Fraternal birth order effect

Another interesting and unexpected phenomenon is the fraternal birth order effect.20 Surveys show without equivocation that homosexual men have a greater number of older brothers than heterosexual men. The estimated odds of being homosexual increase by approximately 33 percent with each older brother. 

Statistical analysis suggests that in approximately i in 7 homosexual men, their homosexuality is related to the fraternal birth order effect. There is no such effect in female homosexuals. Homosexual males with older brothers also have significantly lower birth weights compared to heterosexual males with older brothers. One possible reason for this phenomenon is that as mothers become progressively immunized to male-linked antigens produced by successive male fetuses, the mothers produce antibodies against these antigens. The antibodies then divert the development of the fetal brain from heterosexual male to homosexual male. But this maternal immunity theory remains a speculation. 

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See also in sourсe #XX -- [ , ]

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