Thursday, April 9, 2009

Origins of male homosexuality - Conclusion

What causes exclusive male homosexuality? This is the question I’ve addressed in the last few posts. The answer is still elusive although there seems to be consensus on some points.

One such point is the relative importance of inborn causation versus environmental causation. In men, an exclusively homosexual orientation has a heritability of 30-45%. A genetic cause must therefore be interacting with an unknown but more important environmental cause (Bailey et al., 2000). The genes in question are likewise unknown but may be located near the ones for RH factor (Ellis et al., 2008).

This raises another question. Why would natural selection create a genetic predisposition, however minor, to become exclusively gay? The answer is probably the one put forward by Ed Miller (2000). Human evolution has seen a relatively recent increase in provisioning by men of their mates and offspring, together with a corresponding decrease in polygyny. Even among present-day humans, this evolutionary trend has gone further in some populations than in others. How, then, did natural selection change male behavior over so little time? Miller’s answer: by partially feminizing the male mind (i.e., by impeding cerebral masculinization during prenatal and neonatal development). This is the fastest way with the least genetic change, the only downside being an increased risk that some boys will grow up with the wrong sexual orientation.

For several reasons, I disagree with Miller on one point: I believe that the downside of this rapid natural selection has not been a certain proportion of exclusively homosexual men but rather a larger proportion of weakly heterosexual men. First, as noted earlier, the genetic predisposition is not acting alone. It seems to be interacting with a more important cause of environmental origin. Second, I have trouble believing that a balanced polymorphism could maintain 3-5% of all men in a state of sexual indifference to women. (I incidentally feel the same way about the ‘gay uncle’ theory, where the reproductive cost of indifference to women is balanced by care given to related children—the cost seems too high to offset the presumed benefit). Third, I have trouble believing that 3-5% of all men were sexually indifferent to women before the late 19th century. Male homosexuality is attested in earlier time periods but usually in a facultative form, i.e., older heterosexual men having sex with boys or with males of servile status.

In my opinion, the likeliest scenario is one where a genetic predisposition weakens male heterosexuality but is not enough, in itself, to cause exclusive male homosexuality. Something in the environment has to push some of these heteros over the borderline. If so, there must be a large population of weakly heterosexual men, certainly much larger than the 3-5% who end up being exclusively homosexual.

Alongside this scenario would be a residual of various other causes—random genetic mutations, psychological or environmental stresses during pregnancy, chimerism, etc.—that each occur at such a low rate that natural selection cannot effectively counter any one of them. These residual causes might account for a baseline of exclusive male homosexuality that has always been with us, perhaps less than 1% of all men.

The current level of 3-5%, however, is much harder to ascribe to longstanding causes. I suspect it’s recent, essentially since the late 19th century, and due either to a pathogen that is co-evolving with its host population or to a recent environmental factor that humans have not yet overcome through natural selection.

If a pathogen is responsible, it may have become more prevalent because of the great increase in urbanization at that time or perhaps because some other factor had increased transmissibility. The main problem so far with the ‘gay germ’ theory is simply lack of evidence. Where is the smoking gun?

If a new environmental factor is responsible, it may be some kind of estrogen or estrogen-like compound in the neonatal environment. The late 19th century, however, is too early for most candidates. In fact, there seem to be only two credible ones. One is borax, which was used as a food preservative until the 1950s. The other is estrogen-rich drinking and bathing water from sources contaminated by untreated wastewater. Such effluent greatly increased in volume with the introduction of modern sewer systems in the late 19th century and decreased only with conversion to secondary and tertiary wastewater treatment during the 1970s.

How should we test these different theories? First, we should do what researchers normally do: conduct controlled studies and debate the findings in academic journals. Unfortunately, very little of either is going on. J. Michael Bailey, Lee Ellis, and the people around them, seem to account for much of the current research (and even Bailey runs into a great deal of flak). Most real debate actually seems to be happening in the blogosphere. This may or not be a bad thing, but it does say a lot about the climate surrounding this subject.


References

Bailey, J.M., M.P. Dunne, & N.G. Martin. (2000). Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. Journal of Personality and Social Psychology, 78, 524-536.

Ellis, L, Ficek, C, Burke, D, & Das, S. (2008). Eye color, hair color, blood type, and the rhesus factor: exploring possible genetic links to sexual orientation. Archives of Sexual Behavior, 37(1), 145-9.

