People with dyslexia and other learning difficulties are usually poor at other mental skills as well, according to a study of twins that is expected to reignite the debate over the meaning of intelligence.

In recent years the medical and educational professions have tended to "compartmentalise" mental abilities and disorders, so that children with dyslexia are said to have reading difficulties, but are thought to be otherwise of normal intelligence. The same applies for dyscalculia, which causes problems with mathematics, and other learning difficulties.

This is not usually the case, however, the British Association Festival of Science, in Salford, was told by Prof Robert Plomin, the deputy director of the Social, Genetic and Developmental Psychiatry Research Centre at the Institute of Psychiatry, in London. He has evidence that a common set of brain mechanisms - and genes – underpins a wide range of mental skills.

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Prof Plomin is no stranger to controversy. He left America to work in Britain because of hostility towards his earlier research on the genetic contribution to intelligence, or as he calls it "general cognitive ability", or "g". [See the article below.]

He said the new finding had come from the largest study of twins ever conducted in Britain - the Twins Early Development Study, which began with all twins born in England during 1994-96. It followed 15,000 children at the ages of two, three, four and seven in 7,500 twin pairs.

Analysis of the findings showed that "genes for common learning disabilities - such as language impairment, reading disability and maths disability - are generalists".

If genes were linked with a reading disability, there was a 70 per cent chance that they would also affect maths ability. "In other words, genetic 'diagnoses' differ from traditional diagnoses in that reading and maths disabilities are largely the same thing genetically."

The study also shows that genes that affect common learning disabilities are also responsible for normal variation in learning abilities.

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His work, however, also shows that the environment plays a key role, particularly early in development, giving opportunities for remedial teaching and training to overcome some disabilities.

Prof Plomin's work revives an idea put forward in 1904 by Charles Spearman, who noticed that all cognitive abilities correlated at least moderately. "Spearman referred to this overlap as general cognitive ability or 'g', which is assessed using measures of diverse cognitive processes such as IQ tests," said Prof Plomin.