Univariate and bivariate twin correlations for the CD and AAB symptom counts and the results of tests of differences between correlations are presented in Table 2. For both CD and AAB, the MZ twin correlations were significantly larger than the DZ twin correlations (CD: Δχ2 (1) = 13.46, p < .001; AAB: Δχ2 (1) = 14.12, p < .001), indicating the influence of genetic factors. The male twin correlations for CD and AAB were larger than the associated female twin correlations (CD: Δχ2 (1) = 44.32, p < .001; AB: Δχ2 (1) = 15.92, p < .001), suggesting the possibility of sex differences in the magnitude of genetic and environmental influences on both CD and AAB (quantitative sex differences). There was also evidence of a qualitative sex difference for CD, as the opposite-sex twin correlation for CD was statistically significantly smaller than the same-sex DZ twin correlations (Δχ2 (1) = 8.49, p < .01). Though the opposite-sex twin correlation for AAB was smaller than the same-sex DZ twin correlations, this difference was not statistically significant (Δχ2 (1) = 2.27, p = .13).