Miller, E.M. (2000). Homosexuality, birth order and evolution: Toward an equilibrium reproductive economics of homosexuality. Archives of Sexual Behavior, 29, 1-34.

43 comments:

Tod said...

The Borax content of US food was probably at its peak before 1910. 'Poison Squad' Wiley, aided by a Pure Food movement harnessing the purchasing power of housewives, achieved a ban of borax use in meat products(The Meat Inspection Act of 1906). Heinz Food Co. became market leader though their funding a campaign against borax, which most ketchup manufacturers had stopped using by 1915. (documented in 'Pure ketchup' By Andrew F. Smith)

Counterintuitively, low dose estrogens can have a beneficial effect on fecundity, i.e. improve reproductive success.

The Hormetic Dose-Response Model Is More Common than the Threshold Model in Toxicology

Altered reproductive success in rat pairs after environmental-like exposure to xenoestrogen

"At first, it may appear surprising that 4ngkg−1d−1 EE induced an increase in the reproductive success of exposed pairs. However, a review of the literature shows that such effects are in fact very common. Similar doses of EE increase egg viability in the cunner (Tautogolabrus adspersus, Gutjahr-Gobell et al. 2006), and increase fecundity, i.e. the mean number of spawned eggs per pair, in the fathead minnow (Pimephales promelas, Pawlowski et al. 2004), two species of fish. At present, we do not know the mechanisms that are responsible for the increased fecundity in the EE4 pairs six months after the exposure to EE has been interrupted. Inverted U-shaped dose–response curves where low doses have stimulating effects are in fact commonly observed, a phenomenon called hormesis (Welshons et al. 2003). Hormesis is generally thought to be an adaptive response which may reduce subsequent stress, but in the case of xenoestrogen, the effects are unlikely to be beneficial (Weltje et al. 2005)."

But Wikipedia says "Scientists point out that low dose stimulation can have extremely adverse effects. For example, research by Retha Newbold at the US National Institute of Environmental Health Sciences has shown that while relatively high doses of a xenobiotic estrogen, diethylstilbestrol, during fetal development cause weight loss in adulthood, extremely low doses cause grotesque obesity. Similarly, low doses of the phthalate DEHP cause increased allergic responses to allergens, while higher doses have no effect. Low dose stimulation can have profoundly adverse consequences.

William said...

Interesting series of posts, Peter. I have to say that this, from your lastest post, best sums things up:

"The current level of 3-5%, however, is much harder to ascribe to longstanding causes. I suspect it’s recent, essentially since the late 19th century, and due either to a pathogen that is co-evolving with its host population or to a recent environmental factor that humans have not yet overcome through natural selection."

Yes, after all is said and done, it appears more likely than not that this strange anomaly has an infectious origin, doesn't it?

As for this line from your paragraph above--"due either to a pathogen that is co-evolving with its host population..."

If you wouldn't mind, could you explain how in men who were exclusively homosexual(or almost exclusively) a pathogen would have opportunity to "co-evolve with its host population" since such men would produce no offspring or few offspring.

Are you thinking that carriers of such a pathogen would pass it along through sexual means to young boys, boys who then themselves become homosexual and who years later pass it on? This scenario, of course, is the most politically inflammatory for it involves pederasty/pedophilia.

Or are you thinking that such carriers could pass the germ along through other common means such as coughing, sneezing, etc. to babies/toddlers since the immune system is has not reached its eventual strength until about age 7?

I think Mr. Cochran has supposed that the germ in question is likely to be one that is quite common in childhood and that in only a few % of people does it wind up in the brain, doing selective damage. In this scenario, the damage done is a side effect of the original infection the way sterility and other things are side effects of measles, for example.

Certainly numerous childhood infections ranging from respiratory infections to ear infections to gut problems can wind up causing inflammation of the brain. This we know. In severe cases, of course, such encephalitis would lead to a child's being hospitalized. However, mild CNS inflammation can cause no more than fever, headache, etc. from which the child "recovers," no worse for the wear in the eyes of the doctor or the parent. Perhaps we have not considered that symptoms of headache, fever, etc. were indicative of a much more serious effect.

On the other hand, I suppose it's possible, if not probable, that such a germ might have a strategy of sorts, a life cycle that took it from a host to someone else. I've not read that this is Mr. Cochran's hypothesis. Is it yours?

Such a strategy by such a germ would lead us, then, to ask "How does this germ get passed from one to another?"

