The genetic analyses we conducted are based on the basic assumptions of twin studies. Because MZ twins share all their genes, whereas DZ twins on average share half of their alleles identically by descent, MZ correlations higher than DZ correlations for a behavioral measure suggest a genetic influence. Estimates of the proportions of variance in performance due to additive genetic (A), shared environmental (C), and nonshared environmental (E) influences are obtained through structural equation ACE models (Neale & Cardon, 1992) of MZ and DZ twin covariance matrices that incorporate standard assumptions. Specifically, the correlations between the additive genetic effects (As) are fixed at 1.0 and 0.5 for MZ and DZ twins, respectively. The correlation between the shared environmental effects (Cs) is fixed to one for both MZ and DZ twins, implying equal shared environmental effects for the different twin types. Other implicit assumptions of the model are that genetic and environmental effects are uncorrelated and act additively and that random mating is operating in the parent generation.
