Deficits in reading performance may differ in etiology depending on the IQ of the individuals who have the deficits. According to an article in Behavior Genetics, Professor Sally Wadsworth and colleagues confirmed previous research showing that there is a stronger genetic element in the reading deficits of children with higher IQs (mean = 108.97 ± 6.71) than those with lower IQ (mean = 82.85 ± 6.40). The heritability for the former group is 0.75 ± 0.12, but for the latter it is 0.50 ± 0.10.
In the authors’ summary of their results they caution against using these findings to support diagnoses via discrepancy methods:
The results of the current study suggest that IQ scores may tell us something about the causes of children’s reading deficits. They do not, however, imply that IQ scores should be used for diagnosis via discrepancy scores or strict cut-offs. It has been frequently noted that for both practical and psychometric reasons, the use of IQ and/or discrepancy scores can be problematic…. Our results suggest that the reading difficulties of children with a higher IQ are due substantially to genetic influences. Thus, IQ may help identify those children whose reading deficits may require intensive remediation efforts.
Here is the citation and abstract for the study:
Wadsworth, S. J., Olson, R. K., & DeFries, J. C. (2010). Differential genetic etiology of reading difficulties as a function of IQ: An update. Behavior Genetics, advance online publication. DOI 10.1007/s10519-010-9349-x; PMID: 20333543
In order to test the hypothesis that the genetic etiology of reading disability differs as a function of IQ, composite reading performance data from 308 pairs of identical (monozygotic, MZ) twins and 440 pairs of fraternal (dizygotic, DZ) twins (254 same-sex and 186 opposite-sex) in which at least one member of each pair was classified as reading-disabled were subjected to multiple regression analysis (DeFries and Fulker, Behav Genet 15:467-473, 1985; Acta Genet Med Gemellol 37:205-216, 1988). In the total sample, heritability of the group deficit in reading performance (h (g) (2) ) was .61 (+/-.06). However, results of fitting an extended regression model to reading performance and IQ data suggested that the genetic etiology of reading disability differs as a linear function of IQ (p