In any case, why does it stay at the 3-5% level? Or does it? Are rates of "infection" possibly rising or going in the opposite direction?

If the 3-5% number is stable does that mean that a certain population is biologically vulnerable because of a confluence of factors?

Is it perhaps like the feline leukemia virus, where some cats fall victim because of constant exposure to the virus (shared litter box, a mother cat or sibling who is a carrier). Is there a family member carrying this virus until at some point a child with a particular susceptibility, (blood type, Rh factor, HLA type, depressed immune system) falls victim to it?

So, supposing it is a pathogen, what means of transmission are likely?

As for studying this. Yes, of course, it should be studied. My guess is that while some interest groups are quite upset by such an hypothesis, the average person who has not yet had children would be quite enthusiastic to hear of such an idea, for it immediately leads them to the notion that infection could be prevented.

Tell a man or a woman wanting children that homosexuality can be prevented and you'd have the masses angry at attempts to block such research. It's just that the average bloke has no knowledge whatsoever of such a hypothesis. He has been fed a lot of "genetic" rubbish.

The key to public pressure is public knowledge. The blogosphere itself has spread the notion, but I do think it's time Mom and Dad heard about it. They outnumber the others. Can you imagine an elected official trying to tell the electorate that a common childhood infection could prevent homosexuality, but that he opposed such research?

Anonymous said...

Quite frankly, 3-5% exclusive homosexuality is probably in the noise as far as reproductive success is concerned, surely.

Given that there is good evidence than somethink like 50% of males fail to pass their genes on, what does it matter that 3-5% are exclusively homosexual?

The only impact I can imagine is that it makes it that much harder for females to find an acceptible mate. They have to go further down the pile when otherwise high-qulaity males are homosexual. However, the question then becomes: What is the distribution of exclusive homosexuality compared to the selection function women employ? Is it flat or is it that exclusive homosexuality signals a mixture of genetic and developmental causes that would otherwise make that male a poor choice?

William said...

Sorry, I should have re-read this sentence before submitting: It should read "Can you imagine an elected official trying to tell the electorate that a common childhood infection causes homosexuality, but that he opposed such research?"

The poster after me commented,

"Given that there is good evidence than somethink like 50% of males fail to pass their genes on, what does it matter that 3-5% are exclusively homosexual?"

As I understand it, from an evolutionary standpoint the anomaly is not that such a percentage of men do not mate and pass on their genes; rather, the anomaly is that they fail to try, that they are not just unsuccessful, but are instead, disinterested.

Jason Malloy said...

"In my opinion, the likeliest scenario is one where a genetic predisposition weakens male heterosexuality but is not enough, in itself, to cause exclusive male homosexuality. Something in the environment has to push some of these heteros over the borderline. If so, there must be a large population of weakly heterosexual men, certainly much larger than the 3-5% who end up being exclusively homosexual."

Research does not appear to support the concept of weak heterosexuality. At least not as the term suggests a sort of spectrum of orientation between heterosexual and homosexual-- e.g. Bailey's failure to find a bisexual arousal pattern even among self-identified bisexuals. (Of course, there are spectrums for sex drive and mating effort, but neither high mating effort or sex drive are definitional characteristics of heterosexuality.)

And the data still show that there are less bisexual men (defined by either identity, behavior, and desire) than there are homosexuals, which does not dovetail with a spectrum.

I'm also not eager to interpret female fashion trends as ready guides to male arousal-- they're fleeting, and not obviously optimized to male sexual tastes. The flapper ideal it would seem was consciously trying to make a feminist statement by imitating men in a lot of ways, and flappers probably were more masculine in appearance given their temperaments and relative promiscuity. So even if the flapper look did arouse men, it might've just been an appealing Pavlovian association with casual sex. (A similar effect exists for modern porn stars.)

I'm skeptical that there is much variation in how much facial and bodily masculinity men prefer in women, as opposed to how much they are willing to tolerate or trade-off for. Prisons, etc, suggest many, if not most, men will engage in facultative homosexual behavior if the alternative is no sex partner at all. I assume much the same trade off exists for sex with masculine, old, and ugly women.

Peter Frost said...

Tod,

If I'm not mistaken, diethylstilbestrol, is chemically more stable than natural estrogens, i.e, it tends to hang around in the body for a longer time without being broken down. So a small amount could have a considerable impact.

