Genetics and random environmental factors play a significant role in the development of homosexual behaviour, a new study has said.
Published in the scientific journal Archives of Sexual Behavior, this is the world's largest study of twins to project genetics and environmental factors - which are specific to an individual, and may include biological processes such as different hormone exposure in the womb - as important determinants of homosexual behaviour.
"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," said Dr Qazi Rahman, a leading scientist on human sexual orientation at Queen Mary's School of Biological and Chemical Sciences.
Led by Dr Niklas Langstrom at Karolinska Institutet in Stockholm, the research team conducted a population-based survey of all adult twins (20-47 years old) in Sweden.
The researchers asked 3,826 same-gender twin pairs (7,652 individuals) about the total numbers of opposite sex and same sex partners they had ever had.
They found 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 - was is accounted for by genetics.
"Overall, genetics accounted for around 35 per cent of the differences between men in homosexual behaviour and other individual-specific environmental factors (that is, not societal attitudes, family or parenting which are shared by twins) accounted for around 64 per cent. In other words, men become gay or straight because of different developmental pathways, not just one pathway," said Rahman.
As regards women, genetics explained roughly 18 per cent of the variation in same-sex behaviour, non-shared environment roughly 64 per cent and shared factors, or the family environment, explained 16 per cent.
Based on their observations, the researchers came to the conclusion that genetic influences are important but modest, and that non-shared environmental factors, which may include factors operating during foetal development, dominate.
They said that heredity had roughly the same influence as shared environmental factors in women, whereas the latter had no impact on sexual behaviour in men.
"The study is not without its limitations - we used a behavioural measure of sexual orientation which might be ok to use for men (men's psychological orientation, sexual behaviour, and sexual responses are highly related) but less so for women (who show a clearer separation between these elements of sexuality). Despite this, our study provides the most unbiased estimates presented so far of genetic and non-genetic contributions to sexual orientation," Rahman said.