Science Daily (June 30, 2008) – Homosexual behaviour is largely shaped by genetics and random environmental factors, according to findings from the world’s largest study of twins.

Dr Qazi Rahman, study co-author and a leading scientist on human sexual orientation, explains: “This study puts cold water on any concerns that we are looking for a single ‘gay gene’ or a single environmental variable which could be used to ‘select out’ homosexuality – the factors which influence sexual orientation are complex. And we are not simply talking about homosexuality here – heterosexual behaviour is also influenced by a mixture of genetic and environmental factors.

This study looked at 3,826 same-gender twin pairs (7,652 individuals), who were asked about the total numbers of opposite sex and same sex partners they had ever had. The findings showed that 35 per cent of the differences between men in same-sex behaviour (that is, that some men have no same sex partners, and some have one or more) is accounted for by genetics.