William,

Greg Cochran's argument is that a pathogen induces a homosexual orientation in its male host as a way to facilitate its survival and reproduction. As far as I know, he has not elaborated on this point. It may be that he sees anal copulation with young boys as the pathway by which the pathogen enters new hosts. I really don't know.

Anon,

Fifty percent of males don't reproduce? That may be true today. I'd be surprised if it were true before the 1960s. I believe the figure is closer to 20% for earlier generations.

In any case, those men too are being selected out of the gene pool, they and their genetic characteristics.

Nor do I see how this point could justify a genetic causation for male homosexuality. If the causation is genetic, how does this behavior propogate itself from one generation to the next?

Jason,

You're assuming that weak heterosexuals should be bisexual. I wouldn't make that assumption, for the very reasons that you present.

As I see it, a population of neurons controls sexual orientation. If the neural population increases beyond a certain threshold during prenatal and neonatal development, the child will have a man's search image. If the neural population remains below the threshold, the child will have a woman's search image.

The reason why there would be a threshold (as opposed to a straight linear relationship) is that no situation exists where natural selection would create a bisexual search image. Natural selection would tend to create a mechanism that generates one search image or the other but not both at the same time.

This being said, I can envision a situaton where a search image develops imperfectly, being primarily that of a woman while having some male parameters. This is what I had in mind when discussing the flapper/garconne look of the 1920s.

In the late 1980s, I interviewed elderly people who were young during the 1920s and who remembered this fasion era. None of them saw the flapper look as a feminist statement. Moreover, if we look at the fashion designers who helped create that look, none could be considered feminist. The most prominent one, Gabrielle (Coco) Chanel was either apolitical or rightwing.

Anonymous said...

Peter said...

"Greg Cochran's argument is that a pathogen induces a homosexual orientation in its male host as a way to facilitate its survival and reproduction."

Greg has mentioned that such a pathogen might be like the polio virus which, for reasons we don't understand, wandered into the nerve fibers in a small percentage of people from its common, usually innocuous place in the gut.

Do some viruses lose their way?

Where's a good virologist when you need him?

Anonymous said...

Peter, you say:


Natural selection would tend to create a mechanism that generates one search image or the other but not both at the same time.


However, the DNA of each individual has to code for both search images. Perhaps one is the default (female to male) with the other being the different state, requiring estradiol in the correct cells during development, or whatever.

One can easily imagine things going wrong, such that the wrong search/match criteria develop. Ie, in a genetically male individual, the normal female search image develops. In a genetically female individual, the normal male search image develops. However, in some individuals, perhaps both images are expressed.



One can easily imagine developmental mistakes that

Anonymous said...


Fifty percent of males don't reproduce? That may be true today. I'd be surprised if it were true before the 1960s. I believe the figure is closer to 20% for earlier generations.

In any case, those men too are being selected out of the gene pool, they and their genetic characteristics.


The coalescence times for mtDNA goes back twice as far as that for Y chromosomes. The explanation I have heard for this is that over long periods of time something less than 50% of males have passed on their genes, while something more than 80% of females have done so.

Secondly, yes, those males who are not selected by females are being removed from the gene pool. However, being homosexual is no less of a problem than being simply unacceptable to females.

That is to say, if you are male, being homosexual is the least of your problems!

Anonymous said...

The main problem so far with the ‘gay germ’ theory is simply lack of evidence. Where is the smoking gun?

For obvious political reasons the gay germ theory can not be investigated in the western world. Scientists in Asia will solve this riddle when they get around to it.

Jason Malloy said...

"The reason why there would be a threshold (as opposed to a straight linear relationship) is that no situation exists where natural selection would create a bisexual search image. Natural selection would tend to create a mechanism that generates one search image or the other but not both at the same time.

This being said, I can envision a situaton where a search image develops imperfectly, being primarily that of a woman while having some male parameters. This is what I had in mind when discussing the flapper/garconne look of the 1920s."


Ostensibly you were talking about flappers indicating a population wide shift in male sexual search image, which seems to carry the spectrum logic to me. How many men are you suggesting developed imperfect search image?

And why would search image develop imperfectly only by, say, causing a 10% male search image (slightly mannish preference) in 70% of the male population, and a 100% male search image (total mannish preference) in 3% of the male population?


