A national survey of sexual attitudes in the UK last year came up with lower figures. But most scientists researching gay evolution are interested in an ongoing, internal pattern of desire rather than whether people identify as gay or straight or how often people have gay sex.
Qazi Rahman says that alleles coding for same sex attraction only explain some of the variety in human sexuality. Other, naturally varying biological factors come into play, with about one in seven gay men, he says, owing their sexuality to the "big brother effect". This has nothing to do with George Orwell, but describes the observation that boys with older brothers are significantly more likely to become gay - with every older brother the chance of homosexuality increases by about a third.
No-one knows why this is, but one theory is that with each male pregnancy, a woman's body forms an immune reaction to proteins that have a role in the development of the male brain.
Since this only comes into play after several siblings have been born - most of whom are heterosexual and go on to have children - this pre-natal quirk hasn't been selected away by evolution. Exposure to unusual levels of hormone before birth can also affect sexuality.
For example, female foetuses exposed to higher levels of testosterone before birth show higher rates of lesbianism later on. Studies show that "butch" lesbian women and men have a smaller difference in length between their index and ring fingers - a marker of pre-natal exposure to testosterone. In "femme" lesbians the difference has been found to be less marked. Brothers of a different kind - identical twins - also pose a tricky question. While that's a greater likelihood than random, it's lower than you might expect for two people with the same genetic code.
William Rice, from the University of California Santa Barbara, says that it may be possible to explain this by looking not at our genetic code but at the way it is processed. Rice and his colleagues refer to the emerging field of epigenetics, which studies the "epimarks" that decide which parts of our DNA get switched on or off. Epimarks get passed on to children, but only sometimes. Rice believes that female foetuses employ an epimark that makes them less sensitive to testosterone.
Usually it's not inherited, but occasionally it is, leading to same-sex preference in boys. Dr William Byne, editor-in-chief of the journal LGBT Health, believes sexuality may well be inborn, but thinks it could be more complicated than some scientists believe. He notes that the heritability of homosexuality is similar to that for divorce, but "social science researchers have not… searched for 'divorce genes'. Instead they have focused on heritable personality and temperamental traits that might influence the likelihood of divorce.
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These antibodies bind to the H-Y antigens and prevent them from functioning. Blanchard believes that this phenomenon grows stronger with each boy a woman bears. Studies have found that a man without older brothers has about a 2 percent chance of being gay, but one with four older brothers has a 6 percent chance. Meanwhile, other studies have found the relationship to be weak or nonexistent. According to the report, Blanchard now plans to test mothers of gay and straight men for the presence of these antibodies.
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