Modelling Study Suggests Miscarriages May Explain Older‑Sister Effect on Male Sexual Orientation

A new modelling study proposes that the link between having older sisters and a higher likelihood of male homosexuality could stem from a mother’s immune response to miscarried male embryos. Researchers built a simplified model assuming a constant miscarriage probability across pregnancies and compared its predictions with data from eight populations. The model reproduces the observed pattern: each additional older brother raises the odds of a man being gay by about 30 %, while each older sister increases the odds by roughly 20 %. To account for the sister effect, the model requires that 37–58 % of fertilised eggs be miscarried, a figure within the range of reported miscarriage rates. The authors caution that miscarriage risk varies among women and that the maternal immune hypothesis remains unproven, calling for further research that incorporates detailed miscarriage histories and immune markers.