"In the late 1980s, I interviewed elderly people who were young during the 1920s and who remembered this fasion era. None of them saw the flapper look as a feminist statement"

Feminist or not, flappers had a wider "masculine" agenda and/or set of preferences, that is not easily reduced to a different look to cater to new male sexual tastes.

Even today, fashion is primarily a visual conversation between women, that isn't exactly engineered to please men. Fashion shows certainly aren't trying to cater towards men.

Absolut.Feminist said...

As for germ theory, does this germ have *any* affect other than causing male homosexuality? Is there any other kind of virus with such a narrow range of symptoms?

I heard one explanation for male homosexuality was that gay men were likely to be younger siblings and therefore were part of larger families. Thus, male homosexuality is tied to greater fertility of the mother and homosexuality is actually passed down through a gay man's sisters. But is there any evidence that gay men have more siblings than straight people with no gay siblings?

Is your opinion that male homosexuality, male bisexuality and female homosexuality are all caused by different factors that might have nothing to do with each other?

Anyways, I have some unproven but intuitive insight about what causes female homosexuality. I will write about this in my blog sometime. I do think what causes female homosexuality is the opposing complement to what causes male homosexuality.

Sean_MacCloud said...

"Secondly, yes, those males who are not selected by females are being removed from the gene pool."

If we're talking about the earlier human past and non human mammals, a bigger problem for the males than not being "chosen" by females is not being able to get passed other males.

Only in the modern human western period is the female able to pick or dismiss (eg like birds).

We have been conditioned by politics to not see what I'm getting at.

Simply, that male mammals (and therefore earlier humans) are not what they are because of female choice acting like the prime selector, but rather because the male gauntlet that forms around female sexual value acts like that prime selector.

Female human infidelity (expressing in the modern world as female chicanery, deception, secretiveness etc) is not accouting for the weight of genes passing on once.

Note the large shoulder, large forearmed, bearded and inventive male human... It is simply that large shouldered, forearmed bearded inventive males were winning the fights more often. It wasn't females choice that created those features. It was however her unwitting sexual value and troublesomeness instigating fights though.

I find it fascinating that people who think about this stuff and are intelligent don't see this for themselves.

...The power of La Femme Mystique...

Absolut.Feminist said...

"I find it fascinating that people who think about this stuff and are intelligent don't see this for themselves."

I totally agree. I have always been baffled by the wistful theories about how women desire a strong provider and that such things have driven human evolution.

For thousands of years of recorded history, women are mated to men either because they are married off soon after they reach puberty or because a rival tribe kills all their men and keeps them as concubines. Female choice is almost entirely limited to cuckolding.

This could even partially explain the prevalence of gay men since many societies were such that a male needed children to support him in his old age and the pressure to obtain a wife was not something he could avoid. Perhaps homosexuality is a genetic trait that became possible because so many primitive societies forced gays to marry women and produce children.

Anonymous said...

"As for germ theory, does this germ have *any* affect other than causing male homosexuality? Is there any other kind of virus with such a narrow range of symptoms?"

It may. There is some evidence that gay men may be more neurotic, that they are more prone to compulsivity/impulsivity disorders, more vulnerable to emotional disturbances, but the data are hard to collect, for it's a "touchy" subject, rife with political correctness.

Read about narcolepsy. You see that it results from a very selective destruction of a small population of brain cells.

Sonny said...

People who keep animals can treat them or feed them in such a way that they turn out homosexual sometimes. Sometimes it's scientists doing animal experimentation, using feeds or environments that are extreme deprivations from the natural, that happen to produce homosexual behavior. Do we need to have a long discussion about the evolution of sexual preferences and hypothetical "search images" and sexual dimorphism for each of those species?

To find out whether there's an occurrence of homosexual behavior in humans that's unusual for a mammalian species, would we need to have some humans as a control group, living in natural conditions, eating a natural diet, which we don't have. All we have is the experiment of humans living in captivity or almost so, in artificial environments we create, eating crops we've created and other foods we've processed to unrecognizability. Even the supposed "primitive" peoples still living their pre-civilized ways have crops and cooking and drugs.

Maybe the amount of unnaturalness of human chemical and social environments has been effectively this much, or enough as far as the question of being able to produce homosexuality, for thousands or tens of thousands of years. Then there would be a question whether human cultures had some traditional development toward or away from producing homosexuality, possibly an evolutionarily stable development. However, there's no question that modern industrial culture does produce homosexuality and various other interesting differences from expected biological norms, and isn't an evolutionarily stable system. Living with steel machines and the chemical industries and built environments and diets associated with them is something invented just yesterday, even in terms of the speed of cultural development by tradition.

