Behav Genet (2008) 38:257–265 Homophobia category Males, heterosexual (N = 1,627) Males, non-heterosexual (N = 111) Males (N = 1758)a Females, heterosexual (N = 2802) Females, non-heterosexual (N = 100) Females (N = 2930)a Table 2 Number and percentage of males and females, heterosexuals and non-heterosexuals, for each category of the ordinal homophobia scale (0 is least, 3 is most homophobic) 0 368 (22.6%) 77 (69.4%) 450 (25.6%) 1,011 (36.1%) 78 (78.0%) 1,098 (37.5%) 1 314 (19.3%) 18 (16.2%) 338 (19.2%) 692 (24.7%) 9 (9.0%) 705 (24.1%) 2 392 (24.1%) 13 (11.7%) 408 (23.2%) 580 (20.7%) 10 (10.0%) 600 (20.5%) a Note that for 20 males and 28 females self-reported sexual orientation is missing non-heterosexuals (v12 = 192.51, P \ 0.001). On average, heterosexuals were more homophobic than non- heterosexuals. Finally, it was tested whether age had a signiﬁcant inﬂuence on the distribution of the thresholds. Removing the effect of age from the model did not lead to a signiﬁ- cantly worse model ﬁt (v12 = 1.83, P = 0.18), implying that age has no appreciable inﬂuence on individual differ- ences in attitudes toward homosexuality. Heterogeneity of twin pair correlations Polychoric twin pair correlations for each zygosity group, shown in Table 3, were estimated in Mx by maximum- likelihood. Although the twin pair correlation for MZ males is higher than for MZ females, they were not sig- niﬁcantly different (v12 = 3.18, P = 0.08). Furthermore, as the twin pair correlation for DZ oppo- site sex twin pairs is lower than for the DZ same sex twin pairs, qualitative differences in sources of familial aggre- gation between sexes could be expected. However, the difference in twin pair correlation between the opposite and the same sex DZ twin pairs was not signiﬁcant (v22 = 2.46, P = 0.29). The inability to ﬁnd signiﬁcant differences between the MZ twin pair correlations for males and females, and between the DZ same sex and opposite sex twin pair correlations may, however, be due to a lack of power to detect qualitative and quantitative differences between the groups. The MZ twin pair correlations are signiﬁcant higher than the DZ twin pair correlations (v12 = 15.64, P \ 0.001), suggesting that genetic effects are a source of familial aggregation in attitudes toward homosexuality. Genetic modelling Results of the univariate SEM for the homophobia scores are shown in Table 4. As the DZ twin pair correlations are more than half the MZ twin pair correlations, a model is ﬁtted incorporating additive genetic (A), shared environ- mental (C) and unique environmental (E) effects. Since the DZ opposite sex correlation is lower (although not signif- icantly) than the DZ same sex correlations, a general sex- limitation model is ﬁtted, which allows for qualitative and quantitative differences in the sources of variation in atti- tudes toward homosexuality between sexes (Neale and Cardon 1992). To model the qualitative differences in genetic effects between sexes, the genetic correlation for DZ opposite sex twins (H) is estimated in the model instead of ﬁxed at 0.5 as it is for same sex DZ twin pairs. Subsequently, a common effects sex-limitation model is ﬁtted, by ﬁxing the genetic correlation for DZ opposite sex twins at 0.5. Results show no signiﬁcant deterioration of model ﬁtting (v12 = 1.43, P = 0.23), consistent with the 261 3 553 (34.0%) 3 (2.7%) 562 (32.0%) 519 (18.5%) 3 (3%) 527 (18.0%) HDZOS 0.09 0.50 0.50 123 Table 3 Polychoric twin pair correlations (95% conﬁdence intervals) for the homophobia score for each zygosity group, estimated in Mx MZM 295 pairs 0.61 (0.52–0.69) DZM 182 pairs 0.42 (0.37–0.55) MZF 634 pairs 0.51 (0.43–0.57) DZF 354 pairs 0.39 (0.28–0.50) DZOS 357 pairs 0.29 (0.17–0.41) Correlation 1 2 3 Table 4 Genetic model ﬁtting: Comparison of different genetic models Model General sex-limitation model Common effects sex-limitation model General ACE model -2ll 12,199.93 12,201.36 12,205.88 d.f. 4672 4673 4675 Vs 1 2 Dv2 1.43 4.52 Dd.f. 1 2 P-value 0.23 0.10