The Frontal Cortex

Is Homosexuality An Accident?

Yesterday, PZ linked to a short list of leading evolutionary explanations for homosexuality. On this subject, PZ is an ardent non-adaptationist:

There are really just two classes of explanation [for homosexuality], the adaptationist strategy of trying to find a necessary enhancement to fitness, and the correct strategy of recognizing that not all attributes of an individual organism are going to be optimal for that individual’s reproduction, so don’t even try. Love isn’t hardwired by biology, and it can go in all kinds of different directions.

PZ’s point is well taken, and if humans were the only species to reliably engage in same-sex sexual behavior, his spandrelesque theory would undoubtedly be the only valid explanation. But I think the “gayness is a side-effect of the stochastic nature of love” concept runs into some trouble when confronted with the long list of other animals that also engage in homosexual behavior. Invoking the mystery of love doesn’t exactly explain gay mountain rams, or lesbian oystercatchers.

Last year, I profiled the work of Joan Roughgarden, an interesting if controversial evolutionary biologist at Stanford. She is best known for her book Evolution’s Rainbow, which uses the widespread nature of homosexuality in the animal kingdom to critique Darwinian social selection:

To make her case, Roughgarden filled the text with a staggering collection of animal perversities, from the penises of female spotted hyenas to the mènage à trois tactics of bluegill sunfish. As Roughgarden put it, “What’s coming out [in the past 10-15 years] is to the rest of the species what the Kinsey Report was to humans.”

According to Roughgarden, classic sexual selection can’t account for these strange carnal habits. After all, Darwin imagined sex as a relatively straightforward transaction. Males compete for females. Evolutionary success is defined by the quantity of offspring. Thus, any distractions from the business of making babies–distractions like homosexuality, masturbation, etc.–are precious wastes of fluids. You’d think by now, several hundred million years after sex began, nature would have done away with such inefficiencies, and males and females would only act to maximize rates of sexual reproduction.

But the opposite has happened. Instead of copulation becoming more functional and straightforward, it has only gotten weirder as species have evolved–more sodomy and other frivolous pleasures that are useless for propagating the species. The more socially complex the animal, the more sexual “deviance” it exhibits. Look at primates: Compared to our closest relatives, contemporary, Westernized Homo sapiens are the staid ones.

In other words, if homosexuality is an “accident,” then it’s an accident that has been endlessly repeated (and tolerated) by natural selection. Gayness has occurred in all sorts of distant phyla, from birds to primates. Given this ubiquity, I think it makes sense to look for adaptive explanations.

Here is Roughgarden’s explanation:

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.”


  1. #1 Dunc
    July 13, 2007

    Isn’t there also a possible developmental component to homosexuality? I’m suprised PZ didn’t mention that…

  2. #2 Dunc
    July 13, 2007

    And by developmental, I mean neurologically, not psychologically…

  3. #3 Rich Reynolds
    July 13, 2007

    Jonah (and Dunc):

    Aren’t we making too much of sexual diversity?

    Homosexuality, as Kinsey rightly noted (despite anomalies in his data), is part of the sexual spectrum.

    Sexuality is meant, in a Darwinian sense (even with his caveats), to be creative.

    Even though the natural law (delineated by Aquinas and supported by Freud) states anything outside copulation to procreate is unnatural or a perversion, one can assume that humankind isn’t restricted in its experimentation.

    After all, we are a creative species. and homosexuality has components that intrigue and delight, just as heterosexuality has its own invitations.

    That other species besides man engages in homoerotic behavior indicates that even something more than creativity is at work in the sexual realm.

    What that might be is still an open question. And I don’t think an answer will be come by easily.


  4. #4 Suzanne
    July 13, 2007

    I’m intrigued by Roughgarden’s idea, but wonder about the empirical evidence. In her book she lists a large number of case studies, and certainly there’s a lot of evidence that homosexual behavior plays a role in same-sex coalition formation / appeasement in some species. But her claim that homosexuality is ubiquitous in complex social systems goes far beyond that evidence.

    Does anyone know of any review that supports this assertion statistically? i.e. is homosexuality really statistically associated with social complexity? If so this would seem to support Roughgarden’s assertion of the generality of social selection.

    Also, there’s a huge evolutionary distinction between bisexual and homosexual behavior; bisexuality carries no direct fitness cost and so could easily be an adaptive social lubricant. On the other hand, male obligate homosexuals (i.e., they dislike girls) are behaviorally sterile and adaptive explanations therefore require much stronger social/kin/multi-level selection. So widespread obligate homosexuality would seem like much better evidence for Roughgarden’s argument than bisexuality. Unfortunately, in pair-bonding species it’s hard to distinguish obligate homosexuals from boys who fall in love with boys, so statistical evidence could be confounded by social systems.

  5. #5 Sven DiMilo
    July 13, 2007

    Gayness has occurred in all sorts of distant phyla, from birds to primates.

    a) birds and primates are both members of the same phylum, Chordata. Birds=a traditional class, primates=a traditional order.
    b) I have not read Roughgarden’s book, but I dispute this claim. Homosexual behavior in certain contexts is not the same as human “gayness.”

