A man’s sexual orientation appears to be determined in the womb, a new study suggests.

Past research by Dr. Anthony F. Bogaert of Brock University in St. Catherines, Ontario and colleagues has shown that the more older brothers a man has, the more likely he is to be gay. But it has not been clear if this is a prenatal effect or a psychosocial effect, related to growing up with older male siblings.

To investigate, Bogaert studied 944 gay and straight men, including several who were raised with adopted, half- or step-siblings or were themselves adopted. He reasoned that if the relationship between having older male siblings and homosexuality was due to family environment or child-rearing practices, it would be seen whether or not a man’s older brothers were biological or adopted.

Bogaert found that the link between having older brothers and homosexuality was present only if the siblings were biologically related - this relationship was seen between biological brothers who were not raised together. The amount of time that a man was reared with older brothers had no association with sexual orientation.

“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,” Bogaert writes in his report in PNAS Early Edition.

A woman’s body may see a male fetus as “foreign,” Bogaert explains, and her immune response to subsequent male fetuses may grow progressively stronger.

“If this immune theory were correct, then the link between the mother’s immune reaction and the child’s future sexual orientation would probably be some effect of maternal anti-male antibodies on the sexual differentiation of the brain,” he suggests.

Other lines of research also support the sexual orientation-maternal immune response link, he notes

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SOURCE: PNAS Early Edition, June 26, 2006.