September 17, 2003
Same Genes Cause Normal And Abnormal Mental Ability Differences

Researcher Robert Plomin has found from a study of twins in Britain that genes which have variations which cause learning disabilities also have variations that are responsible for causing differences in normal variations in intelligence.

Research from the largest study of twins ever conducted in the UK shows that genetic influences on common learning disabilities are not specific to each disorder.

Professor Robert Plomin of the Institute of Psychiatry, King’s College London today presents evidence on common learning disabilities to the BA Festival of Science at the University of Salford, Greater Manchester.

The Twins Early Development Study (TEDS) compares identical and non-identical twins born in England in 1994-6 and latest findings are from year-long assessments by teachers of reading and maths at age seven.

Researchers found three main reasons that genes involved in common learning disabilities are generalists in three ways. First, genes that affect these disabilities are the same genes responsible for normal variation in learning abilities. Second, genes are not specific to one aspect of a learning disability but are general to many aspects of the disorder. Thirdly, genes affecting one learning disability also affect others.

Professor Plomin says: ‘Although simple genetic anomalies can lead to specific syndromes, most common problems such as language and reading problems are caused by a range of genetic and environmental risk factors. Many of these causal factors overlap in their effects on different disorders.’

What does this mean? Many learning disabilities may simply be the result of inheriting too many intelligence lowering variations of different genes that contribute to determining mental abilities.

What we consider to be mental abnormalities in many cases may not be special pathologies.

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

"The abnormal is normal - what we call abnormal is merely the low end of the same genetic and environmental factors responsible for normal variation."

Abnormal is just an extreme end of the normal range.

"We found the same genes responsible for ability and disability," he said. "In one sense abnormal is normal. There are no disabilities, just distributions of genes."

This result supports the argument that the general measure of intelligence known as 'g' has a biological foundation. Many of the genes that affect intellectual ability affect ability throughout the brain.

"There is a general set of genes that operates in the brain to affect all learning processes," he said.

Share |      Randall Parker, 2003 September 17 10:32 PM  Brain Genetics

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