Excerpts from a NewScientist article: Bisexuality passed on by 'hyper-heterosexuals' by Tamsin Osborne
Bisexual men might have their "hyper-heterosexual" female relatives to thank for their orientation.
Previous work has suggested that genes influencing sexual orientation in men also make women more likely to reproduce. Andrea Camperio Ciani and colleagues at the University of Padua, Italy, showed that the female relatives of homosexual men tend to have more children, suggesting that genes on the X chromosome are responsible. Now the team have shown that the same is true for bisexuality.
"It helps to answer a perplexing question - how can there be 'gay genes' given that gay sex doesn't lead to procreation?" says Dean Hamer of the National Institutes of Health in Bethesda, Maryland, who was not involved in the work. "The answer is remarkably simple: the same gene that causes men to like men also causes women to like men, and as a result to have more children."
The researchers asked 239 men to fill out questionnaires about their families and their past sexual experiences. On the basis of their answers, the men were classified as heterosexual, bisexual or homosexual. The results showed that the maternal aunts, grandmothers and mothers of both bisexual men and homosexuals had more children than those of heterosexual men.
Camperio Ciani emphasises that, rather than being a "gay gene", this unidentified genetic factor is likely to promote sexual attraction to men in both men and women. This would influence a woman's attitude rather than actually increasing her fertility, making her likely to have more children.
Simon LeVay, a neuroscientist and writer based in West Hollywood, California, describes this as a sort of "hyper-heterosexuality" and explains how it would help to ensure that homosexual behaviour was passed on through the generations. "The positive effect of an X-linked gene on female fecundity tends to outweigh the negative effect of the gene on male fecundity."
According to Camperio Ciani and colleagues, the same genetic factor appearing to be present in both bisexual and homosexual men provides further support for the idea that sexuality is determined by a complex mix of genes and experience.
"We understand that the genetic component has to interact with something to produce different phenotypes," says Camperio Ciani.