Born Gay by Dr Glenn Wilson of the Institute of Psychiatry and Dr Qazi Rahman of the University of East London has been welcomed by gay rights activists, who say it proves there is nothing "unnatural" about homosexuality. But the book has also been criticised for suggesting that men who see themselves as bisexual should be classed as gay.
Born Gay summarises the latest research into the psychobiology of sex orientation, concluding that studies with twins show that around 30 to 50 per cent of sexual orientation is due to genes. The rest is the result of processes in the womb.
The genes involved in sexual orientation produce receptors in the brain which absorb male hormones released by the developing baby's testicles. It is thought that in gay men these receptors are less sensitive or are blocked. The absence of testosterone results in the brain developing certain female characteristics, including attraction to men.
It is also believed that the unabsorbed testosterone affects the body's extremities. This would explain, Dr Rahman said, why the finger lengths of adult gay men tended to be more male-like than those of heterosexual men, in that their index fingers are significantly shorter than their ring finger. (Women's index and ring fingers tend to be of equal length.)
The hormone may also be responsible for increasing the size of the penis, another extremity. A number of studies have shown that gay men are better endowed than heterosexual men.
The scientists also claim that true bisexual men are extremely rare. Studies showed that heterosexual men respond to lesbian pornography, but not to gay pornography. "What's interesting is that bisexual men do not respond to both type of stimuli. Almost 95 per cent of bisexual men respond exclusively to gay stimuli," said Dr Rahman.
According to the researchers, there is much evidence to contradict claims that homosexuality is caused by social factors, such as seduction, or that it can be learnt. "Showing that homosexuality is not due to learning, or seduction, or smothering mothers has very clear implications for social policies," Dr Rahman added. "Clause 28 emphasised the notion that homosexuality should not be talked about because that might encourage young people to experiment."
Ben Summerskill, the chief executive of Stonewall, said he welcomed evidence that made discrimination against gay people untenable.
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