264 sample size needed for equivalent power to a study using continuous data is required. Another important limitation of this study is that no distinction has been made between attitudes toward male versus female homosexuals. Five out of the ten questions are asked in gender neutral terms, while the other ﬁve statements are about homosexual men. The term ‘homo- sexuals’ will by most individuals be interpreted as referring to gay males and not to females (Haddock et al. 1993), so the items will predominantly measure attitudes toward gay men. As previously mentioned, heterosexuals tend to have quite different attitudes toward the two, with lesbians generally being viewed less negatively (e.g. Herek 2002; Kite and Whitley 1996). Hence, to better understand indi- vidual differences in attitudes toward homosexuality, researchers should distinguish between male and female homosexuals in their questions (Herek and Capitanio 1999; LaMar and Kite 1998; Kite and Whitley 1996). A further consideration is whether participation biases inﬂuenced the results of the present study, especially as this survey queried participants about very personal informa- tion. Dunne et al. (1997) have examined this question for the survey used in the current study by comparing the social, psychological and behavioural features of the twins who explicitly consented to join the present study (52%) with those who either explicitly refused (27%) or initially agreed, but subsequently did not return the consent forms (19%). Results indicate that those individuals who explic- itly consented had less conservative sexual attitudes and were more likely to agree with gay rights. However, the effect sizes of these ﬁndings were small, indicating the participation bias probably did not inﬂuence the results to a great extent. Another limitation to consider is the possibility of socially desirable responding. Although it is not possible to exclude social desirability effects, twins were encouraged to be honest by guaranteeing anonymity and by asking them to ﬁll out the questionnaire while alone with sufﬁcient privacy. In summary, this study concerning the aetiology of homophobic attitudes revealed that familial aggregation in attitudes toward homosexuality is accounted for by genes as well as by shared environmental factors. However, when the plausible effect of assortative mating on our estimates is taken into account, familial aggregation for homophobia scores might be almost totally accounted for by genetic effects. More research is necessary to further unravel the sources of variance in homophobia and to determine whether these differ between sexes. This future research will need greater sample sizes—preferably including spouses—and should analyse attitudes toward male and female homosexuals separately. It will also be of interest to compare prejudice toward homosexuals with prejudice 123 Behav Genet (2008) 38:257–265 toward other targets (like racism and sexism), to see to what extent similar results will be obtained and whether the same genetic and environmental sources explain individual differences in prejudice toward different out-groups. Acknowledgements This research was funded by a small grant (R03) to J.M.B. from the U.S. Institute of Mental Health (USA) and a small Commonwealth AIDS Research Grant to N.G.M. Twins par- ticipating in this study were drawn from the Australian NHMRC Twin Registry. K.J.H.V. was supported by an Honours Exchange Master of Neurosciences stipendiat of the CNCR (Amsterdam) and Erasmus Medical Centre (Rotterdam) and a fund from the Bekker-La Bastide- Fonds (Rotterdam). Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which per- mits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited. Appendix A. Attitudes to Homosexuality Questionnaire: Section C. Part 2 of ‘Your Present Personality’ In this part, answer YES if you agree with the statement, but answer NO if you disagree. Again, there are no right or wrong answers, and no trick questions. Work quickly and do not think too long about the exact meaning of questions. Yes No 1. Homosexuality is merely a different kind of sexuality, and is not immoral. 2. Homosexual men should be allowed to work in the following professions: a. Schoolteachers b. Court judges c. Ministers d. Medical doctors e. Government ofﬁcials. 3. Homosexuals are dangerous as teachers or youth leaders, because they try to get sexually involved with children. 4. Homosexuality is obscene and vulgar. 5. Homosexuality is social corruption that can cause the downfall of civilization. 6. Homosexuals should be allowed to dance with each other in public areas. References Bierly MM (1985) Prejudice toward contemporary outgroups as a generalized attitude. J Appl Soc Psychol 15:189–199 Davies M (2004) Correlates of negative attitudes toward gay men: sexism, male role norms, and male sexuality. J Sex Res 41(3):259–266