5 Weak Ideas About The Origin Of Homosexuality: A Reply

Back at the end of last month, Mark Van Vugt presented what he considered to be five candidate selection pressures which might explain how homosexuality as an orientation – the exlcusive preference for same-sex sexual partners – came to both (a) exist in human populations in the first place, and (b) have its existence was maintained in the face of, what appear to be, obvious reproductive disadvantages. In each of these five cases I find the arguments lacking for either theoretical or empirical reasons, and, in most cases, both. Before I get to the science, however, I would like to deal with a troubling claim that Mark makes at the beginning of his post:

“The converging findings suggest that exclusive homosexuality is not a “life style choice” but a perfectly natural sexual orientation….Although these findings make a reasonably strong claim that homosexuality is part of someone’s genotype, there is still much speculation as to how it got there.”

There are three problems I have with those two above statements (admittedly, juxtapositioned for effect) that are worth pointing out. The first is the language used: while I’m in no way about to tell you that being gay is a choice (I happen to think it isn’t one), I will point out that the opposite of “choice” is not “genetic” or “natural”. If it was discovered tomorrow that homosexuality was determined by some environmental variable, say, a specific pathogen affecting development, (Cochran, Ewald, & Cochran, 2000) that would make homosexuality no more or less of a “choice”. The second issue is that all biological traits are equally and entirely codetermined both by environmental and genetic factors. Accordingly, saying homosexuality (or any single other trait) is “part of someone’s genotype” is both trivially and true as well as potentially misleading for those who are not well-versed in genetics.

Finally, the third issue is a cautionary point I made about a year-and-a-half ago: presumably, all the talk about choice, genes, and “being natural” has little to do with statements of fact, but are rather about the moral status of homosexuality. While I fully support the moral acceptance of homosexuality, I would be very wary of basing that support on the notion that homosexuality is a “genetic” trait that people “don’t choose”. Not only do I think homosexuality should be accepted regardless of whether it’s a choice or caused by some environmental factor, but I further would hate to see the arguments for acceptance slip away on the basis of the concordance data not panning out (which we’ll see in a moment, it does not).

Politics and science make awkward bedfellows, not unlike gay men and women…

Now that the political-stuff is out of the way, we can start dealing with some of Mark’s claims. The first thing I’ll take issue with there is the prevalence data. The stable 8% figure that Mark mentions is one that exceeds most every published estimate of homosexuality I’ve seen; those have typically hovered about 1-3%. Bear in mind, one isn’t simply trying to estimate the percentage of people who have had a homosexual experience before; it’s preferences which are of primarily interest. Further, there is little reason to think that such a figure is stable, as Mark implies it is, across time, place, or history. It well might have been, but one would have an awful hard time demonstrating that it is if one wishes to go beyond pure assertion.

As for his next point, yes; there is indeed evidence that twins tend to share a sexual orientation, as they do many other things. However, the concordance rates for a homosexual orientation among identical twins (that is, given that one twin has a homosexual orientation, how often does their co-twin have a similar orientation)  is only about 30%, and about 8.3% for non-identical twin pairs (Kendler et al, 2000). To make matters even more complicated, it’s worth noting that these concordance rates only tells us that some amount of what these twins share – genes, prenatal, and postnatal environments – makes it more likely that both will eventually develop a homosexual orientation, but it doesn’t tell us what that something is. Twin pairs, for instance, are similarly concordant for patterns of infectious disease (Cochran, Ewald, & Cochran, 2000), but it doesn’t mean they inherent genes that function to make them sick.

Finally, before getting to the selection pressures, it’s also worth countering the claim that homosexuality is well-documented in non-human species. Sure, there are some species that will, occasionally, engage in brief interactions more typical of mixed-sex pairs, be those interactions sexual or non-sexual in nature. What needs to be explained when it comes to homosexuality is not homosexual behaviors, but rather heterosexual avoidance. This more-or-less exclusive sexual preference for same-sex conspecifics has only been documented in rams, to my knowledge, and in no other non-human species, much less many or most of them.

