Study: Having Older Brothers Linked to Being Gay

Thu. June 29, 2006 12:00 AM by

Ontario, Canada - A Canadian research study released this week suggests that a man's sexual orientation appears to be determined in the womb.

The study, conducted by Dr. Anthony F. Bogaert of Ontario, asserts that the more biological older brothers a man has, the more likely he is to be gay.

In his investigation, Bogaert studied a sample of 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 regardless of whether a man's older brothers were biological or adopted.

Surprisingly, Bogaert discovered that even if a young man did not grow up in the same house as his older brothers, the fact that he had older biological brothers increased his odds of being gay.

One theory suggests that after delivering a boy, a woman's immune system produces antibodies to male-specific proteins. During subsequent pregnancies the mother's placenta may deliver the antibodies to the foetus, possibly affecting its development.

"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 wrote in his report published in Proceedings of the National Academy of Sciences.

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.

In earlier work, Dr. Bogaert and colleagues estimated that about 28% of homosexual men owe their orientation to fraternal birth-order—"a minority, but not a negligible minority, of all homosexual men."

"Whether this is what is really happening for sexual orientation remains to be seen, but it is a provocative hypothesis," the report stated.

Written By Troy Espera

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