—Full sex-limitation model (a) and nested models (b and c) Note.—A, additive genetic influence; C, shared environment influence; E, nonshared environment influence; Paths a, c, and e, effects of A, C, and E on a trait with subscript “m” for males and “f” for females; rg, genetic relatedness between same-sex twins which is fixed at 1.0 for MZ twins and .5 for DZ twins; rc, shared environment relatedness between same-sex twins which is fixed at 1.0 for MZ and DZ twins; rgO, genetic relatedness between opposite-sex twins; rcO, shared environment relatedness between opposite-sex twins. Opposite-sex twins are represented as twin one male and twin two female, and are linked by rgO and rcO. (a) shows the full sex limitation model that estimates seven parameters: am, cm, em, af, cf, ef, and rgO or rcO. This model allows qualitative sex differences in that the genetic and shared environmental correlations (rgO and rcO) between opposite-sex twins are allowed to be <.5 and 1.0, respectively. The model also allows quantitative sex differences in that the ACE parameters for males and females (am, cm, em, af, cf, ef) can differ. Variances differences between the sexes are also allowed (not shown in the path diagram). (b) shows the common effects model, which is nested in the full sex limitation model and tests for qualitative sex differences by constraining the genetic and shared environmental correlations between opposite-sex twins (rGO and rCO) to be .5 and 1.0, respectively (i.e., the same as same-sex DZ twins). This model estimates six parameters: am, cm, em, af, cf, ef, and allows variance differences between the sexes. (c) shows the scalar and null models of the sex-limitation design which reduces to the basic twin model and thus, in comparison with the other models, tests for both qualitative and quantitative sex differences. This model estimates three parameters: a, c, and e for the sexes combined. In the scalar model, variance differences between the sexes are allowed. In the null model the variances are equated across the sexes (not shown in the path diagram).