Sexual Orientation of
Men Determined
Before Birth
By REUTERS, from the
NYTimes on the Web, June 27, 2006
NEW YORK -- 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.
SOURCE: PNAS Early Edition, June 26, 2006.
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