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... I have heard that there is some evidence that men are more prone to being gay if they follow a long line of older sisters. But is there actually any evidence for this, or is it just apocryphal?
The fraternal birth order effect is the strongest known biodemographic predictor of sexual orientation. According to several studies, each older brother increases a man's odds of having a homosexual orientation by 28–48%. The fraternal birth order effect accounts for approximately one seventh of the prevalence of homosexuality in men. There seems to be no effect on sexual orientation in women, and no effect related to the number of older sisters...The effect has been found even in males not raised with their biological brothers, suggesting an in-utero environmental causation. To explain this finding, a maternal immune response has been hypothesized.