Researchers looked at almost 5,400 men across ten scientific studies and found those with an older brother were 38 per cent more likely to be gay – although it is not clear why.
Having a younger brother appeared not to impact sexuality.
The findings, from scientists led by the University of Toronto, follow previous evidence that each additional older brother increases a man's chances of homosexuality by around a third, while having three older brothers more than doubles the odds.
Dr Ray Blanchard, first author of the study, said only male siblings seemed to affect men's chances of being gay, stating: 'Much prior research has shown that females do not influence the sexual orientation of their younger siblings, and females' sexual orientation is not affected by their numbers of older siblings.'
The study, published in the journal Proceedings of the Royal Society B, compared the chances of the youngest brother in a two-son family being gay against the odds for the oldest son.
It is unclear why having an older brother might have an effect on someone's sexuality.
Some experts have controversially suggested something called the 'maternal immune hypothesis', where a mother who has an older son becomes exposed to 'male-specific substances' from male cells during pregnancy or birth.
The theory is that she then produces antibodies to those cells, which affect the brain cells of a second male child and make his brain work differently.
Professor David Spiegelhalter, a leading statistician from the University of Cambridge, who was not involved with the research, said: 'The fascinating study estimates that having an older brother increases the odds of being gay by 38 per cent, supporting the idea that a mother's immune response to having a male child influences subsequent boys.
'People have endlessly argued about the possible roles of genetics and upbringing, but this clear result fits in neither category.'
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