To test the hypothesis that the genetic etiology of reading disability differs as a function of IQ, composite reading performance data from 223 pairs of identical twins and 169 pairs of same-gender fraternal twins in which at least one member of each pair was classified with reading disability were subjected to multiple regression analysis (DeFries & Fulker, 1985, 1988). In the total sample, heritability of the group deficit in reading performance (h2g) was .58 (± .08). However, when the basic regression model was fitted separately to data from twin pairs with average Wechsler (1974, 1981) full scale IQ scores below 100 or 100 and above, resulting estimates of h2 g were .43 and .72, respectively, a significant difference (p ≤ .03, one-tailed). The results of fitting extended regression models to reading performance and continuous IQ data provide evidence that the genetic etiology of reading disability differs as a linear function of IQ (p ≤ .007, one-tailed). These results suggest that IQ is relevant for the diagnosis of reading disability and that environmental influences may be more salient as a cause of reading difficulties in children with lower IQ scores.
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