luvluv

Read a book by liberal theologian John Spong (don't ask me why) in which he discusses sexual ethics (It's called "Living in Sin?" if you're interested).

Anyway, he suggests that there were studies showing that homosexuality in animals is strongly linked with the absence of certain hormones (usually testosterone) during specific segments of brain development.

As you all probably know, the human body is "sexed" by the introduction of hormones (or lack thereof). The body starts off as semi-default (though it will become female even if no extra X chromosome is contributed by the father). So, for example, the bit of tissue that becomes the clitoris in the female will become the penis in the male if the right hormones are introduced.

Now what Spong was saying was that there were experiments done which show that the brain is sexed by hormones as well during fetal development. Apparently, if these fetuses (in monkeys and rats, which were the test subjects) were deprived of a certain hormone (testosterone?) during a crucial period in development, their MINDS would be sexed as female.

The resultant males, when adult, would present themselves to other males to be mounted and/or attempt to mount other males exclusively.

Now, this book was written in the mid-80's, so it may be quite dated. But I was wondering if anyone has heard of this theory or if there is any recent research supporting it.

It's plausible to me because it clears up one question I've always had: If homosexuality is genetic (which I believe it to be) then why hasn't it been breed out of the race? I know that sounds harsh, and I'm not in anyway suggesting it SHOULD be bred out of the race. I'm simply saying that as a mutation (oy, that sounds hateful... sorry) it shouldn't be able to maintain as much as 10 percent of the population.

But the biochemical explanation would make more sense than either the choice position (who would choose to be gay and suffer that kind of rejection?) and the genetic position (how can a gene which greatly inhibits one's chances to procreate become so widespread.)

Still, I'm expecting to hear that the whole thing is bumpkiss.

Doubting Didymus

I can't help you with the hormonal influence stuff, bit I've been peering at information about homosexuality and genetics myself recently. There appears to be a heritable genetic influence, but certainly nothing deterministic. An interesting nugget that comes out of some fairly recent twin studies (I've seen only abstracts on pubmed, by the way) is that a male who is the identical twin of a homosexual man has 40% probability of being homosexual himself. Note that in this case, not only are the twins genes completely identical, but they also share the same developmental influences in the womb, and in a lot of cases for a long time afterwards to boot (raised in the same environment). The most interesting thing is NOT that there is a 40% chance of sharing your identical brothers homosexuality, but that there is a 60 % chance of not coming across it, even with the exact same genes and similar/same development.

What this suggests, if you take twin studies at all seriously, is that any satisfying answer to the question 'why homosexuality', is going to be a hideously complicated cascade of influences from every which direction, and might very well be attributable mostly to dumb luck.

Regarding the question of why, if genetic, homosexuality has not been selected out might have a variety of explainations. The question itself assumes a variety of things, including that homosexuals never breed. This may be less obvious than it sounds. It also ignores the very real influence of bisexuality of various degrees. There are hypotheses that paint homosexuality as an actual adaptation, a selective advantage, no less. For the record, I tend to find them unconvincing, but thats just me.

More to the point, a brief reflection informs us of a veritable horde of selective disadvantages that are well documented as having a heritable genetic component. This includes such selective menaces as heart disease, which affects far, far more people than homosexuality. The point is, we can't and shouldn't expect natural selection to produce perfect adaptedness in any population in the case of complex traits with multiple influences. In a case where one allele is bad for reproduction and the other is good, sure, we'd expect the unfit allele to piss off quick-smart, but other traits, probably especially personality traits, are nowhere near so simple in their causes and influences, and any quickandfast predictions of their frequency over a given time are therefore almost certainly bound to overlook something.

Viti

I have read a bit about this topic...I thought the twin thing was interesting for a different reason. The womb environment is not identical, they do not receive equal amounts of nutrients etc hence one twin is usually stronger and larger than the other...couldn't hormones or other biological agents be distributed unevenly? Also, often the stronger twin takes so much away that the weaker twin is born needing medical attention...could this lead to a different environment as far as bonding and attention? The smaller or weaker twin is coddled or fussed over more?

Just some musings I have had since I read about the biochemical developmental theory of homosexuality. Nothing concrete here.

Lefthandedness is also being studied as possibly developmentally related and happens in roughly the same percentage as homosexuality.

Doubting Didymus

Good points. I wonder if any twin studies have focused on that element? The abstracts I have read, (which can be found by searching for 'homosexuality twin study' on pubmed, if anyones interested), only made the correlation between identical male twins, without mentioning if the factors you point out are taken into account.

