Modelling Study Links Older Sisters to Miscarried Male Embryos — SkimNews
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- Michel Raymond at the University of Montpellier led a modelling study proposing that older sisters act as a proxy for their mother's prior pregnancies, including miscarried male embryos that may trigger an immune response against male fetal brain proteins.
- Raymond's team compared the model's predictions against data from eight populations, finding each additional older brother linked to ~30% higher odds of a man being gay and each older sister to ~20% higher odds.
- The model calculated that 37-58% of fertilised eggs would need to be miscarried to fully account for the older-sister association — well above the 10-20% known-pregnancy miscarriage rate cited for the US.
- Ray Blanchard at the University of Toronto, who first proposed the older-sister-as-proxy hypothesis, said the study "came up with results indicating… we're within the realm of what is possible."
- A 2017 study found mothers of gay sons with older brothers carried higher antibody levels against NLGN4Y, a Y-chromosome-linked protein that Elena Jazin at Uppsala University documented in 8-to-11-week-old embryos.
- Jan Kabátek at the University of Melbourne found birth order also predicts women's odds of entering same-sex unions — a pattern he says can't be readily explained by antibodies to male-specific proteins alone.
- Raymond cautioned the maternal immune hypothesis may still be wrong: "it is also possible that the maternal immune hypothesis is not correct, and at this stage, it is not possible to decide."
Why it matters: For decades, researchers have known older brothers correlate with male homosexuality, but older sisters remained an unexplained piece of the puzzle — Raymond's modelling study offers a quantified test of whether maternal immune reactions to miscarried male embryos could account for the gap, though the 37-58% miscarriage rate it requires far exceeds the 10-20% known-pregnancy rate.
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