Taken From:
http://news.yahoo.com/s/nm/20060627/ts_nm/sexual_orientation_men_determined_before_dc;_ylt=AqNaP5__zLB6vN4GETlL76p34T0D;_ylu=X3oDMTA0cDJlYmhvBHNlYwM-
Tue Jun 27, 12:38 AM ET
NEW YORK (Reuters Health) - 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.
Additional Readings:
1) http://news.yahoo.com/s/hsn/20060626/hl_hsn/homosexualityagainlinkedtobiologicalfactors
2) http://www.latimes.com/news/science/la-sci-brothers27jun27,0,407199.story
