http://www.sciencenews.org/articles/20060701/fob1.asp
Birth order may steer some men toward homosexuality in a process that perhaps begins before birth. A new study finds that homosexuality grows more likely with the greater number of biological older brothers—those sharing both father and mother—that a male has.
Men display this tendency toward homosexuality even if they weren't raised with biological older brothers, finds psychologist Anthony F. Bogaert of Brock University in St. Catharines, Ontario. No gay connection appears in men raised with half-brothers, stepbrothers, or adoptive brothers, all deemed non-biological by Bogaert.
"The mechanism underlying this fraternal birth-order effect remains unknown," Bogaert says. It's possible that succeeding pregnancies with male fetuses trigger a maternal immune response. A mother's immune system may treat male fetuses as foreign bodies, attacking them with antibodies that alter sex-related brain development, the Canadian psychologist suggests.
Scientists haven't yet looked for any specific immune reaction during pregnancy that targets later-born boys who become homosexual.
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