This is dry stuff; you’ve been warned. :-)
As I left off (here), having older brothers increases the chance that you will be gay (by about 33% per older brother). In fact, of those with an older brother, it’s calculated (by statistical magic ;-)) that 24% of them may attribute being gay to that fact (43% for those with two older brothers and so on) (1). Anyway, that effect is there but how does it work? (I’ll be greatly following the review in reference 1, but will deviate)
The theories here may be broken down into pre and postnatal causes.
To recap, evidence has accumulated to the extent that the hypothesis of a postnatal fraternal birth order effect is becoming relatively defunct (2). Early sexual experience (3,4,5) and any other effect of being raised with an older male child (6) has greatly and recently been ruled out. In fact, the older brother need not even be present in the home. Only a womb need be shared, strongly suggesting the womb is the place to look for a cause.
Another important indication that a prenatal cause is in play came surprisingly from measurements of birth weight (1). Blanchard and Ellis seem to have been the first to look at such a relationship in gays (7). They analyzed the birth weights of 2599 heterosexual females, 1111 heterosexual males, 125 homosexual females and 208 homosexual males some with the same biological mother. It was found that:
1. Men with older brothers weighed less at birth than men with older sisters.
2. Gay men with older brothers weighed less at birth than heterosexuals with older brothers.
3. Gay men with no older brothers weighed the same as heterosexuals with no brother.
With Zurcher, these findings were repeated (8), in part. The first finding was not found in one subsequent study, but, as the authors admit, this may be due to the fact that their controls were all suffering from clinical disorders and found from a psychiatric hospital. One has to be careful from where one picks their “average” human ;-). But that first finding was repeated by Cote (a bunch of crazy French accents on that) (9).
It may also be important in finding this mechanism to note that gays seem to run along the maternal line more frequently (10). I’ve got’em on both sides. This could indicate there’s something passed in the genes from mother to male child, but it could also be evidence that the maternal line has characteristics that cause the mother’s biology to influence her child towards homosexuality.
Regarding these findings a “single-mechanism” hypothesis has been proposed. Here there is one cause from the mother for both the low birth weight and homosexuality. So, if this mechanism isn’t present (no older brothers) there’s no effect on gayness or weight (#3 in the above findings). A bit more of this mechanism and baby boys are slowed in their growth, and made more effeminate (#1 above, along with Reference 1). Finally, a high dose of this mechanism leads to homosexuality (#2 above).
The front-runner as to what this mechanism could be seems to be the maternal immune hypothesis (11,12). That’s not to say others are not possible. Neither is that to say there aren’t other prenatal effects, such as hormone exposure (to which I’ll eventually get but I’ll limit the focus in this post to the cause of the fraternal birth order effect only).
It is known that a variety of cell types from the fetus enter the mother during a typical pregnancy, and do more so during birth and throughout abnormal pregnancies (21-23). In the maternal immune hypothesis, such wondering male cells are more readily recognized by the mother as foreign, due to their sex. With each exposure, each pregnancy, she may develop a progressive immunization. The related antibodies may then cross the placental barrier in future pregnancies to attach to the same male markers in the first child’s younger brother, thus altering his development, attacking what would have made him fully male (thus no such effect in lesbians). Importantly, in this theory each older brother would increase the chance that the next will be gay, as we see in the data.
Now, this is the prevailing theory, no one has found a smoking gun in humans, and how to do so ethically, in a simple manner, may be problematic. But such a theory is not without it’s evidence. As early as the first days of a pregnancy, a H-Y immune response (an response to molecules expressed by male but not female cells) has been shown in mice and cattle (24-26). This, of course, also means this effect may be in play even if you have no older brother, and it may be even stronger than the current data suggests, which only uses live births. Also, immunization to paternal antigens have been known to affect birth weight in rats (13), and mice (14, 15). Unfortunately, such experiments on humans would be frowned upon by anti-science fanatics ;-).
In humans, though, it’s not a theory without a real, and well-understood example: hemolytic disease or HDN. A woman who is Rh-negative and carrying a Rh-positive child (due to a Rh-positive father) develops an immune response to the child’s blood, which enters her system primarily during birth. Subsequent Rh-positive children are affected more and more by this attack, as seems to happen for the fraternal birth order effect. The first child is fine, the second may be anemic, and the third may never make it to term, as the mother’s immune system destroys their blood (16). It’s also important to note, adding strength to the maternal immune hypothesis on gays, it’s been shown the immune response in HDN is more likely to occur in male children, indicating mothers recognize and attack male cells more often as foreign than female cells, as one might expect (17-19).
But HDN affects the blood, and we’re talking about the brain. Here too there is evidence of a mother’s immune system playing a role. Serum from women who’ve produced autistic or dyslexic children, when injected into pregnant mice, produce mice with mental deficits (27,28). Also, in humans, boys with cognitive disorders show the same fraternal order effect that we see in gays, but girls do not (29, 30).
As to the specifics of this theoretical immune response, a number of H-Y antigens have been discovered. Of those, we do know of some good candidates. Of the 27 known proteins and protein families encoded on the Y chromosome, 12 are expressed throughout the body, and three are known to be specific to the brain and two of those on cell surfaces (20). Both of those are instrumental in cell-cell adhesion and thus the structure of the male brain. But who knows? It could be a protein encoded on another chromosome and only regulated by the Y chromosome. Much is to be done.
Let’s end it there. Though there are a couple of other variants on this theme involving the placenta and some hormonal effects, that’s plenty for now. To sum up, we see this fraternal birth order effect as one of the more prominent predictors of homosexuality, and it seems we are closing in on its mechanism. It will be interesting to see how it all plays out. For example, right now we have treatments for HDN, preventative injections. Maybe the question will someday be there for mothers with such antigens: Do you want to stop your infant from being gay?
I’ll take a break from the research for a while.
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