S. J. Wadsworth1, R. K. Olson1, B. F. Pennington1,2, & J. C. DeFries1
In order 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-sex fraternal twins in which at least one member of each pair was reading-disabled were subjected to multiple regression analysis (J. C. DeFries and D. W. Fulker, 1985, Behav. Genet.15, 467-473; J. C. DeFries and D. W. Fulker, 1988, Acta Genet. Med. Gemellol, 37, 205-216). Heritability of the group deficit in reading performance (h2g) across the full range of IQ scores was .58. When the basic regression model was fitted separately to data from twin pairs with full-scale IQ scores below 100 or 100 and above, resulting estimates of h2g were .43 and .72, respectively, a significant difference (p < .03, one-tailed). Results of fitting extended regression models to reading performance and IQ data from the entire sample provided 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 low IQ.
Address: Sally J. Wadsworth, Institute for Behavioral Genetics, Campus Box 447, University of Colorado, Boulder, CO 80309, Telephone: 303-492-6795, FAX: 303-492-8063, e-mail: wadswort@ibg.colorado.edu
1Institute for Behavioral Genetics, University of Colorado, Boulder, Colorado 80309 2University of Denver, Denver Colorado, 80208 3Supported by NICHD grants HD-11681 and HD-27802