MZ twin correlations for the estimated Neuroticism and Extraversion scores ranged between 0.39 and 0.54 (Table 1). DZ correlations were typically smaller than half the MZ correlations, suggesting non-additive genetic effects on variation in Neuroticism and Extraversion. Significant sex differences in same-sex twin correlations (p value <0.01) were found in the MCTFR, Finnish Twin and STR cohorts, but not in the NTR and two QIMR cohorts. The NTR and QIMR cohorts included opposite-sex twins. Table 1 shows that in the NTR and in the QIMR adolescent cohorts, the opposite-sex twin correlations are not significantly different from the same-sex DZ twin correlations, nor are the male same-sex DZ twin correlations different from the female same-sex DZ twin correlations. Only in the QIMR-adult cohort, there is some evidence of a larger same-sex DZ correlation for Neuroticism in females compared to males.