Selection Pressures: Kin Selection
The first of the five selection pressures that Mark mentions is kin selection: helping others who share your genes reproduce. As Mark correctly points out, there’s little evidence for his hypothesis being correct, but the issues are much larger than that. For starters, the relationship coefficients don’t work well here: for each offspring a homosexual individual doesn’t produce, they would need to ensure that a full sibling produced an additional two that they otherwise wouldn’t have had for them to just break even and for this hypothesis to work. This would require an intense level investment that, if it existed, would be plainly obvious to any observer. It’s not enough that a homosexual individual is occasionally or even often nice to their relatives; they would need to be utterly devoted around the clock.

More to the point, though, is the seemingly apparent point that having a same-sex sexual attraction does nothing to help you invest in kin. Sure, maybe an asexual preference would work, if you wanted to save the time otherwise spent pursuing sex; a facultative heterosexual preference would probably do just as well, if not better. A homosexual orientation, on the other hand, is a complete waste of time; it would be a pointless distraction from the investment issue. Unless seeking out same-sex sexual relations was somehow functional in terms of increasing investment (or a rather odd byproduct), this explanation makes little sense.

Ants are very helpful, yet not very gay…

Selection Pressures: Group Selection
Group selection – the idea that a trait can spread if it offers group-wide advantages despite being individually detrimental – is a conceptual nonstarter, running counter to everything we know about how evolution works. Since I’ve written about this matter before on several occasions, there’s little need to continue beating this theoretical horse which has been dead since the 1960s. As Mark, again, points out, he knows of no evidence in favor of this hypothesis either, so there’s little less to say about it, other than that it doesn’t sound like a very “big” idea.

Selection Pressures: Sexual Attraction
This one, I admit, is probably the strangest of the selection pressures Mark posits. The idea here seems to be that because women might find homosexual men sexually attractive, this could give homosexual men a reproductive advantage. Now, perhaps I’m misunderstanding the basic idea in some fundamental way, but if an individual with a homosexual preference is found to be attractive by opposite sex individuals, it would seem to not matter much, as it’s quite unlikely that the two will ever end up having sex at all, let alone frequently. Provided these increased opportunities for sexual encounters even exist(Mark says there’s no evidence available that they do), they wouldn’t seem to do much good if the urge to take them is all but absent.

In case the problems aren’t plainly apparent at this point, imagine a hypothetical species of bird, like a peacock. In this species, males grow elaborate ornaments that females find to be attractive, generally speaking. Growing these ornaments, however, carries a cost: it makes the males sterile. In this case, no matter how attractive a male is to the females, his genes will never be benefited because of it. Attractiveness only matters so much as it leads to reproduction. No reproduction, no selection.

Selection Pressures: Balanced Selection
This argument at least poits that homosexuality is reproductively detrimental. These detriments are made up elsewhere, though, in the form of benefits to other carries of the genes. In essence, this argument says homosexuality is a lot like sickle-cell: harmful in some cases, but beneficial in others. There’s nothing theoreticall wrong with this possibility, but there are some serious practical hurdles. Specifically, if homosexual orientations ensured that 1-8% of the population was, effectively, sterile, there would need to be tremendous compensating benefits. Sickle-cell, for instance, is only common in areas that have a ton of malaria – which can kill huge minorities of populations and leave even more severely harmed – and pretty much the only known byproduct of its kind with a fitness hit as great as homosexuality (Cochran, Ewald, & Cochran, 2000). It also doesn’t fit well with the concordance rate data. So, while this explanation is theoretically possible, it’s highly improbable. At the risk of sounding like a broken record, there’s also no known evidence for this being the case.

Selection Pressures: Sexually Antagonistic Selection
This brings us to the final selection pressure. Here, the idea is that a gene is detrimental when it’s inherited by one sex, but beneficial in the other. This is another theoretically plausible suggestions with some consistent evidence behind it (but the account isn’t anywhere near complete, and only considers male homosexuality). Unfortunately for this suggestion, like the above hypothesis, it also suffers from the concordance rate data. It would also require that females consistently more than make up for the detriment to the male offspring, reproductively. Remember, this isn’t just a matter of slight disadvantages; this is a matter of effective sterility. Further, such sexually antagonistic issues tend to be weeded out over time, as any new modifications that can avoid the costs associated with expression in males will be selected for. Even if this was a viable account, then, it would still be far from a complete one, as it would not be able to explain why some of the twin pairs turn out concordant, but most don’t, why these reproductive costs have yet to be eliminated, and it’s missing an account of female homosexuality.

