Genetic etiology of reading difficulties in boys and girls.

Although it has been suggested that genetic influences on reading difficulties may differ in boys and girls, results obtained from previous analyses of data from same-sex twin pairs have failed to provide evidence for a differential genetic etiology of reading disability (RD) as a function of gender. However, results of a recent study in which data from both same-sex and opposite-sex twin pairs were analyzed indicated a higher heritability for reading difficulties in boys (Harlaar et al., 2005). Because the current sample of twin pairs tested in the Colorado Learning Disabilities Research Center is substantially larger than that analyzed for our previous report (Wadsworth et al., 2000), this hypothesis was tested more rigorously using data from both same-sex and opposite-sex twin pairs in our current augmented sample. Composite reading scores from 634 twin pairs were subjected to DeFries-Fulker sex-limitation analysis using the model-fitting approach of Purcell and Sham (2003). Analysis of data from the combined sample of male and female twins indicated that genetic influences account for more than half the proband reading deficit (h2g = .58). When this model was extended to test for gender differences in the magnitude of genetic influences on RD, h2g estimates were somewhat higher for females than for males (.63 and .53, respectively), but the difference was nonsignificant (p > .3). A test for qualitative gender differences was also nonsignificant. Thus, these results provide little evidence for a differential genetic etiology of RD in boys and girls.