Boys with older brothers are more likely to be homosexual than those with sisters, younger brothers or no siblings at all, according to research.

Gay men had more older brothers, on average, than straight men, a study has found.

For every older brother a man has, the chances of him being gay increases by 33 per cent.

However, the researchers think their findings are linked to what happens in the womb, rather than the effects of growing up among older boys.

For decades, academics have argued over nature versus nurture - whether individuals are born gay, or become gay because of where and how they grow up.

Now Canadian psychologist Ray Blanchard says the answer depends on how many older brothers a man has.

He studied the family backgrounds of 302 gay men and the same number of straight men. All were white and none were twins, he told the journal New Scientist.

The gay men, had, on average, 1.32 older brothers compared to heterosexual men, who had an average of

0.96 older brothers. There was virtually no difference in numbers of other siblings - suggesting there is no truth in the theory that a lone boy with lots of sisters is more likely to grow up gay.

Dr Blanchard, of the Centre of Addiction and Mental Health in Toronto, said he initially thought the idea that being gay was more likely if a man had older brothers seemed 'absurd, egregious pseudoscience'.

But his studies showed those with an average of 2.5 older brothers were twice as likely to be gay as those with no older brothers. Those with four older brothers were three times as likely.

His research was backed up by John Manning at the University of Central Lancashire in Preston, who said: 'It's one of the few reliable correlates of homosexuality that I know of.'

The exact cause of the link is unknown. But Dr Blanchard said a mother's body changes every time she gives birth to more than one son.

Second, third and subsequent unborn male babies in the womb are exposed to more testosterone - something which may go back to when younger siblings needed to be stronger to survive if food was scarce.

There are also higher levels of antibodies in the womb for subsequent sons which could affect the orientation of the brain.

But not all the experts believe Dr Blanchard's theory because there is no evidence that Roman Catholics and Mormons, who tend to have large families, produce more gay men.

Social scientists also point out that the theory would mean the trend for smaller families in the Western world would mean fewer gay men, too, and that doesn't seem to be true either.

Dr Blanchard said, however: 'The theory that early childhood experiences lead to homosexuality has had 100 years and its supporters have come up with zip.'