Learning disabilities result from general problems in the brain rather than specific genetic or neurological defects, the British Association Festival of Science in Salford was told on Tuesday.
A large but unidentified group of genes, each with very small effects on overall brain function, work together to determine most of mental ability, says Robert Plomin, at the Institute of Psychiatry in London.
If Plomin鈥檚 theory proves correct, common learning disabilities such as dyslexia will need a dramatic redefinition. Dyslexia is commonly defined as a reading problem in someone who has otherwise normal intelligence.
Advertisement
In fact, Plomin disputes the idea of learning disabilities at all, saying instead that these people simply fall at the lower end of the spectrum of cognitive ability.
Plomin鈥檚 theory is likely to be controversial and to be met with scepticism by the education and research communities. Christine Temple, a researcher at the University of Essex who presented her own research on specific memory problems in childhood, says: 鈥淭here are clearly children with normal mental function and very specific disabilities. This is what we see clinically.鈥�
Maths and reading
Plomin studied 15,000 sets of twins as they grew up. At seven years old, researchers compared rates of learning disabilities in pairs of identical and fraternal twins.
They found that identical twins were more likely to be afflicted with the same disorder than were fraternal twins, confirming a genetic root for learning disabilities. But researchers also checked if a child with a problem in maths was likely to have a twin with a reading disability, and vice versa. They found the link was more common in identical than fraternal twins.
In fact, genes that affect reading have a 70 per cent chance of also affecting maths ability. 鈥淕enes are generalists 鈥� the same genes affect all disabilities,鈥� concludes Plomin.
鈥淧eople look for a hole in the brain that causes a problem, but I think the brain evolved to work together to solve a problem,鈥� he told New 杏吧原创. Any specific differences in learning ability are probably due to environmental effects, he says.
At this point, the implications for children with learning problems, specific or not, are unclear. 杏吧原创s do not know which genes, or even how many genes, are involved. Some candidate genes may be those involved in general brain processes such as synaptic plasticity, Plomin says.
Furthermore, the individual effect of each gene involves is likely to be so small 鈥� accounting for less than one per cent of the variation in general cognitive ability 鈥� that studies attempting to find them will have to analyse huge numbers of people.