Men who have an older brother are more likely to be gay, says a new study.
Scientists at the University of Toronto looked at 5,400 men across ten different scientific studies and found that men with an older brother are 38% more likely to be gay.
The study did not, however, conclude why. It also found no effect of having a younger brother on peoples sexuality.
Published in the Proceedings Of The Royal Society Journal, the study looks at the fraternal birth order effect or (FBOE.)
It is the finding that older brothers increase the probability of being gay in later-born males.
This is also put alongside the female fecundity effect (FFE,) which is the finding that the mothers of gay men produce more offspring than the mothers of straight men.
It’s not the first time researchers have explored this phenomenon, with previous research suggesting it is something to do with a protein some babies will have while in the womb. Indeed at least 30 different studies over the past three decades have researched the topic.
However, Professor David Spiegelhalter, a leading statistician from the University of Cambridge, remarks that though the study is fascinating, adds:
'People have endlessly argued about the possible roles of genetics and upbringing, but this clear result fits in neither category,' reports the Daily Mail.
The study does attempt to draw a reason for their result, but they do note in the study – as many other have – that parts of the Y-chromosome of fetuses cause an immunological reaction in some mothers.
Because these antibodies remain in her body, and later affect the brains of any future male fetuses in her womb in a way that makes them more likely to be gay.
This means that with further pregnancy with a Y-chromosome child, this makes this maternal reaction stronger.