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 (0.67 [plus or minus] 0.07, p less than 0.001), and that shared environmental influences are significantly higher for members of twin pairs than for those of twins and their nontwin siblings (viz., 0.25 versus 0.17, p = 0.02). (Contains 2 tables and 2 figures.)