For decades, researchers have known that the more older brothers a man has, the greater his likelihood of being gay. More recently, this has also been linked to having older sisters. Now, a modelling study suggests that the latter link could be explained by older sisters being a proxy for their mother previously having multiple pregnancies, some of which may have involved miscarried male embryos.
The scientists have stressed, however, that this is just an idea and further research is needed. Carrying boys during pregnancy is thought to cause an immune response towards specific male proteins involved in fetal brain development, which becomes stronger with successive male pregnancies. If the resulting antibodies enter the brain of a male fetus, they could influence his sexual orientation later on.
But that doesn’t explain the link to older sisters. Is there an evolutionary reason for same-sex sexual behaviour? One proposed explanation is that older sisters provide clues about a mother’s reproductive history.
The idea goes that the more children a woman has, the more times she has been pregnant, some of which could have ended in miscarriages, including extremely early ones. If male embryos were miscarried, they may have sensitised her immune system against those male proteins. To explore this, Raymond and his colleagues built a model to uncover whether miscarriages could generate an older-sister effect for male sexual orientation.
For simplicity, it assumed that all women have the same probability of miscarriage for each pregnancy, says Raymond. In reality, miscarriage risk probably varies among women, even when accounting for age, which should be investigated in further research, he says. The researchers compared the model’s predictions with data from eight populations.
This suggested that each additional older brother is associated with about a 30 per cent increase in the odds of a man being gay, while each additional older sister is linked to about a 20 per cent increase. The team calculated that between 37 and 58 per cent of fertilised eggs would need to be miscarried to fully explain the older sister association. Information about whether the participating mothers included non-binary people and trans men wasn’t routinely collected across the eight population datasets.
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