Research published in the journal the Proceedings of the National Academy of Sciences (PNAS) suggests a link between male sexuality and conditions in the womb.
Initial research by Dr. Anthony F. Bogaert and his colleagues at Brock University, Ontario revealed a possible link between the number of elder brothers a boy has and the likelihood he is to be gay. Whether this correlation be attributed to nature through the prenatal effects of the womb or to nurture through the psychosocial effects of growing up with elder siblings, however, remained unclear.
To further his research Bogaert studied 944 heterosexual and homosexual men. The men were either "biological" brothers and shared the same mother or "non-biological" brothers through adoption or as half or step siblings. Boegart reasoned that if the link between homosexuality and elder brothers was due to the nurturing effects of the family environment it would show regardless of any biological relationship.
The study showed, however, that the link was found only in biological brothers. The amount of time an individual spent being raised with elder brothers had no bearing on sexual orientation; the relationship was even found to be true in brothers who were raised apart from one another.
Bogaert writes in this month's PNAS: "These results support a prenatal origin to sexual orientation development in men and indicate that the fraternal birth-order effect is probably the result of a maternal 'memory' for male gestations or births."
He suggests that each male foetus prompts a progressively stronger immune reaction in the mother's body. The antibodies that are produced by this reaction may account for sexual differentiation of the brain.
Scientists from Michigan State University added in an accompanying article: "These data strengthen the notion that the common denominator between biological brothers, the mother, provides a prenatal environment that fosters homosexuality in her younger sons." 
"Increasingly, credible evidence appears to indicate that being gay is genetically determined rather than being a so-called lifestyle choice. It adds further weight to the argument that lesbian and gay people should be treated equally in society and not discriminated against for something that's just as inherent as skin colour," said Andy Forrest, a spokesman for gay rights group Stonewall. However; Dr. Bogaert's study made no explanation for the origin of lesbianism.
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Conditions in the womb determine male sexuality