Gay men may be born and not made, new research indicates. A study of 1,000 heterosexual and homosexual men in Canada has found that the single most important factor that can be linked with male homosexuality is the number of elder brothers a man has in his family.

Although this has been known for some years, the latest study demonstrates that the link appears to have nothing to do with whether a man is brought up with his elder brothers, and is therefore psychologically influenced by them. The increased chance of being gay seems to be the result of whether or not the mother had already given birth to boys - in other words, a biological link from the womb rather than psychological links from childhood. But it is still a mystery why for every elder brother a boy has, his chances of growing up gay increase by about a third.

Anthony Bogaert, of Brock University in Ontario, who helped to discover the link between homosexuality and elder male siblings, looked at gay men who had not been raised with their elder brothers, but still found that there was a link between homosexuality and the number of elder male siblings they had.

One explanation could be the "maternal immunisation" hypothesis, which suggests that mothers may produce antibodies against a male foetus's Y chromosome while in the womb, and that these antibodies have an accumulative effect on subsequent pregnancies, which could affect the sexuality of boys.

Marc Breedlove, of Michigan State University, a colleague of Dr Bogaert, wrote in the Proceedings of the National Academy of Sciences: "[Sigmund] Freud thought a distant, emotionally cold father might prevent a boy from identifying with dad and steer him to homosexuality. How much stranger it will be if, instead of the father's psychological rejection, it is the mother's immunological rejection that inadvertently but actively makes her son gay?"