The last male child in a family of many boys is likely to be gay, Canadian researchers found in a study published on Monday in the United States Proceedings of the National Academy of Sciences online journal.
The study, led by Anthony Bogaert of Brock University near Toronto, found that ”the most consistent bio-demographic correlate of sexual orientation in men is the number of older brothers [one has]” and not social influences.
”Only biological older brothers, and not any other sibling characteristic, including non-biological older brothers, predicted men’s sexual orientation, regardless of the amount of time reared with these siblings,” Bogaert said.
Researchers did not investigate the mechanism at play, but the results support a ”fraternal birth order effect” theory on homosexuality in men prescribed by others, he said.
According to this theory, a mother’s immune system interprets and remembers male — but not female — foetuses as foreign and creates ”maternal anti-male antibodies” that affect the part of the brain that determines sexuality.
The effect is cumulative, thus is more likely to cause homosexuality in the last of several sons born to the same mother.
Female foetuses are not affected because mothers are themselves female and so no antibodies are created during the gestation of daughters.
Other theories suggest that a mother’s age or stress level during pregnancy may have an impact on men’s sexuality.
A total of 944 Canadian men were interviewed for the study. – AFP
