The Frontal Cortex

Selective Abortion and Homosexuality

Last week, gay-rights activists led a protest against research being done on sheep at Oregon State University. Andrew Sullivan reports:

The researchers have been adjusting various hormones in the brains of gay rams to try to see if they can get them to be interested in the opposite sex. The indifference of many rams to otherwise attractive and fertile ewes is a drag on sheep-breeding, it seems. We don’t have any peer-reviewed studies yet, but reports of success in manipulating the sexual behaviour of some rams have led to an outcry.

Sullivan is worried that this research will lead parents to start aborting their homosexual infants. (He presupposes that we will one day have a genetic test for homosexuality.)

We already have widespread gender-selective abortion, with fewer and fewer girls being born in the developing world. And most parents across the globe are far more hostile to the idea of a gay child than of a daughter. Tests that could infer even a slightly higher probability of homosexuality in foetuses could lead to the equivalent of a “final solution” to the existence of gay people — the dream of bigots for millenniums.

Obviously, this nightmarish scenario won’t happen anytime soon. Despite the best efforts of geneticists, there is virtually no evidence of a “gay gene”. Homosexuality is a complicated phenotype, and is clearly not reducible to a short script of DNA. (Sullivan uses Down Syndrome as an example of a phenotype that has been subject to “selective abortion”. Needless to say, trisomy is much easier to detect than a trait like homosexuality, which is most likely encoded for by dozens, if not hundreds, of different genes working in concert with the environment.)

But let’s play devil’s advocate, and imagine a world where homosexuality has been selected against. My guess is that such a civilization would be an uncivil nightmare, a Hobbesian dystopia. Last summer, I profiled the theories of Joan Roughgarden, an evolutionary biologist at Stanford. She has constructed a plausible theory about the adaptiveness of homosexuality:

So how might homosexuality be good for us? Any concept of sexual selection that emphasizes the selfish propagation of genes and sperm won’t be able to account for the abundance of non-heterosexual sex. All those gay penguins and persons will remain inexplicable. However, if one looks at homosexuality from the perspective of a community, one can begin to see why nature might foster a variety of sexual interactions.

According to Roughgarden, gayness is a necessary side effect of getting along. Homosexuality evolved in tandem with vertebrate societies, in which a motley group of individuals has to either live together or die alone. In fact, Roughgarden even argues that homosexuality is a defining feature of advanced animal communities, which require communal bonds in order to function. “The more complex and sophisticated a social system is,” she writes, “the more likely it is to have homosexuality intermixed with heterosexuality.”

So here’s my hypothesis: if you select against homosexuality in a biological community, you will also be selecting against our instinct for solidarity. The same genes that give rise to gayness might also give rise to cooperation. When scientists create a population of all heterosexual sheep – this would be a boon to ranchers, since a high percentage of male sheep are gay – they will find that their sheep are now more violent as well.

The truth of the matter is that homosexuality is clearly an adaptive trait. It has been documented in more than 450 different species. As Roughgarden points out, “a ‘common genetic disease’ is a contradiction in terms, and homosexuality is three to four orders of magnitude more common than true genetic diseases such as Huntington’s disease.” Selecting against homosexuality isn’t just immoral and unethical: it’s also just a terrible idea, driven by bad biology.

Note: Sullivan has an scientific update on his column. (The original reporting on this was rife with errors.) Thanks, Larry.


  1. #1 Larry
    January 10, 2007

    Andrew S. has since posted a follow-up, with the text of an email from one of the researchers involved in that study. Apparently, the article Sullivan quotes is rife with errors — any of us who work in science and deal with the press know this is quite common.

    It’s definitely worth a look.

  2. #2 brtkrbzhnv
    January 10, 2007

    Razib wrote about this a while ago and got some interesting comments:

    Given that the number of older brothers seems to be the best correlate of male homosexuality, I think we would be more likely to get hormonal or some other chemical treatment for the mother, rather than selective abortion (based on genetic analysis) as a way to reduce the likelihood of producing gay sons. This would mean that the negative effects of selecting against gay genes with positive social correlates would be avoided.

  3. #3 Luke Hoekstra
    January 10, 2007

    “The truth of the matter is that homosexuality is clearly an adaptive trait.”

    I don’t follow your reasoning here. Just because a behavior is prevalent in a lot of species doesn’t make it adaptive. You seem to say yourself that it could be a byproduct of selection on cooperative behavior in social species. I think you do the gay community more of a disservice by trying to explain away homosexuality on shaky biological grounds.

