Genetic and environmental etiologies of reading difficulties: DeFries–Fulker analysis of reading performance data from twin pairs and their non-twin siblings

Reading performance data from 254 pairs of identical (MZ) and 420 pairs of fraternal (DZ) twins, 8.0 to 20.0 years of age, were subjected to multiple regression analyses. An extension of the DeFries–Fulker (DF) analysis (DeFries & Fulker, 1985, 1988) that facilitated inclusion of data from 303 of their nontwin siblings was employed. In addition to providing estimates of heritability, this analysis yields a test of the difference between shared environmental influences for twins versus siblings (Astrom et al., 2011). Results suggest that proband reading deficits are due substantially to genetic factors (.67 ± .07, p < .001), and that shared environmental influences are significantly higher for members of twin pairs than for those of twins and their nontwin siblings (viz., .25 versus .17, p = .02).

► We employ a novel extension of DeFries–Fulker analysis to include data from twin pairs and their non-twin siblings.
► Our analyses included data from 674 twin pairs and 303 non-twin siblings.
► Results confirm substantial genetic influences on reading deficits.
► Significant effects of “special twin environment” are also suggested.
► Possible implications for power are discussed.