258 homophobic individuals in general seem to be higher in authoritarianism, are more religious, hold traditional atti- tudes toward family and gender roles, have relatively low levels of education and have had fewer interactions with homosexuals (Herek 1988, 2002; Herek and Gonzalez-Ri- vera 2006). As Kite and Whitley (1996) stated, these data offer some insight into the proﬁle of homophobic individu- als, but the existing research is only descriptive in nature and hence provides little theoretical insight into prejudices regarding sexual orientation. There is considerable support for the existence of gen- eralized prejudice (see Ekehammar et al. 2004). Different types of prejudice (e.g. racism, sexism and prejudice toward homosexuals) have been shown to be highly cor- related (e.g. Bierly 1985; Ekehammar and Akrami 2003), implying that people who reject one out-group will also tend to reject others. As Ekehammar et al. (2004) describe in their article, two major theories have risen to explain why some individuals are more prejudiced than others. In the ﬁrst theory, individual differences in prejudice are considered to be due to stable factors within the individual (their personality characteristics); in this case prejudice is seen as an expression of personality (Ekehammar and Akrami 2003; Heaven and St. Quintin 2003). The second theory implies that individual differences in prejudice are caused by factors linked to the outside world, like inter- group relationships and social life (see e.g. Guimond et al. 2003; Guimond 2000; Reynolds et al. 2001). Earlier twin studies have demonstrated that individual differences in personality are substantially heritable (e.g. Jang et al. 1996; Loehlin et al. 1998), and therefore, it is possible that there are genetic inﬂuences on homophobia as well. On the other hand, a study concerning personality and social attitudes (including general conservatism and attitude to sex and politics) in a large sample revealed that, besides genetic effects, family (shared) environment also has a considerable contribution to social attitudes (Eaves et al. 1999). Conse- quently, shared environmental inﬂuences on attitudes toward homosexuality can also be expected. Such results would ﬁt the second theory that emphasizes social processes. Accordingly, the aim of the present study is to determine the contribution of both genes and shared environment to individual differences in attitudes to homosexuality, using data from a large twin sample. Findings will result in further insight into the aetiology of variation in homophobia. Methodology Participants 123 All participants were drawn from the Australian National Health and Medical Research Council Twin Registry Behav Genet (2008) 38:257–265 (ATR). Subjects for this study were recruited in two phases from a large twin-family study of alcohol use and abuse. In phase one, all twin pairs (N = 4,269, aged between 18 and 25 years) participating in an extensive Health and Lifestyle Questionnaire (HLQ) were asked whether they were willing to participate in an anonymous study about sexual behaviour and attitudes. All those who agreed were mailed the sex questionnaire between July 1991 and October 1992. To ensure anonymity, twins were not asked about their name or date of birth, but to make up a 10 digit number. Both twins of a pair had to use the same number, which made it possible to connect the responses of both twins. Additionally, informed consent was obtained sepa- rate from the questionnaires. In the second phase, an additional group of twin pairs in an older cohort (aged between 27 and 52 years old) was asked to participate in the sexual behaviour and attitudes study. Procedures for this data collection were the same as for the younger cohort, as described above. Twins who expressed willingness to participate were mailed the questionnaire between April and August 1992. For a more extensive description of the data collection procedure, the study sample, and the pairing of the returned questionnaires (see Kirk et al. 2000). In total, 1,907 complete pairs and 1,090 twins from incomplete pairs completed the questionnaire (1,824 male and 3,080 female individuals). However, due to missing data and ambiguous zygosities the effective study sample used for the homophobia analyses is lower than 4,904, as described in the result section. Zygosity The zygosity of the same-sex twins was determined during completion of the HLQ, based on their response to standard items about physical similarity and being mistaken for each other. Ambiguous replies were clariﬁed by telephone call. According to Ooki et al. (1990) and Martin and Martin (1975), concurrence on zygosity between discriminant analyses of questionnaire scores and DNA typing is at least 95% and telephone clariﬁcation will have increased accu- racy. In the present study the zygosity of the twin pairs was premarked on their questionnaire and the twins were asked whether they did or did not agreed with the assessment. Measures Data used in this study were 10 items from a larger part of the sexual behaviour and attitudes questionnaire called ‘Your Present Personality’. This part of the questionnaire contained 47 items, selected from three former instruments, namely: Inventory of Attitudes to Sex (30 items; Eysenck 1976), Attitudes to Homosexuality (10 items; Klassen et al.