A new study shows that men who have older biological brothers, are more likely to be gay than those who don't. The more older brothers he has, the greater the likelihood is.
"The link between having older brothers and homosexuality has long been established, but the new findings indicate firmly that conditions in the womb before birth, and not the subsequent family environment, are responsible."
It's yet unknown what those conditions may be, though there is speculation they may be related to the way the mother's immune system reacts to carrying male foetuses. The Y chromosone in the male foestuses may be recognized as foreign and generate a stronger immune response, which increases which each subsequent pregnancy.
Below is the article from the Times on the study.
Gay men are 'born with big brothers'
By Mark Henderson
From Times Online: http://www.timesonline.co.uk/article/0,,11069-2244488,00.html
Some sexual orientation may be determined by the maternal response to carrying male foetuses
A MAN’S sexual orientation can be determined before he is born, according to 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, but only when these brothers are true biological siblings.
The link between having older brothers and homosexuality has long been established, but the new findings indicate firmly that conditions in the womb before birth, and not the subsequent family environment, are responsible.
This suggests that in at least a proportion of gay men, their orientation is heavily influenced by biological factors they experience before they are born, and not by the way they are brought up or choices they make later in life. Nature, and not just nurture, has an important role to play.
“These results support pre-natal origin to sexual orientation development in men,” said Anthony Bogaert, of Brock University, Ontario, Canada.
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, 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 fraternal birth order effect — since 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 background.
If family nurturing were 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.
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 is that it reflects the way a mother’s immune system reacts to carrying lots of 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 generate a strong immune response.
Other research has shown that this response can strengthen with each subsequent male pregnancy. This may affect the way that 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 change 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.