After stating my opinion in response to the debate here, I'm looking at the post again, thinking of answering what it's really asking about.

There are cultures that do care about increasing the chance children will be normal heterosexuals within the definitions of those cultures. Most of them aren't scientific enough and affluent enough to do research into what might be causing homosexuality at a greater rate since the industrial revolution, or if that is so. However, conservative Catholics and Mormons are some likely groups to fund research on that through hospitals or universities, if there is a prospect of some result that will be useful advice for their members.

Anonymous said...

Does either BYU or Notre Dame have quality biology/microbiology programs/facilities? I truly don't know.

Anonymous said...

Peter,

I understand concordance rates for MZ and DZ twins, but I don't really understand how the following was determined:

"In men, an exclusively homosexual orientation has a heritability of 30-45%."

Can you give me a brief explanation of how "heritability" was determined in this case?

Tod said...

Jason, Flappers were trying to attract men because - as Peter has pointed out - at that time "there were more single women than single men in all age groups. If a woman was still unmarried at 25, people considered her doomed to spinsterhood."

Consider: black, previously considered a scandalous colour for women to wear, became fashionable in the twenties. Previously only servants and women in mourning wore black.

Reduced masculinity = weak hetrosexuality?
---------------------
Do Gentlemen Really Prefer Blondes?: Bodies, Behavior, and Brains--The Science Behind Sex, Love, and Attraction Gives your theory a mention(not for blue eyes though)

Jason Malloy said...

Jason, Flappers were trying to attract men because - as Peter has pointed out - at that time "there were more single women than single men in all age groups. If a woman was still unmarried at 25, people considered her doomed to spinsterhood."Todd, women don't try to secure husbands by demonstrating how loose they are and how much they like to party.

Flapper masculine behaviors extended far beyond appearance to myriad lifestyle aspects like driving and smoking. A theory that postulates flapper masculine fashion was intended towards shifted male visual tastes does not account for the fact that flappers adopted all kinds of consonant masculine affectations that had nothing to do search-image relevant appearance.

You can't take the fashion out of its broader lifestyle context. The whole package must've had a different underlying set of motivations.

Tod said...

You can't take the fashion out of its broader lifestyle context. The whole package must've had a different underlying set of motivations. Maybe so, but the fact that it took off as a look says something; flappers were attractive to men despite being a change from everthing previously.


"Hemlines rose - calves and (gasp!) knees were visible; much more skin was shown. With all the men who died in the war, there was a shortage of them. This compelled women to “advertise”, in a way: they wore rouge, lipstick, and eyeliner (which, up to this point, had been saved for the loose women.)" here “By spending too much of their time flirting, Flappers outraged feminists"The entertainment sex symbols of their day were flappers
"[...}actresses, such as Clara Bow, Louise Brooks, Colleen Moore, and Joan Crawford, would soon build their careers on the same image, achieving great popularity"
The movie moguls catered to what was attactive to male customers.

Peter Frost said...

Jason asked:

"Ostensibly you were talking about flappers indicating a population wide shift in male sexual search image, which seems to carry the spectrum logic to me. How many men are you suggesting developed imperfect search image?

And why would search image develop imperfectly only by, say, causing a 10% male search image (slightly mannish preference) in 70% of the male population, and a 100% male search image (total mannish preference) in 3% of the male population?"

Jason,

I'm arguing that the male population varies in its closeness to the threshold for homosexual orientation. Even if the hypothetical environmental cause acts similarly on all men, the effect would vary because some men are more vulnerable than others.

In most men, the search image would still be recognizably female, albeit with faulty inputs here and there. In a minority of men, so many of the inputs would be faulty that the most stable configuration would be recognizably male.

I don't believe true bisexuality exists. For such a thing to exist, there would have to be two separate and fully functioning search images. It would be like a man with two pairs of nipples: one male and the other female. I suspect that most 'bisexuals' are simply hypersexuals with low thresholds for sexual excitation.

Anon,

The heritability estimates come from Bailey's work. If I remember correctly, twin studies tend to produce estimates at the high end whereas family studies produce low end estimates.

Anonymous said...

I don't believe true bisexuality exists. For such a thing to exist, there would have to be two separate and fully functioning search images.
I agree with this. It seems highly unlikely that two sets of search image machinery would be assembled, and that one was same-sex and the other opposite sex.

