A man's sexual orientation can be fixed before he is born, according to new research that provides the strongest evidence yet of a biological basis for male homosexuality.

Scientists in Canada have discovered that the probability of a man being gay rises significantly according to the number of elder brothers he has, when these brothers are born of the same mother.

While the link between having older brothers and homosexuality has long been established, the new findings indicate that it is conditions within the womb before birth, and not the subsequent family environment, which are responsible for the effect.

This suggests that in at least a proportion of gay men, sexual orientation is heavily influenced by factors experienced in the womb, and not by the way they are brought up.

"These results support pre-natal origin to sexual orientation development in men," said Anthony Bogaert of Brock University in Ontario, Canada, who conducted the research.

The idea that having a large number of older brothers might influence male sexuality was first raised in 1997 by a study led by Ray Blanchard of the Centre for Addiction and Mental Health in Toronto.

Dr Blanchard found that gay men were more likely to have lots of elder brothers than both straight men and lesbians. Each older brother increases the probability of being homosexual by a third, though as the starting probability is small - most men with lots of elder brothers are still heterosexual.

This effect, which has since been confirmed by 14 other studies, does not apply to having older sisters.

Though the original work raised the possibility that the effect was biological, probably as a result of conditions in the womb, it left open a rival explanation: that the social, family and environmental consequences of having lots of older brothers can influence male sexuality.

This alternative hypothesis has now been dismissed by Dr Bogaert’s study, which is published today in the journal Proceedings of the National Academy of Sciences.

It sought to investigate this by comparing men who have older biological brothers — full or half siblings born to the same mother — with men who have older step or adopted siblings, to whom they have no biological relationship, but a shared family background.

If family nurturing was responsible for the birth order effect, both groups would be expected to have similar rates of homosexuality. Dr Bogaert, however, found that only biological brothers had an impact: growing up with older step-brothers or adoptive brothers had no influence at all.

The increased chance of homosexuality applied even where men had older full brothers who had been raised separately in a different home, offering further evidence for a biological effect.

"Only biological older brothers (reared with or not) and no other sibling characteristic predicted men’s sexual orientation," Dr Bogaert said. "If rearing or social factors associated with older male siblings underlies the fraternal birth order effect, then the number of non-biological older brothers should predict men’s sexual orientation, but they do not."

The mechanism by which having older biological brothers affects male sexuality remains unknown, but the most popular theory among scientists is that it reflects the way a mother’s immune system reacts to carrying male foetuses.

As males have a Y chromosome and females do not, a mother’s body may be more likely to recognise a male foetus than a female one as foreign, and to generate a strong immune response.

Other research has shown that this response can strengthen with each subsequent male pregnancy. This may affect the way the brain develops sexually. Sisters have no impact, and there is no effect on girls, as female foetuses do not provoke the same reaction.

"If this immune theory were correct, then the link between the mother’s immune reaction and the child’s future sexual orientation would probably be some effect of maternal anti-male antibodies on the sexual differentiation of the brain," Dr Bogaert said.

It is also possible that successive male pregnancies changes the way that foetuses are exposed to the male hormone testosterone in the womb. This, however, would also be expected to influence female sexuality, on which having older brothers appears to have no effect.

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