A study conducted by researchers at the University of Toronto in Canada has found that you’re more likely to be gay if you have an older brother. This is not the first time this finding has been noted. The study included a comparative analysis of ten previous studies.
In total, it looked at 5,400 men and found that if a man has an older brother, they were 38% more likely to be gay than a man without an older brother.
The findings have not been replicated in women, where no link has been found between sexuality and the number of siblings someone has.
Previous studies have found that the more older brothers you have, the more likely you may grow up to be gay. If you have three older brothers, it’s been suggested you are twice as likely to be gay than if you have no older brothers.
This study looked primarily at families with one or two sons only. It found that men with younger brothers showed neither an increase or decrease in the likelihood of being gay. The results were published in the biology journal Proceedings of the Royal Society B.
It is unknown what causes this phenomenon.
Some scientists have speculated that each time a woman carries a male child in her womb, she produces a slightly different set of antibodies – prompted by her exposure to ‘male-specific substances’ in her unborn child.
The different antibodies she produces can impact the brain development of the next male child she carries. The controversial theory has been dubbed the “maternal immune hypothesis.”
A second hypothesis proposes that if there are genes that make men gay, the same genes in women make them more fertile and therefore likely to produce more offspring. Hence, the genes remain present in the human population despite gay men having fewer offspring than straight men.
The authors of this study say their findings lean more towards the first hypothesis than the second, but that’s not to be considered as proof. They suggest more research is needed into the second hypothesis – specifically research that counts, “the offspring of homosexual men’s sisters, maternal aunts, maternal grandmothers and so on, in addition to analyzing the offspring of their own mothers.”
Professor David Spiegelhalter, a statistician from the University of Cambridge unconnected with the research, told The Daily Mail: “The fascinating study estimates that having an older brother increases the odds of being gay by 38%, 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.”