“Would you care for a sixth cup of weak tea?”

Out of the five “big” ideas, then, four seem to be basically dead in the water and the fifth, while potentially plausible, is by no means conclusive or complete. In my experience, poor outcomes like these can be seen frequently when people attempt to use scientific research to justify some political or moral opinion: any available evidence that can be potentially interpreted in a favorable light is seized upon, no matter how weak or nonsensical the underlying connection between the two is. The goal, after all, doesn’t appear to be accuracy, but rather persuasion; to the extent that the former helps with the latter, all the better for the persuader, but their need not be any necessary connection between the two goals.

References: Cochran, G., Ewald, P., & Cochran, K. (2000). Infectious Causation of Disease: An Evolutionary Perspective. Perspectives in Biology and Medicine, 43 (3), 406-448 DOI: 10.1353/pbm.2000.0016

Iemmola F. & Ciana, A. (2009). New evidence of genetic factors influencing sexual orientation in men: Female fecundity increases in the maternal line. Archives of Sexual Behavior, 38, 393-399.

Kendler, K.S., Thornton, L.M., Gilman, S.E., & Kessler, R.C. (2000). Sexual orientation in a U.S. national sample of twins and nontwin sibling pairs. American Journal of Psychiatry, 157, 1843-1846

15 comments on “5 Weak Ideas About The Origin Of Homosexuality: A Reply

  1. There are two other hypotheses.

    (1) Genes are not usually related to a specific behavior. For instance the same combination of genes might result in one being a CEO and another a criminal in a prison. I mean genes that lead to some kinds of psychopathic behavior. Thus some genes might result in homosexual behavior in some cases and in some other behavior in other cases.

    (2) If mutations are random and not costly, the mutated gene might keep existing for long periods of time. After all some animals have organs and behaviors that do not have any survival benefit.

    I’m not a biologist so ignore my reaction if I said something stupid.

    • Jesse Marczyk on said:

      Genes are not typically related to specific behaviors in a 1-to-1 fashion, no. Genes are, however, necessarily related to all behaviors in some way. That first hypothesis would be, basically, some kind of balancing selection, and would have the same criticisms that I outlined above.

      As for two, the concordance rates tell us (though we shouldn’t need them to) that there’s more to the development of a homosexual orientation than genes. It could be the case that genes that predispose people towards the development of a homosexual orientation are typically not harmful, so they stick around. That’s all well and good, but what that doesn’t tell us is anything about how or why those genes predispose one towards homosexuality. Basically, the developmental story is missing here, and that story is going to turn out to be very important.

  2. gringojay on said:

    DNA gene codes for the protein order of amino acids, both hydro-phobic & hydro-philic parts of the protein.
    DNA does not determine the functional 3D shape configuration that protein subsequently assumes under multiple influences.
    (Even astrologers hedge their theories by saying the stars impel, they do not compel….)

  3. bernard on said:

    If the prevalence of exclusive homosexuality is down at the 2% mark, maybe it’s just a kind of brain malfunction that doesn’t require an adaptive explanation. Though we would then have to explain why only the human brain seems to be susceptible to this kind of malfunction.

    • bernard on said:

      I remember reading somewhere that the reason humans are susceptible to mental illnesses like schizophrenia and bipolar disorder that are not seen in other animals could be due to the hastily way the brain has been expanded and modified over the past couple million years.

      These illnesses usually have a prevalence of about 1 or 2 % in a population. Could exclusive homosexuality (and exclusive paedophilia for that matter) be another malfunction the human brain is susceptible to for this reason?

      This may be the rather unPC truth: Homosexuality is essentially a mental illness.

      • Jesse Marczyk on said:

        The conclusion, I think, hinges on the meaning of the word “illness”. In my case, I do not equate “caused by an infectious agent” with “illness”. For instance, consider a viral infection that, if acquired at a certain point during development, resulted in something positive (say, an increase in IQ). Most people would not consider a high IQ to be a hallmark of mental illness.

        One can certainly say that an outcome like exclusive homosexuality would be maladaptive, but that is a separate matter.

  4. SciCommenter on said:

    On Sexual Attraction, it seems to me that an adaptation that increases sexual attractiveness but occasionally is over-expressed in the form of homosexual behavior could be adaptive. Your dismissal of some of these ideas appears to be based on an all-or-nothing approach to whatever factors may lead to expression of homosexual preferences.

