Genetic abnormalities play a major role in reading difficulties, a study of twins has found.
Richard Olson, a leading researcher on reading, said: "The further you get into reading instruction, the more genetic factors count, and the less environmental factors count, including what families did in pre-school."
Mr Olson, associate director of the Colorado Learning Disabilities Research Center, told the Canadian Language and Literacy Research Network conference in Toronto this summer that he was surprised by the findings of his on-going study of 172 identical twins and 153 fraternal twins between eight and 18 years old.
The study, which controlled for IQ and income levels, tested the students before they started school and found that environmental factors, such as whether or not parents read to their children, strongly correlated to the presence of problems with pre-reading skills.
But tests on these students after kindergarten found environmental factors had declined and genetics were the predominant factor.
The correlation between genetics and reading difficulties was even stronger after grade 1 (age five to six). Between 60 and 75 per cent of the reading difficulties could be linked to genetic factors shared by the twins.
Researchers do not believe that there is one single gene that is the key to genetically caused dyslexia.
But biology is not destiny, said Mr Olson. Structured programmes that tell students immediately whether they have understood a word or sentence correctly can raise the reading levels of those with genetic reading problems.