allelic effects add up across loci) and those that are nonadditive (i.e., due to dominance or epistasis). In twin studies, however, it is not possible to estimate both additive and nonadditive genetic effects (d2) simultaneously with shared twin environment effects. The twin correlations summarized in Table 8.2 can be used to estimate the genetic and environmental influences to aggressive behavior. Twice the difference between the MZ and DZ correlations provides an estimate of the relative contribution of additive genetic influences to aggressive behavior [h2 = 2(rMZ –rDZ)]. The contribution of the nonadditive genetic effects due to dominance or epistasis (d2) is obtained by subtracting four times the DZ correlation from twice the MZ correlation (d2 = 2rMZ–4rDZ). The proportion of the variance that is due to shared environmental influence is given by subtracting the MZ correlation from twice the DZ correlation (c2 = 2rDZ–rMZ). Finally, the contribution of the nonshared environmental influences can be obtained by subtracting the MZ correlation from unit correlation (e2 = 1–rMZ) (Posthuma et al., 2003). Many twin studies do not specifically examine or test for nonadditive genetic effects and instead report heritability estimates based on additive effects only. However, some twin studies compare models with