    • Jesse Marczyk on said:

      Sure, it’s a possibility. That’s what you typically see in species that contain sexually-selected traits, like peacock tails. Having the larger tail is both more costly to grow and maintain, and it makes the carrier more vulnerable to predation. However, these costs are offset by increased reproductive benefits during the peacock’s lifespan. I’m dismissive of the claim here because, (a) according to the way Mark presented it, women might find homosexual men more appealing because those men aren’t trying to have sex with them, which means no reproductive advantage and (b) there’s no evidence for some sexually-selected byproduct in the case.

  5. —> Sexually Antagonistic Selection <—

    Some interesting work has come out recently that supports this view, even if tentatively.

    1) VanderLaan DP, Forrester DL, Petterson LJ, Vasey PL (2012) Offspring Production among the Extended Relatives of Samoan Men and Fa'afafine. PLoS ONE 7(4): e36088. doi:10.1371/journal.pone.0036088

    "This study compared the reproductive output of the paternal and maternal line grandmothers, aunts, and uncles of 86 Samoan androphilic males, known locally as fa'afafine, and 86 Samoan gynephilic males. Reproductive output was elevated in the paternal and maternal line grandmothers, but not aunts or uncles, of fa'afafine. These findings are consistent with the sexual antagonism hypothesis and suggest that male androphilia is associated with elevated reproduction among extended relatives in both the maternal and paternal line. Discussion focuses on how this study, in conjunction with the broader literature, informs various models for the evolution of male androphilia via elevated reproduction on the part of female kin."

    2) Camperio Ciani A, Pellizzari E (2012) Fecundity of Paternal and Maternal Non-Parental Female Relatives of Homosexual and Heterosexual Men. PLoS ONE 7(12): e51088. doi:10.1371/journal.pone.0051088.

  6. bernard on said:

    What about a buffer theory?

    A few years ago i had the thought that genes for homosexuality may have been spared extinction by institutions of marriage and heterosexuality. Even the most primitive societies have some tradition of marriage so that homosexual people end up marrying and reproducing. For thousands of years these institutions would have buffered “gay genes” from natural selection to some extent and allowed them to survive.

    Just an idea i had.

    • Jesse Marczyk on said:

      For starters, it certainly wouldn’t explain homosexuality in the rams. Further, even if, say, a homosexual man was married to a homosexual woman, a disinclination towards engaging in intercourse with their partner could lead to maladaptive outcomes like not caring about the quality of partner one has, not engaging in otherwise adaptive intercourse with that partner, passing up on otherwise adaptive affairs, and so on down the line.

      A buffer hypothesis would need to posit that homosexuality – an aversion towards pursing members of the opposite sex – has always been, essentially, reproductively neutral with respect to an interest in pursing members of the opposite sex. It just doesn’t sound particularly plausible, anymore than a preference for eating rocks would likely be reproductive neutral, relative to eating food with nutritional value.

      • bernard on said:

        It wouldn’t explain homosexuality in rams but we shouldn’t expect homosexuality to arise in different species for the same reasons and i’m not proposing that this is the complete explanation for why it has arisen in our species.

        It’s just a thought i had that if a behavior necessary for survival and reproduction is enforced by social rules or institutions that the selection pressures that gave rise to that behavior would become a bit “flabby” and make it possible for drives contrary to that behavior to arise since the behavior itself is enforced by the social rules.

        If you get what i mean.

        • Jesse Marczyk on said:

          Why should we not expect homosexuality to arise for the same reasons? Given that we already don’t have a good account for it in humans (the pathogen hypothesis being the strongest current contender), I think it would be premature to suggest what we ought to expect from other species (or at least rams, as there aren’t other species I know of that display this kind of pattern).

          As for the second part, homosexuality is not a behavior; it looks like preference. Unless every adaptive mating behavior associated with that preference was enforced equally-well, we should likely not expect the preference to be neutral. That is, social pressures do not force people to have the same amount of sex, pursue the same quality of mates, and pursue extra-pair copulations regardless of their desire to do so. In many cases, social pressures cut in the opposite direction (don’t have affairs and don’t pursue the mates I’m pursing).

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