Behav Genet (2008) 38:257–265 DOI 10.1007/s10519-008-9200-9 O R I G I N A L R E S E A R C H Genetic and Environmental Inﬂuences on Individual Differences in Attitudes Toward Homosexuality: An Australian Twin Study Karin J. H. Verweij Æ Sri N. Shekar Æ Brendan P. Zietsch Æ Lindon J. Eaves Æ J. Michael Bailey Æ Dorret I. Boomsma Æ Nicholas G. Martin Received: 21 June 2007 / Accepted: 28 February 2008 / Published online: 18 March 2008 The Author(s) 2008 Abstract Previous research has shown that many hetero- sexuals hold negative attitudes toward homosexuals and homosexuality (homophobia). Although a great deal of research has focused on the proﬁle of homophobic individ- uals, this research provides little theoretical insight into the aetiology of homophobia. To examine genetic and environ- mental inﬂuences on variation in attitudes toward homophobia, we analysed data from 4,688 twins who com- pleted a questionnaire concerning sexual behaviour and attitudes, including attitudes toward homosexuality. Results show that, in accordance with literature, males have signif- icantly more negative attitudes toward homosexuality than females and non-heterosexuals are less homophobic than heterosexuals. In contrast with some earlier ﬁndings, age had no signiﬁcant effect on the homophobia scores in this study. Genetic modelling showed that variation in homophobia Edited by Hermine Maes. K. J. H. Verweij (&) S. N. Shekar B. P. Zietsch N. G. Martin Genetic Epidemiology Laboratory, Queensland Institute of Medical Research, 300 Herston Road, Brisbane 4029, QLD, Australia e-mail: karinverweij@hotmail.com K. J. H. Verweij D. I. Boomsma Department of Biological Psychology, VU University, Amsterdam, The Netherlands L. J. Eaves Virginia Institute for Psychiatric and Behavioral Genetics, Virginia Commonwealth University School of Medicine, Richmond, VA, USA J. M. Bailey Department of Psychology, Northwestern University, Evanston, IL, USA scores could be explained by additive genetic (36%), shared environmental (18%) and unique environmental factors (46%). However, corrections based on previous ﬁndings show that the shared environmental estimate may be almost entirely accounted for as extra additive genetic variance arising from assortative mating for homophobic attitudes. The results suggest that variation in attitudes toward homo- sexuality is substantially inherited, and that social environmental inﬂuences are relatively minor. Keywords Attitudes Genetics Heritability Homophobia Twin study Homosexuality Introduction Homophobia can be deﬁned as ‘the fear of, aversion to, or discrimination against homosexuality or homosexuals’ (Merriam–Webster’s Medical Dictionary 2007). Although the social and cultural status of homosexuality has improved, Kite and Whitley (1996) concluded from their meta-analyses of 112 studies that many heterosexuals still hold very neg- ative attitudes toward homosexuals and their sexual behaviour. A great deal of research has focused on the per- sonality and demographic features of homophobic individuals. Numerous studies indicated that men tend to hold more negative attitudes toward homosexuals than women, especially toward gay men (e.g. Davies 2004; Herek 1988; Herek and Gonzalez-Rivera 2006; Kite and Whitley 1996; LaMar and Kite 1998; Lingiardi et al. 2005; Steffens and Wagner 2004). The most common explanation for this sex difference in homophobic attitudes is that men tend to hold more traditional gender beliefs, which encourage them to be more negative toward homosexuality, particularly toward gay men (Kite and Whitley 1996). Furthermore, 123