There appears to be an increase in news relating to the science of sexual orientation. Why is this?
Starting in the early 1990’s there was a renaissance of interest in biological research on sexual orientation. To some extent this was probably a function of new technologies coming online, which allowed a lot of old questions to be reexamined.
Anti-gay activists claim there is no evidence for a biological influence on sexual orientation. What is your response?
At this point, people who claim there is no evidence at all for biological influences on sexual orientation are simply saying things for which there is no basis in fact. The cumulative evidence which has been building since the early 1990’s has all pointed in the direction of some kind of biological influence on sexual orientation.
Pseudo-scientific groups like NARTH blame parents for causing their children to be gay. What does modern science say about this view?
Notions that the childhood environment or the rearing styles of parents influence sexual orientation have now been around for the better part of 100 years, if not for 100 years. This is a very long time that people have had to bring forward hard data that would persuade everybody that there is at least some contribution of early family environment. And yet, in all these decades, nobody has come forward with any hard data that is proof that there is a positive contribution of family environment.
What does the latest research say about the origins of sexual orientation?
In the field of genetics there are studies that compare similarity of sexual orientation in identical and fraternal twins and point to some genetic contribution. The studies that have shown that homosexuality tends to run in families point towards some biological contributions. And now that the DNA research is starting to be published, this is bench science confirmation that there is probably some genetic contribution to homosexuality.
Similarly, there are some kinds of research that bear on hormonal explanations. In particular, research that is now pretty reliably indicates masculinized finger length ratios in lesbians points towards some kind of prenatal event that is associated with homosexuality and argues that the homosexuality itself probably also has prenatal origins. And the work that I’ve been doing, which is work on older brothers and homosexuality is also beginning to point towards some kind of prenatal, congenital influence on sexual orientation.
The research has pretty consistently shown that gay men tend to be very slightly shorter. A difference in height between gay and straight men per se would not be of much interest. It is only of interest in the sense that it’s an indirect marker that whatever happens to cause homosexuality has some biological cause, some prenatal cause.
Your research shows that the more older brothers one has, the more likely he is to be gay. Talk about your work.
I knew a hot research finding when I saw one and I realized that although I really didn’t know what could it possibly mean, that gay men had more older brothers than straight men, I thought it had to mean something and that it was interesting and worth pursuing.
It took me a long time to formulate what I thought might be a reasonable theory of why older brothers increase the odds of homosexuality in later born males. And, the theory that I later arrived at had been suggested in rudimentary forms by earlier researchers, but hadn’t received much attention and really wasn’t influencing my own thinking that much. I felt that almost certainly had to be happening was some kind of a maternal immune response…it looks as if the mother’s body is in some way remembering the number of male fetuses she has carried and ignored the number of female fetuses. The only part of the maternal body that has that kind of memory, except for the brain, of course, is the immune system. So, I felt that the epidemiological data indicated that there had to be something in the mother’s immune system that was clocking down the numbers of male fetuses she had previously carried and was somehow changing the uterine environment for later males.
The data shows that the older brother effect is very real. Talk a bit more about your theory on how it works.
The full-blown theory goes like this. We know that there are certain proteins in male cells that either have counterparts in females that are different or have no counterparts in females. We also know that fetal material gets from the fetus into the maternal circulation. Especially at childbirth where there is a lot of tearing of maternal tissue and a lot of fetal material gets into the woman’s circulation. So, I hypothesize that tissue from cells or cell fragments from a male fetus get into the mother’s circulation. The mother perceives these male substances as foreign and develops an immune response to them in the form of anti-bodies. These anti-bodies now cross the placenta barrier when she has another male fetus. And affect the male fetus in such a way as to partially affect differentiation of the brain in a male typical fashion. So, that the part of the fetuses brain that controls sexual orientation is left in the default position of sexually preferring males rather than females.
The older brother effect only works if one has older brothers. What about only children — such as myself?
Many times when I’ve presented this research to audiences somebody says to me, “well what about only children? Or, gay guys who have older sisters but no older brothers? How would your theory account for that?” And, the answer is, my theory doesn’t account for it, and I’ve never tried to account for it. I have been very explicit in my research to say that I think that the fraternal birth order effect only accounts for some proportion of gay men. I did two studies to get at this question. One study estimated the proportion of gay men who are gay because of older brothers at around 15% and a second study which used a different sample and different statistical approach and estimated the proportion of gay men who are gay because of older brothers at around 29%. I don’t think exact percentages matter. But, the take home message here is that the fraternal birth order effect accounts for a minority, but not a trivial minority of gay men. So, first born gay men are obviously caused by the other seventy or eighty five percent of factors that cause homosexuality, which could be many different genes acting together or possibly hormone levels that fluctuate at just the right time in the right way. My theory is not intended to explain all of homosexuality in men. It is only to explain some portion that got there via an older brother via immunization root.
It still seems odd that the older brother effect exists. Were you surprised by it in your research?
Even after the second or third time that I found results indicating this excess of older brothers in gay men, I still had a hard time with it, because it just seemed so bizarre to me. So, I kept doing the same study in effect over and over again. After I had done it five or six times, I was always looking for one more sample to look at it again. Because I needed a lot of subjective…convincing myself that something this odd could be true. It wasn’t like at some moment I said “a-ha, I’ve shown it.” I had to talk myself into believing it, so there came a point in which people were, friends were kind of making fun of me like I couldn’t stop. I kept looking for another sample to show the same thing over and over again. And they would say to me, like, “you’ve shown it already you can stop now.” But, I always wanted to show it one more time. Because I thought something this odd requires an exceptional amount of data to believe it.
So much more data has built up showing the relation between older brothers and homosexuality. And from so many different countries, that people kind of have to say, yes, this must be true. And, along with the data now seeming inevitable, people became more accepting of the idea that a maternal immune response was a plausible explanation. However, there still has not been a laboratory demonstration of this phenomenon, and in fact the very first study that would approach this in a laboratory way is only now being conducted.