Anthony F. Bogaert, Ph.D. and Sven Bocklandt, Ph.D., Proceedings of the National Academy of Sciences ( 2006) June 26-30
Adding to the theory that homosexuality may be hardwired and not the product of environmental factors, new research again confirms that the more older brothers a male has, the more likely he is to be gay.
Researchers have known for years that a man's likelihood of being gay rises with the number of older biological brothers. But the new study found that the so-called "fraternal birth order effect" persists even if gay men were raised away from their biological families.
Study author Anthony F. Bogaert said the findings add to a growing body of evidence that links homosexuality to nature, not nurture.
 "The research suggests that the development of sexual orientation is influenced before birth," said Bogaert, a professor of community health sciences at Brock University in St. Catharines, Ontario, Canada.
Bogaert and a colleague first reported the older-brother effect a decade ago. According to Bogaert, men with no older brothers have about a 2 percent to 3 percent chance of being gay. If they have three or four older brothers, the rate goes up to about 5 percent.
About 20 studies have reinforced the link between fraternal birth order and male homosexuality, Bogaert said. However, no similar link has been found in lesbians.
Bogaert said he launched his new study to better understand the fraternal birth order effect. "Is it a biological phenomenon? Or is it psychological or have to do with rearing?" he said.
To find the answer, Bogaert examined surveys of 944 Canadian men, both gay and straight, about their sexuality and their families.
The older-brother effect was constant regardless of whether the men were raised with natural, adopted or stepbrothers. It also didn't matter if they weren't raised with their biological mothers.
If gay younger brothers and older brothers don't have the same home environments, what do they have in common? "They shared the same uterus, the same womb, the same mother," Bogaert said.
Bogaert said he concluded the effect was biological by comparing men with biological brothers to those with brothers to whom they were not biologically related.
The increase in the likelihood of being gay was seen only in those whose brothers had the same mothers, whether they were raised together or not, he said.
Men raised with several older step- or adopted brothers do not have an increased chance of being gay.
"So what that means is that the environment a person is raised in really makes not much difference," he said.
What makes a difference, he said, is having older brothers who shared the same womb and gestational experience, suggesting the difference is because of "some sort of prenatal factor."
One possibility, he suggests, is a maternal immune response to succeeding male fetuses.
The mother may react to a male fetus as foreign but not to a female fetus because the mother is also female.
The theory suggests that mothers during childbirth may develop antibodies to proteins made by their firstborn son's Y chromosome, and subsequent pregnancies may stir up those antibodies in an immune reaction that affects the development of a male fetus. "Whether this is really what is happening ... remains to be seen, but it is provocative hypothesis,'' said Breedlove.
So far, scientists have found no similar relationship between birth order and the probability that a girl will grow up to be lesbian. That could be because a female baby has the same double X chromosomes as her mother and is less likely to provoke an immune reaction during childbirth.
Breedlove stressed that these biological "perturbations" possibly affecting male fetuses should not be confused with disease or a birth defect. They are simply biological effects that steer development. "It just means there is a variation,'' he said.
Breedlove said that he is surprised that Bogaert's original findings about the fraternal birth order effect are not more widely known, because the work is so interesting and has been replicated by other researchers. One reason may be that homosexuality occurs in about 4 to 5 percent of the population, so the increase noted among boys with several older brothers is a small effect involving a small percentage of all people.
Similarly surprising research has found that the fraternal birth order effect is limited to younger boys who are right-handed. In other words, if a younger boy has many older brothers but is left-handed, he does not have an elevated chance of being gay.
"We never dreamed of such an association,'' said Breedlove, a co-author of that study.
The right-handed exception to the fraternal birth order effect was particularly surprising because other research had previously uncovered another puzzler: Both men and women who are left-handed are slightly more likely to be gay.
"These data strengthen the notion that the common denominator between biological brothers and the mother, provides a prenatal environment that fosters homosexuality in her younger sons."
"The results support a prenatal origin to sexual orientation development in men."
"A man's sexual orientation may be determined by conditions in the womb".
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