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.