Boys are more likely to be gay if they have older brothers, if new research is to be believed.
Scientists say the more older brothers a man has the greater the chance of them being homosexual.
It is believed to occur because women mount an immune response when pregnant with a boy, which attacks part of the foetus' brain linked to sexual orientation.
The strength of this 'immune response' is said to increase with each son, raising the odds of homosexuality.
Scientists from Brock University in Ontario collected data from almost 1,000 men, including some who had been adopted or brought up with stepbrothers and stepsisters.
They found having stepbrothers did not have an impact on sexual orientation, but having biological brothers did – even if they were brought up separately. They claim this shows the influence of nature over nurture on sexuality, and brings traditional arguments into question.
They found each older brother raised the odds that a man was homosexual by a third. But scientists found no direct link between girls having lots of sisters and being lesbians.
The study has been printed by New Scientist, which show the impact of siblings on a person's psychological traits – where for example older siblings are said to be more intelligent but more cautious than younger brothers and sisters.