Standard univariate twin models were extended to test for quantitative and qualitative sex differences in the genetic and environmental influences on CD and AAB. Quantitative sex differences refer to sex differences in the magnitude of genetic, shared environmental and non-shared environmental influences (Figure 1: am ≠ af, cm ≠ cf, em ≠ ef), whereas qualitative sex differences refer to sex differences in the actual genetic or shared environmental factors influencing the phenotype. Qualitative genetic sex differences are apparent when the genetic correlation (Figure 1: rA) among opposite-sex twin pairs is estimated to be less than .50 – the expected correlation for DZ twin pairs given their genetic relatedness. Thus, a genetic correlation of less than .50 among opposite-sex twin pairs suggests that different genetic factors are operating for males and females. Qualitative shared environmental sex differences are apparent when the shared environmental correlation (Figure 1: rC) among opposite-sex twin pairs is estimated to be less than 1 – the expected correlation for MZ and same-sex DZ twins. Thus, a shared environmental correlation among opposite-sex twin pairs of less than 1 suggests that opposite-sex twins have fewer shared environmental experiences than same-sex twins.
