The more older brothers a man has, the more likely he is to be gay. And there is now evidence that indicates this effect is caused by immunological conditions in a mother’s womb.

Those are the results of two new studies, which provide stronger evidence for a phenomenon known as the fraternal birth order effect.

“Two early lines of research on human sexual orientation focused on the sibship composition of homosexual men,” explained researcher Ray Blanchard, a professor of psychiatry at the University of Toronto.

“One line, which originated in Germany in the 1930s, indicated that homosexual men have an excess of male siblings. Another line, which originated in England in the 1950s, indicated that homosexual men have an excess of older siblings. I was dimly aware of this work but I put no stock in it, until I inadvertently replicated these findings while doing something completely unrelated.”

“A few years later, I and my then postdoc, Tony Bogaert, showed that the German and British investigators had simply been looking at different facets of the same phenomenon, namely, that homosexual men have an excess of older male siblings,” Blanchard told PsyPost.

“Older brothers increase the odds of homosexuality in later-born males. Tony Bogaert found that biological brothers increase the odds of homosexuality in later-born males, even if they were reared in different households, whereas stepbrothers or adoptive brothers have no effect on sexual orientation. Thus, the available evidence indicates that the effect is prenatal.”

Blanchard published research in the Archives of Sexual Behavior that provides evidence that fraternal birth order is linked to men’s sexual orientation. The research used a statistical method known as a meta-analysis, allowing the researchers to examine a large amount of data recorded from previous studies.

“I recently investigated the reliability of this phenomenon — the fraternal birth order effect — in two non-overlapping meta-analyses. The first meta-analysis was carried out on 30 convenience samples from 26 studies, totalling 7,140 homosexual and 12,837 heterosexual males,” Blanchard explained to PsyPost.

“The second meta-analysis was carried out on 6 probability samples from 5 studies, totalling 2,386 homosexual and 445,301 heterosexual males. The homosexual males had proportionately more older brothers than the heterosexual controls in 35 of the 36 total samples (although the difference was not statistically significant in every individual study).”

Blanchard’s meta-analytic study prompted seven commentaries from other researchers. Northwestern University psychology professor J. Michael Bailey, for example, cautioned that publication bias could have led to Blanchard finding a positive result.

“Scientists are incentivized to produce positive (i.e., statistically significant) findings,” Bailey wrote in his commentary. “They are especially incentivized to produce positive, surprising findings. Surprising findings are those we would be prone to doubt without compelling evidence. Such findings are especially unlikely to be true, all else being equal. The fraternal birth order effect qualifies as a surprising finding.”

Blanchard responded with a commentary of his own, defending his findings, and also published an addendum to show that older sisters did not have the same effect on sexual orientation as older brothers.

Blanchard and his colleagues believe the fraternal birth order effect has a biological basis in maternal immune responses in the womb — which they call the maternal immune hypothesis (MIH).

They now have evidence that the effect is immunological in origin. A study led by Bogaert, which was published in the journal Proceedings of the National Academy of Sciences, found that a maternal immune response to a Y-linked protein important in male fetal brain development was linked to men’s sexual orientation.

“Tony, I, and a team of other psychologists and immunologists recently reported the first laboratory test of the MIH. We drew blood samples from 16 women with no sons, 72 mothers of heterosexual sons, 31 mothers of gay sons with no older brothers, 23 mothers of gay sons with older brothers, and an additional control group of 12 men,” Blanchard told PsyPost.

“We conducted immunoassays to measure the mothers’ antibody reactivity to two cell-surface proteins that are found only in males and are expressed in fetal brain: PCDH11Y and two isoforms of NLGN4Y. There were significant differences for both isoforms of NLGN4Y tested.”

“When the total number of pregnancies was controlled for, mothers of homosexual sons (especially those with older brothers) had significantly higher anti-NLGN4Y levels than did the control samples of women, including mothers of heterosexual sons,” Blanchard explained. “Thus, the findings for NLGN4Y were consistent with the predictions of the MIH. Of course, it is very important that this study be replicated by an independent team using a fresh sample.”

The maternal immune response is just one factor among many that influences sexual orientation. Not every man with a large number of older brothers is gay.

“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 said.

The two studies are titled, “Fraternal Birth Order, Family Size, and Male Homosexuality: Meta-Analysis of Studies Spanning 25 Years” and “Male homosexuality and maternal immune responsivity to the Y-linked protein NLGN4Y“.