Lobstrosity

In my sexual psychology classes I heard the same thing--that studies bear out that hormones influence both physical and psychological development in the womb. Incorrect balances of hormones can lead to sexual developement (mental and/or physical) that does not necessarily correspond to chromosomal sex. This doesn't mean it is the sole cause of bisexuality/homosexuality, however.

There could potentially be a slightly less complex solution to this. As LadyShea points out, womb environments are not identical for twins. As such, what if one twin is fully homosexual whereas the other still retains some heterosexual urges (ergo is actually bisexual). Given the stigma placed on homosexuality, some bisexuals could very easily repress any homosexual tendancies and convince themselves that they are fully "straight" (or simply flat-out lie to others so as to remain in the proverbial closet). Such people could skew twin studies significantly (considering how small of a sample group such studies have access too). The problem is that these studies require perfect honesty about what can often be a very sensitive subject and accurate introspection--two things that are by no means guaranteed.


I also disagree with the notion that homosexuality is an adaptation and instead favor the notion that it is simply a "price" we pay for the remarkable economy of sexual differentiation. Simply put, sexual differentiation is achieved by subjecting a prototypical more-or-less gender-neutral human to a rather simple set of stimuli--a very economic process that as a result isn't completely robust (not all men come out wanting to mate with women and vice versa). The explanation is just that the benefit of the economy outweighs the small decrease in reproduction rate due to any non-breeding homosexuals. As an analogy, think of a computer program:

Option 1) You can write one that's quick and dirty, taking only a few minutes and few lines to achieve your desired goal. However, such a simple program will probably not be very robust and not have much error-checking. You would write it under the assumption that it would be used a precise way: perhaps there's a certain format with which the user must enter data or maybe it's only written to handle a specific file format. Well what happens when a user comes along who doesn't fully understand what the program expects? What happens when he types in data with the wrong syntax? Well, a very simple program would likely crash or at least produce erroneous output. The upside is that the program was very easy to write (very economic) but the downside is that it will only work properly if used a very specific way. Any deviations from this way cause problems.

Option 2) You can write a program that's just chock-full of error-handling--one that's as robust as you can get. It can handle almost any input syntax imaginable and if it doesn't understand what it's getting it will querry the user for more precise input. On top of this, it's been beta-tested and had nearly every bug worked out. In short, no user will be able to break the program and the output is almost always what is desired. That's the upside. The catch is that such a program is very complicated. It's very long and takes quite a long time write.

DNA is a code programmed over the millennia through nothing more than trial and error. It naturally must balance the pros and cons of options 1 and 2 above. It could get long and complicated to assure flawless results or it could stay short and dirty and produce bugs when external conditions interfered with performance. This is an optimization problem and I personally feel that the optimal solution from a cost-analysis standpoint lies somewhere between options 1 and 2. I view this as the most likely source of homosexuality, but that's just my personal opinion on the matter.

ps418

The difference in prenatal environments, whether they lead to different nurture responses by mother or not, would serve to make the two twins dissimilar rather than similar. In that case, the 40% MZ concordance would actually underestimate the genetic influence. However, if I recall correctly, more recent twin studies show a far smaller genetic influence than this.

However, I don't know how dissimilar the prenatal environment can be for the two twins specifically with respect to the concentration or androgens. Also, are you aware of any evidence that differences in maternal behavior play a causal role in adult sexual orientation? Just some thoughts.

Patrick

ps418

As far as rodents go, it is quite apparent from many experiments that prenatal androgens play a profound role in shaping sexual behaviors. You can literally get females to engage in mounting (the typical male sexual posture), and males to engage in lordosis (the typical, arched-back female sexual posture), by altering the levels of prenatal and early postnatal sex hormones. You can also get females to display more male-typical levels of agression by altering levels or these hormones. You can find lots of abstracts on this type of research here. On the other hand, what I have read about the relationship between sexual behavior in humans leads me to believe that while sexual orientation MAY be influenced by hormone exposure factors, this is probably not the whole story.

Patrick

Dr Rick

A recent study has found that polycystic ovary syndrome (PCOS), an endocrinopathy that affects women and increases androgenic (ie testosterone) hormone levels, is much more common in lesbians than straights.

Viti

Not at all...was just commenting on a previous post and some theories I have read that the developmental stages include prenatal environment as well as infancy.

I'll see if I can find it again, but one study was hypothesizing that homosexuality and left handedness have the same developmental causes.

Truly I know nothin bout nothin except that homosexuality is not a lifestyle choice. I only have looked into these things when debating with fundies on the issue (Christian Forums favorite topic seems to be homosexuality)

Viti

Here's an abstract (can I post the whole abstract or only excerpts? What is the copyright on abstracts?)

Just to be safe here are excerpts