Study confirms link between the number of older brothers and increased odds of being homosexual

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Groundbreaking research led by a team from Brock University has further confirmed that sexual orientation for men is likely determined in the womb.

In the first-ever laboratory study of mothers of gay men, the research was prompted by more than two decades of statistical data examining the 'older effect' which shows that biological older brothers—but not older sisters—increase the odds of homosexuality in later-born males.

The study, "Male Homosexuality and Maternal Immune Responsivity to the Y-Linked Protein NLGN4Y," was published Monday, Dec. 11 in the journal Proceedings of the National Academy of Sciences.

Brock Health Sciences Professor Tony Bogaert, lead researcher on the project, said the new study has produced some of the most significant findings in men's sexual orientation research in the past 10 or 15 years. The team included researchers from Harvard and the University of Toronto.

"The implications of this study, especially if and when it is replicated by an independent team, are profound," said Bogaert. "Along with more deeply understanding the exact origin of the older brother effect, it helps solidify the idea that, at least in men, there's a strong biological basis to sexual orientation.

"This is the culmination of more than 20 years of research where we started looking at the older brother, or fraternal birth order, effect. The current study adds to the growing scientific consensus that homosexuality is not a choice, but rather an innate predisposition."

Bogaert, an internationally recognized expert in human sexuality, said the study is groundbreaking for at least two major reasons:

  • It supports the conclusion, suggested by previous studies, that genes alone do not completely account for .
  • It suggests that immunological factors should be considered along with genetic and hormonal factors as possible biological influences on sexual orientation.

Joining Bogaert from Brock were Health Sciences Assistant Professor Adam MacNeil, Health Sciences PhD student José Gabrie, Psychology PhD students Malvina Skorska (now at University of Toronto) and Mark Hoffarth (now at New York University). The team also included Harvard's Chao Wang and University of Toronto's Doug VanderLaan, Ken Zucker and Ray Blanchard.

Bogaert said that since the older brother effect was first discovered two decades ago, many studies have replicated it, including cross-cultural ones. But when a Bogaert research project 10 years ago strongly indicated that the older brother effect in men was likely biological in origin, he saw the need to look at prenatal factors, and set out to test the major biological theory explaining this effect through a lab study.

The team of psychologists and immunologists tested 16 women with no sons, 72 mothers with heterosexual sons, 31 mothers of gay sons with no older brothers, 23 mothers of gay sons with older brothers, and a control group of 12 men.

The women's antibody reactivity was measured to two proteins (PCDH11Y and two forms of NLGN4Y) found only in males, both of which are expressed in the male fetal brain.

The team found that mothers of gay sons, especially those with older brothers, had significantly higher antibody levels to both forms of NLGN4Y than did the control samples of women, including mothers of heterosexual sons.

"It seems that some women during their first male pregnancy, or just after their first male birth, begin to detect this foreign substance (the NLGN4Y protein) and start to develop an immune response. And then later, with further male pregnancies, the high levels of antibodies directed toward this substance may change brain development in these later born males," Bogaert said.

This confirms that the older brother effect is very likely immunological in origin.

"We think it's very important to understanding male sexual orientation," said Bogaert. "It speaks to the mechanisms of development."

However, the psychologist cautions that the effects are modest and the likelihood of a male child being born gay is still small—even if they have multiple male siblings.

"The vast majority of men with older brothers are still heterosexual, but it says something very broad about sex and gender development."

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More information: Anthony F. Bogaert et al. Male homosexuality and maternal immune responsivity to the Y-linked protein NLGN4Y, Proceedings of the National Academy of Sciences (2017). DOI: 10.1073/pnas.1705895114
Provided by Brock University
Citation: Study confirms link between the number of older brothers and increased odds of being homosexual (2017, December 12) retrieved 21 July 2019 from
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Dec 12, 2017
A certain subset of researchers really, really want to discredit the role that choice plays in sexual behavior. Anthony F. Bogaert appears to have spent all of his working career on this effort.

Dec 12, 2017
When it comes to 'sexual orientation,' choice is not a factor. People go in the direction that their biological urges send them. Sexual behavior and sexual orientation are two different things.

Dec 12, 2017
Which brings up the interesting question: if, in fact, the mother creates antibodies to the production of these male fetal proteins, can the antibodies be medically treated when a mother is known to be carrying a younger son to minimize their effect on the fetus?

Dec 12, 2017
A demonstration of the technique of using a diversionary fraud.
In fact, the idea of homosexuality being innate or learned was never so important. The fact that homosexuality, like all the sex deviances, is depraved and sick is. Rather than address the issue of the malignant qualities of the sex deviances, scammers and swindlers instead focused on whether it develops in the womb or by choice. Many conditions, from aggressiveness to recklessness, can develop in the womb, but that doesn't make them desirable! All the sex deviances have common factors, depression that lasts longer than a year; extreme anxiety disorder; self loathing higher than in any other group; an inability to control themselves not to engage in things they find foul; lack of impulse control; drug use higher than other groups; suicide rates higher than any other groups. Developed in the womb or not, they cannot validly be declared normal and necessarily beneficial.

Dec 12, 2017
Anything to take the finger away from the hormone mimicking chemicals being spewed on us by the GOP, your president, (he's not mine), the Ag, Petroleum & Plastics industries.
The rest of the now ranked 27th in the world in math & science educated USA public might not know what "endocrine disruptor" means but I do.

Dec 12, 2017
Perhaps there is an evolutionary connection here. Males choose the sex of a child, so perhaps women have a role in choosing the sexual orientation of thier male offspring as a means of competitive selection. While there were defiantly women warriors, and many women today participate in combat and war, the evolution of males is set to be the physical provider. Males are competitive driven via testosterone more so than women, and so as an evolutionary precaution, or perhaps evolutionary selection (considering our now chemical environment as a possible absorption pool), women change the sexual orientation of thier descending male offspring to reduce competition and increase the chances of their off spring surviving. After all, being gay or bi does not reduce the function of ones anatomy or ability to reproduce. Given the scenario of mankind's survival, I have my doubts that any person, gay or bi, would let our species die out.

Dec 12, 2017
Well said Misseau. I might add that the male fetus that triggered the hormonal response may have a hand in this selection also. Like the alpha male in the animal kingdom who makes sure it is HIS seed that continues down the line, maybe the first male fetus does the same with his seed [hormonally]. If the second/third son does not bear children because of homosexuaity then the first son is assured that his seed will continue down the line. Plus there is an added bonus. Should anything adverse happen to the first son's spouse or mother, the homosexual second son often has innate nurturing skills which could be used to care for the first son's offspring should circumstances warrant it.

Dec 12, 2017
Does this apply only to live births ? Or include always-tragic miscarriages etc ??

Dec 14, 2017
One the broad subject of 'cause' of homosexuality in basic terms: As if we needed more proof that it is biological. It has be determined behaviorally to be biological. Those that don't understand that are simply incapable of interpreting the considerations rationally And also probably irrationally biased and prejudiced.

Dec 18, 2017
julianpenrod, I find your bigotry and homophobia far more depraved and sick than any homosexual I know.

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