  4. #4 Jonah
    January 10, 2007

    Thanks for your comments. Luke: I agree that homosexuality might largely be a by-product of other adaptive behavioral traits. Nevertheless, I still think there’s persuasive evidence that homosexuality isn’t just a genetic by-product. In my article on Roughgarden, I cite research done on Japanese macaques:
    Macaque society revolves around females, who form intricate dominance hierarchies within a given group. Males are transient. To help maintain the necessary social networks, female macaques engage in lesbianism. These friendly copulations, which can last up to four days, form the bedrock of macaque society, preventing unnecessary violence and aggression. Females that sleep together will even defend each other from the unwanted advances of male macaques. While this lesbianism probably decreases reproductive success for macaques in the short term, in the long run it is clearly beneficial for the species, since it fosters social stability. “Same-sex sexuality is just another way of maintaining physical intimacy,” Roughgarden says. “It’s like grooming, except we have lots of pleasure neurons in our genitals. When animals exhibit homosexual behavior, they are just using their genitals for a socially significant purpose.”

    In this case, homosexuality is itself an adaptive trait, and fosters social solidarity within female macaque society. It’s more than just a by-product of other genes favored by natural selection. Of course, Roughgarden may be completely wrong about this, but I still think her hypothesis is a compelling one. (You might also be interested in her 2006 Science paper on the European oystercatcher.)

  5. #5 Epistaxis
    January 10, 2007

    The sticky question here seems to be whether homosexuality itself is the hypothetically adaptive trait or it’s just tightly linked to some other one. But I think there’s a more fundamental problem that has to be addressed first.

    It sounds here, and in your profile of Roughgarden, like the individuals that build social solidarity by having sex with other individuals of the same sex (like the macaque does) are actually the same ones that have sex with members of the opposite sex to keep the population up. So “homosexuality” itself (as opposed, in crude dimorphism, to heterosexuality) isn’t what’s hypothetically adaptive; it’s simply temporary homosexual behavior performed by individuals who also engage in heterosexual behavior at other times.

    At least where I grew up, there’s an agreed-upon disjunction between homosexual and heterosexual people; supposedly, no member of either group ever has impure feelings about the other, not even in the name of social cohesion (fraternity life notwithstanding). That could be a discussion in its own right, but in order to apply the “casual gay sex as social glue” hypothesis to humans, we have to make a big assumption there. That is, if your hypothesis (as I understand it) is true, it would seem to imply that the generally understood dimorphism, or polymorphism, of human sexualities is a purely social construct without a basis in our biological instincts. Otherwise, that hypothesis can’t really address what’s going on with humans.

  6. #6 MattXIV
    January 11, 2007

    I have a couple problems with Roughgarden’s hypothesis that vertibrate homosexuality has a common origin in fostering social bonding.

    First is that the social structures which utilize it that way vary widely across species in a way that doesn’t seem to match with the idea that they are all variations on a common strategy. It seems more likely that homosexual behaviors emerged and disappeared based on how useful they’d be in specific species’ social structures. One of the strongest arguments for this is probably the immense variation in how human societies have regarded and practiced homosexuality.

    Second, it seems to supposes certain things are true about the heritability and cognition of sexuality by treating homosexuality as having a single biological root that I don’t think have been established. The struture of attraction that is assumed in Roughgarten’s hypothesis appears to be that there is a hetrosexual base state which is modified by the heritable factors that enable homosexuality.

    Two other hypotheses seem at least as credible for explaining sexuality. First is that there is a bisexual base state where attraction to one of the two genders is suppressed by other genes that are expressed dependent on sex. In this case, if the regulation based on sex isn’t perfect, the distribution of sexuality could be dictated by tradeoffs in making each of the supression mechanisms stronger – for example, making it easier to suppress attraction to males may reduce the number of homosexual males, but it may also reduce the number of hetrosexual females. This would allow obligate homosexuality to be maladaptive on its own but still presist without being linked to any additional adaptive trait.

    The second is that the hertiable phenomena are traits which are found attractive or unattractive and a mechanism for suppressing the attractiveness of certain traits based on gender. In this case, homosexuality and hetrosexuality aren’t fundamental traits, but groupings based on how the composite of these trait attractions works out in practice. This hypothesis has the advantages of being able to explain not just homosexuality, but all kinds of potentially maladaptive sexual preferences of small fractions of a population as stochastic variation.

    Has anybody come across any good arguments for or against these hypotheses? I think I’ve heard the first of the two advanced seriously before, but the second is more or less of the top of my head.

  7. #7 Luke
    January 13, 2007

    I agree this is an interesting topic and hypothesis. And so is the debate over the adaptiveness of the female orgasm. However, if you’re thinking specifically about macaques I would recommend checking out some of Paul Vasey’s studies. He showed that females don’t choose female partners based on their ability to be a good ally. I think his current hypothesis is that the homosexual behavior in macaques is not necessarily adaptive in itself, it just feels good. Isn’t that enough?

  8. #8 beepbeepitsme
    January 15, 2007

    RE: homosexuality

    Homosexuality Is Natural Part 2

New comments have been disabled.