many genes rather than just one, the idea has disturbed social scientists concerned by the ethical implications.

But by no means all scientists believe intelligence is innate: estimates of IQ heritability still range from below 40 per cent to 80 per cent. British biologist Steven Rose says methods of calculating heritability are dubious because genetic and environmental factors are so difficult to separate. He argues that recent evidence showing rising overall IQ scores and declining differences in scores between populations suggest environmental factors must be crucial. Leon Kamin, an American psychologist, is a similarly outspoken critic of IQ tests as a useful measure of intelligence and of the idea that intelligence is in the genes. He was the first to suggest that British educational psychologist Cyril Burt had fabricated data from twin studies to prove his theory that about 80 per cent of intelligence was inherited.

Fraud is not the only unpleasant allegation to enter the intelligence debate. Charges of racism have dominated since soon after Binet created his tests. Experiments in the early 20th century by H. H. Goddard, director of research at an American school for the "feeble-minded", showed that four-fifths of Hungarian, Italian and Russian immigrants arriving at New York's Ellis Island were of below average intelligence. He advocated preventing "morons", as he termed them, from bearing children. Tests carried out by Robert Yerkes for entry to the US army during the first world war showed similarly low immigrant scores, with black Americans coming out bottom. The tests contributed to tough new immigration laws.

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In 1969, Arthur Jensen published an article in the Harvard Educational Review that argued that genetic as well as social and environmental factors explained the fact that black Americans regularly score 15 per cent lower than whites in IQ tests. His views, which prompted an outcry, were taken up by psychologist Richard Herrnstein and social scientist Charles Murray in The Bell Curve: Intelligence and Class Structure in American Life , published in 1994. This argued that economic and social power was determined by g, that most social problems were rooted in low intelligence and that genetic differences between blacks and whites accounted for IQ differences and therefore for the way blacks were overrepresented in the underclass.

The racism debate rumbles on, those opposing The Bell Curve view insisting that racial differences are due predominently to cultural bias in the tests and differing socialisation. Curiously, there has been no similar debate over gender IQ differences despite the apparent structural differences between male and female brains. IQ tests are controlled for gender, and men and women perform broadly the same - although there are more men at the top and bottom of the scales and men tend to perform better in spatial questions, while women perform better in verbal tasks.

These differences in type of task become more significant for critics of the g concept, who argue that there is not just one general intelligence but several. One of the first to put forward this view was Louis Thurstone, an American electrical engineer and psychologist in the middle of the 20th century. He identified a number of primary mental abilities that an individual needed to survive and succeed. More recently, Howard Gardner, a professor of education and neuropsychologist at Harvard University, has suggested seven types of intelligence. By studying individuals with disabilities, he localised parts of the brain needed to perform particular tasks and found seven different areas of the brain relating to, for example, musical, mathematical and linguistic functions. American psychologist Robert Sternberg has narrowed down these multiple intelligences to just three - analytic, creative and practical. He also argues that psychometric tests can measure only the first of these intelligences.

In the past few years, researchers in the field have been studying yet another kind of intelligence - the artificial kind. British cybernetics professor Kevin Warwick has suggested that "it is difficult to think of any area of human intelligence (even including that intelligence solely associated with being human) in which a machine will not soon be able to outperform us". Cambridge physicist Stephen Hawking has also warned that machines could one day develop intelligence and take over the world. He suggests that humans should be genetically engineered to improve their own intelligence and so prevent this happening.

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But machines are a long way from acquiring one type of intelligence at least. In Emotional Intelligence , published in 1995, journalist and former academic Daniel Goleman suggested that a high IQ did not necessarily equal worldly success. In fact, individuals with very high IQs were often held back by emotional weaknesses. More useful as a life tool, he argued, was emotional intelligence, which he defined as a mixture of motivation, empathy and the ability to manage emotions and relationships.

His ideas, while hugely popular with the general public and businessmen, tend to be dismissed by academics as unscientific. Which of these judges is the more valid depends, perhaps, on your definition of intelligence.

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