Researchers at Yale School of Medicine in New Haven, Conn., and the University of California, Davis, find IQ and reading in typical students not only track together, but also influence each other over time. However, in children with dyslexia, IQ and reading are not linked and do not influence one another.
This explains why a dyslexic can be bright and still unable to read well, the researchers say.
"For the first time, we've found empirical evidence that shows the relationship between IQ and reading over time differs for typical compared to dyslexic readers," Dr. Sally Shaywitz of Yale says in a statement.
The findings "support the concept that dyslexia is an unexpected difficulty with reading in children who otherwise have the intelligence to learn to read."
Shaywitz and colleagues base their study, scheduled to be published in Psychological Science, on 12 years of data from the ongoing Connecticut Longitudinal Study of cognitive and behavioral development in a representative sample of 445 Connecticut schoolchildren. Each child was tested each year in reading and every other year for IQ.