262 same sets of genes inﬂuencing homophobia scores for males and females. Third, the ﬁt of a general ACE model, in which pro- portions of A and C are constrained equal in males and females, is compared with the ﬁt of the common effects sex-limitation model. Results show no signiﬁcant deterio- ration of model ﬁt (v22 = 4.52, P = 0.10), indicating there are no magnitude differences in effects of A, C and E on variance in homophobia scores between sexes. Under the general ACE model, additive genetic and common environmental effects together account for between 48% and 59% of the variance in homophobia (see Table 5). It is impossible to distinguish between AE and CE models, as removing the effects of either A or C causes signiﬁcant deterioration in model ﬁt. Accordingly, the most parsimonious explanation for the observed pattern of MZ and DZ twin pair correlations is the general ACE model, where there are no qualitative and quantitative differences in sources of variance between sexes. The twin pair correlations, however, point to pos- sible differences in the sources of variance between sexes. The inability to detect these qualitative and quantitative differences between sexes might be due to insufﬁcient power from using a threshold model, necessitated by the extreme skewness of the raw data (Neale et al. 1994). Based on the general ACE model, estimates of the inﬂuence of A, C and E on the variance in homophobia scores for both sexes are 36%, 18% and 46% (see Table 5). In Fig. 1, the graphical illustration of the general ACE model (a) and the general sex-limitation model (b), including the path coefﬁcients, are presented. Discussion Analyses of the homophobia scale showed that, in accor- dance with former literature (e.g. Kite and Whitley 1996), males have more negative attitudes toward homosexuality than females and heterosexuals are more homophobic than non-heterosexuals (Jellison et al. 2004). In this study, age had no effect on attitudes toward homosexuality, whereas Behav Genet (2008) 38:257–265 some others have found that younger individuals have more favourable attitudes regarding homosexuality than older persons (Herek 2002; Steffens and Wagner 2004). It should be noted however that the distribution of age in this study was highly skewed, with the majority of participants (70.9%) in age group 19–35 and only 29.1% of all par- ticipants aged between 35 and 52 years. The aim of the present study was to determine the contribution of genes and environment to individual dif- ferences in attitudes toward homosexuality. Based on the general ACE model, estimates of the inﬂuence of A, C and E on the variance in homophobia scores are: 36%, 18% and Fig. 1 Graphical presentation of the general ACE model (a) and the general sex-limitation model (b) along with the standardised path coefﬁcients. In the general sex-limitation model path coefﬁcients for males are presented on the left and for females on the right side. Note that in the general sex-limitation model the correlation between opposite sex twins for additive genetic effects DZ^OS is estimated in the model Table 5 Proportions of variance (95% conﬁdence intervals) attributable to A, C and E effects for both sexes for the general sex-limitation model, the common effects sex-limitation model and the general ACE model Amales Cmales Emales Afemales Cfemales Efemales HDZOS General sex- 0.38 (0.08–0.68) 0.23 (0.00–0.49) 0.39 (0.31–0.44) 0.23 (0.00–0.49) 0.28 (0.04–0.49) 0.49 (0.43–0.57) 0.09 limitation model (0.00–1.00) Common effects sex- 0.55 (0.16–0.69) 0.07 (0.00–0.47) 0.38 (0.30–0.47) 0.20 (0.00–0.54) 0.30 (0.00–0.51) 0.50 (0.43–0.58) 0.50 limitation model General ACE model 0.36 (0.19–0.54) 0.18 (0.02–0.32) 0.46 (0.41–0.52) 0.36 (0.19–0.54) 0.18 (0.02–0.32) 0.46 (0.41–0.52) 0.50 123