A study out yesterday lends support to the fraternal birth order theory of male homosexuality. Also called the older brother effect, the effect has been hypothesized to relate to an immune response in mothers of sons. The more sons a woman has, the more likely her younger sons will not be heterosexual. Although without support up to now, supporters of the immune theory believe that having sons for some women create antibodies which react against proteins which effect brain development. Presumably, the brain development would be an areas which effect sexual orientation.
From the press release:
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 brother 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 homosexuality.
It suggests that immunological factors should be considered along with genetic and hormonal factors as possible biological influences on sexual orientation.
The study did not link the proteins with specific neural outcomes but did find that mothers of gay sons demonstrated a different immune response than mothers of straight sons.
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.
If the work is replicated and it can be demonstrated that these antibodies influence brain structures associated with non-heterosexuality, e.g., the hypothalamus, then the evidence for the immune connection would be even stronger. Since antibodies in response to multiple male births would only effect a small number of gay males, there must be other pathways to non-heterosexual brain development. As I review several lines of research, I believe there are other ways in addition to the immune hypothesis (e.g., hormones, epimarks) that the neural basis for non-heterosexuality could come to be.
Genetic association study
Just last week, a genome association wide study found yet more ways in which the DNA of gay males differ from straight males. Alan Sanders and his team reported that they found those differences in a gene which is involved in helping to form the hypothalamus. The hypothalamus repeatedly shows up in studies where gay and straight males differ.
Charles Roselli has studied the hypothalamus in rams and found that “gay rams” (male oriented rams) demonstrate size differences in the preoptic area of the hypothalamus. Here is a 2013 video where he described his research: