One of the questions that has dominated the nature vs. nurture debate is that of sexual orientation -- are there physiological differences that determine who is gay and who is lesbian? Though researchers have detected differences in the brains of gay and heterosexual men, evidence of biological variations in women had never been reported until last week, when researchers at the University of Texas released their findings that the auditory systems -- specifically the inner ears -- of lesbians seem to operate more like those of men than those of heterosexual females. The findings were the result of a study in which microphones were placed in the external ear canal of 240 people who had previously been questioned in detail about their sexuality.
Salon talked with the head of the study, Dr. Dennis McFadden, a professor of experimental
psychology at the University of Texas-Austin, about the implications of his findings.
So you've found the first strong evidence of a physical difference
between straight and lesbian women?
To my knowledge, it is the first physiological concomitant to homosexuality in females. There are a few claims, as you may know, of differences in males in various brain sites. But this is, as far as I know, the first time for females.
What exactly did you find?
A normal inner ear, a cochlea, actually makes sounds -- as well as receives and processes sounds -- and these are known as "otoacoustic" emissions. The particular kind of otoacoustic emission we were studying is very much like an echo. You put in a very brief sound, a click, like the snap of a light switch, and there will be an echo given off by a normal-hearing ear. They are very idiosyncratic. The strengths or magnitude of the sounds differ from one individual's ear to another's, although they're quite constant within the individual's ear. Females tend to have much stronger otoacoustic echoes or emissions than males.
Why are females' echoes stronger?
Females tend to have much better hearing sensitivity than males. Their inner ears also tend to make stronger sounds, although the sounds have no real purpose in themselves.
I was interested in the origins of this. Something we did years back was to measure the otoacoustic emissions in opposite-sex twins. And what we found was that those females had otoacoustic emissions that were like those of males, not other females. Since we knew from other research that otoacoustic emissions are reasonably stable through life -- infants and children have them in the same pattern and manner that adults do -- the implication was that those females with male twins had in fact been born with weaker cochlear amplifiers, that is, with weaker click-evoked emissions. So that leads you to try and imagine what kinds of mechanisms might have been operating prenatally to produce this effect. What we came up with was the suggestion that those females who had male twins were exposed to higher levels of androgens than females ordinarily get in the womb. And where those androgens came from was from their male twins.
But female co-twins are not more likely to be lesbian, I assume. So
what does this say?
Right. There's not a greater incidence of homosexuality among females with male twins. But it still led us to wonder if there might be a difference between homosexuals and heterosexuals. So we did this experiment, and in fact we did find that the strengths of these echoes were smaller in homosexual women than in heterosexual women -- that is, the magnitude of their echoes was between that of heterosexual females and heterosexual males.
So why aren't more female twins lesbian?
Well, we aren't sure why. But if the suggestion that we're offering is correct -- that there's been some prenatal "masculinization" in the womb to account for both the changes in the auditory system and later on in some brain centers in homosexual and bisexual women -- it suggests that the opposite-sex female twins are not getting quite as strong a dose of the androgens, and whatever brain centers are responsible for sexual preference are not being affected by them.
There's any number of possible reasons why the latter might occur. It may be a matter of timing -- that the high dose of androgen comes along in homosexual and bisexual women at the right time to masculinize the brain centers for sexual preference, but is off some for female twins, so their auditory systems get masculinized but not the other brain centers. There's also the question of androgen sensitivity -- that receptor sites can be hypersensitive in some situations. So it may not be that homosexual and bisexual women are exposed to more androgen, but that they're more sensitive to it and are absorbing more of it.
How dramatic was the difference between the echoes in homosexual women and those in heterosexual women?
It's a big difference -- a monstrous difference. The difference between heterosexual males and females is five or six decibels, while homosexual and bisexual females are halfway in between -- that is, about three decibels weaker than heterosexual females. Remember though, we are talking about group differences, how each group differs on average. It's a continuum. You can't predict the sexual preference of an individual with any accuracy from knowing the size of the echo response. In the same way, while men tend to be taller than women, you can't predict whether an individual is a man or woman from that person's height.
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Can you give us an example of how conditions might come together in the womb to produce a lesbian fetus? I mean, why might this higher level of androgen or higher sensitivity in the fetus occur?
There are lots of possibilities. Things can happen in the mother's life that can lead to more androgen produced by the placenta. The developing fetus could be producing more androgen throughout its development -- although that's unlikely since there are no signs of a masculinized body in homosexual or bisexual women. Or androgen could be produced at certain times when it only affects the brain sites for sexual preference and the cochlea.
What could be happening in the mother's life to produce more androgen?
Stress. There may be other things too, but stress is one that's been pointed to for decades. People have tried to account for sexuality by looking at that. There are some classic studies looking at children born during the bombing in Dresden. Those mothers were under one heck of a lot of stress and it was reported that there was a greater incidence in homosexuality, particularly among female kids, if I recall correctly. More recently there have been some contradictory findings -- in part because it's not quite that simple, as every woman will respond differently to the stress physiologically.
So is the idea now discredited?
No, not thoroughly discredited. It's true, to my knowledge, that women do produce more androgen when they're under stress. That's a fact. The question is whether those androgens are responsible for producing more homosexual females or not.
What else, other than more androgens, might be a factor in producing this masculinization?
I have absolutely no idea.
Did the finding -- this dramatic difference -- surprise you?
No, because it was a logical outgrowth of other research that we had done. I've been studying the auditory system for 30 years, otoacoustic emissions for 15 and sex differences for half a dozen years, and homosexuality for two years, so we knew the fields pretty thoroughly. It's kind of an off-the-wall fact when you just hear it for the first time. It just sounds bizarre. Why in the world would the ears of homosexual women be any different from the inner ears of anybody else? But if you approach it from the context of the research that we've been doing, it makes more sense.
When I started the experiment, we didn't have very high expectations of finding physiological differences between homosexual and heterosexual women -- they had never actually been found before -- but it was worth a gamble to me. I'm glad that we did see a difference because it has a number of different implications.
It gives us additional insight to the origins of the sex differences.
But clearly your findings will stir debate and controversy. Even if you're not that interested in studying homosexuality, those who do might use your findings to further their own agendas.
Yes. I've been hoping I won't get run out of town. I did have some apprehension before I started the research, but since it's been published I've only heard positive reaction from the homosexual community, many of whom have told me that they have felt they were different for years. To hear there might be some physiological concomitant is welcome. But I'm certainly not saying that homosexuality always has a biological or physiological basis. I accept that some women may have made a decision that is solely social or political. It might well be that there is more than one type of homosexuality, and that's an area for future research.
Is it fair to say that this adds new support to the theory that
sexual orientation may be predisposed before birth?
Predisposed is a strong word. Most neuroscientists believe that just about everything about human traits and behaviors is a complex mix of heredity and environment. But the default condition of humans when they develop is female. If you don't have certain biochemical events occur at particular points during prenatal development, then you will have a female. Those events are, of course, exposure to androgens -- or what we know as sex hormones such as testosterone -- produced by developing male fetuses. But if they get those androgens from some other external source, even if it were an XX organism -- something that would have otherwise been a female -- it will be masculinized.
I know that some people take exception to the term masculinization. This a technical term. It's just descriptive of the process that can occur during prenatal development. It's not meant to be some pejorative term. It's the fact that during prenatal development some fetuses turn male. How do they do that? They're exposed to androgens.
So why might the sex hormones vary in the womb?
Individual fetuses could very well produce differential amounts of androgens. The mother herself may produce some androgens. The adrenal glands actually have the capability of producing androgens, as well as the testes. And you have to understand that everything is a mix. Both sexes have some of each, both during development and later in life.
There are also certain maladies that can produce different amounts. One is "congenital adrenal hyperplasia," when a lot of androgen is produced in a fetus by the adrenal gland and the gland goes bad -- it malfunctions and leads to the production of more androgen. When this happens in a female fetus, a fetus with XX chromosomes, you end up with what is known as an inter-sex infant. This is an infant who has what are called "ambiguous" genitalia, and surgeons usually get into the act pretty quickly and try to reverse this. They take the person back to a female appearance because, in this case, they're genetically female.
We're not talking about a malady when we're talking about homosexuality, a condition that comes about reasonably often. The estimates on homosexuality vary hugely, from 1 percent of the population to 10 percent. Most scientists believe the 1 percent figure is a gross underestimate; it's hard to determine it exactly because historically people have not been open about it. But exactly what would produce this masculinization of the auditory system isn't known, though we think it's most likely excess androgens. It's not the masculinization of the auditory system that's particularly important, but rather that it is an indication that some other brain sites that are responsible for sexual preference were also masculinized at the same time. That's the implication of the work.
How likely is it that the masculinization is the result of excess androgens?
Well, scientists are generally a cautious bunch, but it seems to be a real strong working hypothesis. I'd like to think more than 50 percent, likely. It's what we scientists call a parsimonious idea. It's a simple explanation that fits with other known facts and it explains the outcome very tidily. But could we be wrong? Absolutely!
Future research should look at androgens then?
Absolutely. I hope other researchers become interested in this and will redouble their efforts to find those brain centers that are responsible for sexual preference and sexual orientation in females.
Which might be where?
The hypothalmus is a center that gets a lot of attention from sex researchers looking for centers that are driving these behaviors. But in a number of cases they've searched the hypothalmus for differences between those who are homosexual and heterosexual and only found them in males. Maybe it's the wrong place to look.