Differential genetic etiology of reading disability as a function of IQ

To test the hypothesis that the genetic etiology of reading disability diff ers as a function of Ia composite reading performance data from 223 pairs o f 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 s ubjected to multiple regression analysis (DeFries & Fulker, 1985, 1988). In the total sample, heritability of the group deficit in reading performance (h(g)(2)) 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 h (g)(2) were .43 and .72, respectively, a significant difference (p less tha n or equal to .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 less than or equal to .007, one-tailed). These results suggest that I Q is relevant for the diagnosis of reading disability and that environmenta l influences may be more salient as a cause of reading difficulties in chil dren with lower IQ scores.