Behav Genet (2008) 38:257–265 46%. However, although no signiﬁcant qualitative and quantitative differences in sources of variation between males and females were found, twin pair correlations do suggest possible differences between the sexes. Under a general sex-limitation model, estimates of the inﬂuence of A and C on variation in homophobia scores are 38% and 23% for males and 23% and 28% for females, respectively. The additive genetic correlation for the DZ opposite sex twins is estimated at 0.09, considerably lower than the 0.5 assumed for DZ same sex twins. This ﬁnding would sug- gest that genes inﬂuencing homophobia in males are to a large extent different from those in females. However, since both A and C effects are present in both sexes, the parameterisation of sex limitation as genetic in origin is arbitrary. If it were parameterised as a lower common environmental correlation between males and females, the same model ﬁt would be obtained. Moreover, there could be sex limitation of both A and C inﬂuences, but since there is only one degree of freedom to estimate them— obtained from the DZ opposite sex twin pair correlation— the two are completely confounded. In either case, differ- ences in saliency of homosexuality between males and females make it unsurprising that there are distinct causes of variation in homophobia in the two sexes. A limitation to our conﬁdence in the variance compo- nents estimated above is that strong assortative mating has been repeatedly found for social attitudes (e.g. Eaves et al. 1999; Hatemi et al. 2007; Martin et al. 1986). This means that the relative genetic variance may have been underes- timated. If spouses are correlated for homophobia through the assortment process, then both MZ and DZ twin corre- lations will be raised to the same degree. Mimicking the effect of shared environment this extra resemblance would yield higher estimates of ‘C’. As our sample does not include spouses, we rely on previous estimates of assorta- tive mating to determine its effect on our estimates. Recently, Hatemi et al. (2007) investigated spouse corre- lations on social attitudes, including an item on gay rights, for which they found a spouse correlation of 0.58. Assuming that this value is a reasonable estimate for assortative mating in our sample, it is possible to correct our C estimate for assortative mating by using a procedure described by Martin (1978).1 After correction, the contribution of shared environment on individual differences in homophobia scores falls from 0.18 to 0.03, while the genetic estimate increases to about 0.51. This ﬁnding suggests that variation in attitudes 1 Martin (1978) used the following formula to correct C for assortative mating: ca2dj ¼ c2R h2RA=ð1 AÞ, where h2R and c2R are the relative genetic and common environmental effects as estimated by the model, and A is the correlation between additive genetic values of mates, and is a function of the observed value for assortative mating (l) and h2R; A ¼ 0:5 1 p 1 4lh2R . 263 toward homosexuality is substantially inherited, while social environmental inﬂuences are relatively minor. When applying the assortative mating correction to the results of the general sex-limitation model, the contribution of shared environment on individual differences in homo- phobia scores for males falls from 0.23 to 0.04 and for females from 0.28 to 0.24, while the genetic estimate increases to about 0.57 for males and 0.27 for females. As such, based on the general sex-limitation model familial aggregation for homophobia scores for males might be almost totally accounted for by genetic effects, while for females both genes and shared environment seem to have about equal effects. If this would be the case, and there is little or no C inﬂuence on homophobia scores for males, sex limitation for homophobia scores can only have a genetic origin. Another important consideration is that the genetic contribution to homophobia might partly reﬂect genetic inﬂuences on related traits such as intelligence, personality, or social class. In our data, the polychoric correlation of the homophobia measure with educational level was -0.27 (P \ 0.01) and with social class was -0.14 (P \ 0.01), so these traits would only account for a modest amount of variance. While individuals’ attitudes toward homosexuality are partly inﬂuenced by genes, shared environmental factors also contribute a modest proportion of variation in homo- phobic attitudes. This ﬁnding suggests that shared environmental inﬂuences, such as norms and values taught by parents, are aspects in forming positive or negative attitudes toward homosexuals, and possibly also toward other out-groups. The main source of variance is unique environment (46% under the general ACE model). While an unknown fraction of this will be due to measurement error, the number suggest a great inﬂuence on attitudes to homosexuality is the unique experiences of the individual shaping his or her attitudes independent of family inﬂu- ences. This study shows that one’s own sexual orientation is one such inﬂuence, although previous research has shown that sexual orientation itself is substantially genet- ically inﬂuenced (e.g. Kendler et al. 2000; Kirk et al. 2000). Although the effect of sexual orientation on homophobia is large at the individual level, the percentage of non-heterosexuals is so low, it accounts for only a trivial proportion of population variance in attitudes to homosexuality. There are a few important methodological limitations to be considered for this study. As already mentioned, the most important limitation of this study was a lack of sta- tistical power. Although the sample size is fairly large, due to the fact that the homophobia scale had to be analysed as ordinal data, it is still insufﬁcient. Neale et al. (2004) showed that for a threshold study at least three times the 123