It would be like a man with two pairs of nipples: one male and the other female.
I didn't realise that there were male and female nipples. How wonderful evolution is.

Sean MacCloud said...

"If we're talking about the earlier human past and non human mammals, a bigger problem for the males than not being "chosen" by females is not being able to get passed other males.

Only in the modern human western period is the female able to pick or dismiss (eg like birds).

We have been conditioned by politics to not see what I'm getting at."

I'm fascinated by human political conformity.

Is my above posted comment (re posted in part immediately above) is wrong?

Is there a reason why it was ignored by all by the feminist female?

Human group conformity is fascinating.

Jove said...

Seriously? Homosexuality caused by a viral vector?

We're getting ridiculous.

Have you considered any models of inclusive fitness, male variability advantage (Miller, 2000), or the fertile female hypothesis (Rahman, 2008)?

I realize that a large amount of the environmental variability remains unexplained, but this pathogen idea just sticks as completely ridiculous to me. Homosexuality is present across cultures and geographical areas in similar frequencies. This doesn't map to any pathogen I'm familiar with. Compared to HbS/HbA and sickle cell anemia, this is just pure fluff. It's even weak compared to the argument that CF or Tay-Sachs may have co-evolved with tuberculosis or typhoid.

Let's get real.

Anonymous said...

"Seriously? Homosexuality caused by a viral vector?

We're getting ridiculous."

Jove,

Sounds silly right off the bat maybe, but
I can see you haven't done a lot of recent reading in the field of neonatal and childhood infections, the infectious etiology of chronic disease, infectious etiologies of behavior, and that you aren't well-versed in evolutionary/genetics basics.

Start by reading someone like Paul Ewald.

Yes, seriously, it is the most likely explanation.

Jove said...

Um, did you even read the last half of my comment? I am well-versed in evolution and genetics, and even cited two examples of pathogens affecting human evolution.

I'm sorry, but this argument just doesn't hold water for me for the reasons I mentioned above.

Anonymous said...

Someone said,

"Homosexuality is present across cultures and geographical areas in similar frequencies."

Oh, really? How would any of us know anything about pre-history?

And no, it is not known among some hunter gatherers.

And, sickle cell? Come on, that's a genetic adaptation that increases fitness among those with one allele, a great defense to combat the world's greatest killer in those who live where that killer is common.

What's that got to do with a pathogen that is probably common and doesn't leave behind any or very little damage in most of whom it infects? Could be as simple and common as a rhinovirus or a lung infection like RSV, pathogens that were spread to all but the most isolated parts of the world.

Jove said...

I didn't say anything about prehistory; I'm speaking about currently. I don't know what the frequencies of homosexuality was in prehistorical societies.

As to your refutation of my sickle cell argument, you actually made my point for me. Why would there be some type of response to a pathogen that had a side effect of homosexuality if the pathogen itself caused very little damage to its host? Sickle cell became prevalent precisely because malaria was so devastating to the areas it was most concentrated in. Yet what selective advantage is there in being resistant to a virus that doesn't greatly negatively affect the fitness of its host? Furthermore, why would homosexuality be the byproduct of this adaptation?

Even if we do grant these two things, then the "homosexual allele" would be more prevalent in areas where this virus was its most prolific.

I would encourage the reading of:

Rahman, et al. (2008) Maternal Inheritance and Familial Fecundity Factors in Male
Homosexuality. Archives of Sexual Behavior, Vol. 37, 962-969.

To see an alternative explanation for the possible maintenance of male homosexuality in populations, despite its obvious selective disadvantages.

Anonymous said...

"Why would there be some type of response to a pathogen that had a side effect of homosexuality if the pathogen itself caused very little damage to its host..."

Some thoughts I have gleaned from reading about this.

1. Consider the polio virus...present in the gut in about 57-61% of people where it does no damage (not that we know of anyway). Before innoculations were available, in a small % of the population, the polio virus made its way to the nerve sheathings. Why just in some people? We don't know?

2. Consider streptococcus...the cause of all kinds of infections, right? Small % of kids who have had a recent strep infection suddenly exhibit behavioral changes accompanied by tics, compulsions, preoccupation with urination, "clingy" behavior, abandonment fears, etc. Reason: the strep infection, actually the immune reaction to the strep infection. It's called Pediatric Autoimmune Neurological Disorders Associated with Streptococcus. Acronym=PANDAS. Google it. Fascinating stuff.

