260 dominance and epistasis. Common environmental variance (C) is that resulting from environmental inﬂuences shared within twin pairs, whereas unique environmental variance (E) results from non-shared environmental inﬂuences and includes measurement error. Estimates of A, C, D and E can be derived from the resemblance between monozygotic (MZ) twins who are genetically identical and dizygotic (DZ) twins who share, on average, 50% of their segregat- ing genes. Accordingly, if MZ twins resemble each other more than DZ twins on a particular trait, this is an indicator that it is partly inﬂuenced by genetic effects. Further, if the DZ within-pair correlation is higher than half the MZ within-pair correlation, this implies that common envi- ronmental effects contribute to individual differences in the homophobia scores. By means of SEM, different combi- nations of A, C, D, and E can be evaluated to obtain the most parsimonious explanation for the observed pattern of MZ and DZ twin pair correlations. Note, however, that C and D cannot be estimated simultaneously (Martin et al. 1978) and that the choice for an ACE or ADE model depends on the pattern of MZ and DZ correlations. Results Descriptive statistics Of the 4,904 respondents to the questionnaire, 136 were omitted as they had more than two missing values on the 10 homophobia items and an additional 80 single twins were deleted due to ambiguous zygosity. Hence the total number of individuals utilised for the analyses is 4,688, comprising 2,846 twin families (1,822 pairs and 1,044 singletons). Complete twin pairs included 295 MZ males, 182 DZ males, 634 MZ females, 354 DZ females and 357 DZ Behav Genet (2008) 38:257–265 opposite sex pairs. The age range of the study sample was 19–52 years for both sexes, with a mean age for males of 30.6 (S.D. = 8.3) and for females of 31.1 (S.D. = 8.5). Descriptive statistics for each homophobia statement for all individuals are presented in Table 1. Using the imputed data set, item scores were added together and subsequently recoded into a four-point ordinal scale ranging from 0 (not homophobic at all) to 3 (highly homophobic). Table 2 shows the number and percentage of individuals within each homophobia category. To determine whether sexual orientation inﬂuences homophobic attitudes, individuals were asked which sexual orientation they considered themselves. Of the 1,758 male participants, 1,627 (92.5%) rated themselves as hetero- sexual, while 111 (6.3%) considered themselves to be non- heterosexual [20 responses (1.1%) were missing]. Of the 2,930 females, 2,802 (96.6%) rated themselves to be het- erosexual, while 100 (3.4%) considered themselves to be non-heterosexual [28 responses (1.0%) were missing]. In Table 2 the number and percentage of heterosexuals and non-heterosexuals within each homophobia category is shown. Hypotheses concerning differences in threshold parsimony Within sexes, thresholds for MZ and DZ twins could be equated without signiﬁcant deterioration in model ﬁt (v122 = 8.60, P = 0.74), indicating no evidence for social interaction between twins. However, results show a sig- niﬁcant difference in the distribution of attitudes toward homosexuality between sexes, such that males were more homophobic than females (v32 = 80.41, P \ 0.001). Furthermore, a signiﬁcant difference in distribution of thresholds was found between heterosexuals and Table 1 Percentage of agreement (yes), disagreement (no) and missing answers on the 10 homophobia statements for males and females separately Males (N = 1,758) Females (N = 2,930) Homosexuality is not immoral Allow as school teachers Allow as court judges Allow as ministers Allow as medical doctors Allow as government ofﬁcials Dangerous as teachersa Homosexuality is obscenea Homosexuality is social corruptiona Allow to dance with each other in public 123 a Note that agreement on these statements indicate a homophobic attitude Yes 50.9 53.0 65.2 56.9 58.0 70.8 34.4 48.0 29.2 49.1 No 48.2 46.9 34.7 43.0 41.9 29.1 63.4 51.3 69.3 49.2 Missing 0.9 0.1 0.1 0.1 0.1 0.1 2.2 0.7 1.5 1.7 Yes 64.0 69.4 78.4 67.4 70.1 81.8 20.3 30.3 19.9 62.2 No 35.1 30.5 21.4 32.2 29.8 18.1 77.8 68.6 77.7 35.2 Missing 1.0 0.0 0.2 0.4 0.1 0.1 1.9 1.1 2.4 2.6