We used to think the blood brain barrier pretty much protected the brain from infection. While it still is a very effective barrier, particularly in keeping out most bacteria, which are larger than viruses, it's not as bullet-proof as we once thought.

In addition, part of the brain, part of the brain suspected of being involved with sexuality, the hypothalamus, is not fully protected by the BBB.

Lots of conditions are looking as if the body's immune response goes into overdrive. Consider narcolepsy. Palsies.

Consider all the things we once thought were the result of genetics which have turned out to be triggered by pathogens, like cervical cancer, atherosclerosis, stomach cancer, ulcers, probably colorectal cancers, etc.

We know the side effects (at least some--very likely we don't know all ) of things like measles. We know the serious side effects of mumps, chicken pox in some kids. Only now are we beginning to realize that early infections, even infections that didn't make us that ill at the time, probably have had serious side effects in many, actually, all of us.

RSV, which all kids get by the age of 2, and that most have several times in childhood, may seem to have harmlessly resolved itself, yet now looks to be related to developing asthma later on.

It's highly unlikely any of us escaped having damage of some kind that's been caused by a childhood infection or even an infection contracted later. We are only now discovering what those things are.

Lack of attraction for the opposite sex caused by destruction of certain cells may be just one of all kinds of side effects/collateral damage, as it were, from infections. Many are probably the result of autoimmune responses. In the case of SSA, it's the brain that has been affected rather than, say, the arterial system.

We evolve. The bugs evolve. Lots of times they out evolve us.

I am confused by your comparison to sickle cell. Sickle cell is the result of an evolutionary strategy to beat malaria. In the pathogenic hypothesis of SSA, there is no evolutionary strategy. The hypothetical infection does what lots of infections do--destroys cells. (Refer back to polio, narcolepsy references).

We have not escaped the effects of many infections, even once we are "over" the symptoms, if we recognized the symptoms at all, of course.

Interesting too is that there is evidence of a bit of clustering in some families of SSA. Infections or susceptibilities to infections or certain autoimmune responses to infections are similar among family members.

Anonymous said...

"Even if we do grant these two things, then the "homosexual allele" would be more prevalent in areas where this virus was its most prolific."


I should have been clearer in responding: the pathogenic hypothesis argues against a genetic cause of homosexuality. Thus, there are no gay alleles, no gay genes--it's about infection and cell damage done by infection or by an autoimmune response to the infection.

As for the increased female fecundity hypothesis. Yes, I have read a few articles layng it out, and a few refutations of the math involved.

Anonymous said...

We see more and more of things like this every day in research:

http://www.sciencedaily.com/releases/2009/03/090305141639.htm

Both types of diabetes are being linked to viruses actually.

Peter had it right: so far, there is no "smoking gun"--no hard proof, only a hypothesis that universities and review boards, at least in the US, don't want to test--too hot to touch, careers in the balance. Research might come from outside the country.

Best guess--just a matter of time.

In reading about all the new brain stem cell projects, it may be we accidentally repair the damage trying to fix something else before we even know the exact cause/pathogen.

Or, could be that as childhood illnesses are one by one innoculated for (vaccine for RSV should be ready in 5-10 years last I read, and one for some flu strains w/in the next few years)we will see a decrease in the % of those with SSA.

Anonymous said...

Peter,

This series you posted on male homosexuality was very thought-provoking.

Chimerism, estrogens in the water, pathogens, plus the older hypothesis of hormonal imbalances in the womb --all these are fascinating to consider as biological origins of what certainly has to be considered an odd proclivity, given natural selection.

While I do discount the "Freudian" explanations of homosexuality and agree there must be a biological cause(s), I am just as perplexed by another odd proclivity and that is pedophilia.

I am not equating pedophilia with homosexuality other than to state the obvious, that both have to do with sexual interests and both seem maladaptive, but I wonder this: do you and other serious scientists who believe in a biological explanation for exclusive homosexuality, also believe there must be a biological causation for interest in pre-developed children?

If Freudian explanations of one are now viewed as silly, it would seem Freudian explanations of the other just as silly. Yes? No?

I'd appreciate your thoughts on the matter. Have you read much about pedophilia? Does anyone really know much? Any new hypotheses out there?

Ghost said...

PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects

http://www.pnas.org/content/105/27/9403.full

A new study.

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