  6. #6 Taylor Murphy
    July 13, 2007

    Let’s suppose this “it’s good for the community” explanation is true.

    There are still a few problems that I can’t really think of:
    First, group selection would have to be a fairly large evolutionary force. We have to have (through all social vertebrates, apparently) groups constantly dying off because they don’t have enough homosexual members. The pressure has to be there.

    Not only that, but there has to be a way for it to trump selection on a genetic level. I remember reading The Selfish Gene awhile ago, and a portion of the book was dedicated to explaining how in a community of people who risk their lives to save their group, the individuals who stop one step short (at the risk of their non-kin peers) will have a genetic advantage over the rest, and whatever genetic factors there are for this will become more numerous.

    If the biological basis for homosexuality is detrimental to the genes which give rise to it, but help the entire groups wellbeing, it should dwindle out of any population over time and all species should go extinct, and it is just the most amazing miracle that so many groups have lucked out. The pressure is there, but through some Invisible Hand these pressures have mysteriously not had any effect. It simply must be advantageous to it’s own propagation, unless we can find the Invisible Hand.

    But that doesn’t mean it’s inexplicable, as the author being quoted says. It just means that homosexuality isn’t advantageous in a vacuum, all by itself, not considering any other related traits. I bet it’s a by-product or side effect of more reproductively advantageous traits. Or it may not even be that, it might just be a statistical inevitability for creatures with plastic nervous systems. It seems very unlikely that homosexuality is advantageous all by itself, even if we declare that group selection and an Invisible Hand is a possible solution.

    July 13, 2007

    why are all these evolutionary changes pz and roughgarden write about behaviorally based? why has there been no development of a physical difference between homosexuals and “heterosexuals”?
    animals do many strange things. alot of dogs will mount just about anything;does this mean some canines have a “trouser leg fetish”?
    what is the “adaptive”advantadge of pedophilia,sado-masochism,bestiality etc.?
    as for “all those penguins”,the media focused on a pair(they`ve since broken up,by the way)and the story lingers like an urban myth.

  8. #8 Dr. Tim
    July 17, 2007

    Hi Jonah —

    Thought you might be interested to know about a lecture Professor Roughgarden made this weekend as part of our Annual Meeting:

    Good News! Online: Roughgarden Lecture: History in the Making

    As you can see, we’re planning to make her lecture available as a DVD or online. Please let us know if you’d like to go on our waiting list.

  9. #9 rp
    July 18, 2007

    I think the homosexuality-must-be-adaptive argument has a lot of weaknesses. In addition to the problems mentioned in the comments above, there’s the big one raised in PZ’s blog when he first raised this issue back in 06: the assumption that homosexuality is somehow heritable. Where’s the evidence for that? And to assume that because it exists, it _must_ be heritable, we all know (or should) that that’s BS. Of course, you could fall back on the position that, at least evolution hasn’t selected against it–that it could have hardwired critters to be homophobic, but it didn’t. But that’s a different argument, with its own problems.

  10. #10 Daniel
    July 18, 2007

    It is my observation that in human beings at least, a same-sex preference among gay people is not their only distinguishing characterisitc, but in fact, gay people seem to have a whole collection of non-sexual traits that might fall under the category of “homosexual syndome.” Perhaps it is one of these other traits associated with same-sex attraction that accounts for its survival and continued propogation, generation after generation.

  11. #11 PennyBright
    July 18, 2007

    Hmm. It’s my opinion, given what I know of the current research, that we will find that homosexuality has a number of potential origins of variable heritability, and that not every homosexual person is homosexual for the same reasons.

    It’s difficult to claim a trait as either universally adaptive or non-adaptive when there are likely multiple sources for that trait.

  12. #12 David Harmon
    July 18, 2007

    The imprecision of neurological development development in general would seem to be a pure negative; in practice, it yields behavioral diversity, allowing a population to have behavior vary among individuals, without juggling a zillion behavior-related genes. (Not that there aren’t a lot of genes involved, but not enough to separately select for/against every possible behavior pattern!)

    This implies PZ is wrong about the dualism — it’s not just a choice between “adaptation” and “accident”. I would say that rather than being specifically selected for, obligate homosexuality is most likely to be “covered” under the more generic category of “reduced-fertility behavioral sports”. That broader category can be supported by any of the usual group-selection scenarios (non-breeding workers, population control, social stabilization, et al.), but it can also be offset by the generic advantages of behavioral diversity!

    And this interpretation makes it clear that the question of “obligate vs. optional” is really irrelevant — the point is that any species with complex social behavior is self-adapted — it has developed a social pattern that accomodates “most” of the behaviors actually found in that species, regardless of causation. Every individual that “has to” be killed or ejected from the group is a charge against the group’s success; every individual whose efforts continue to benefit the group is a plus for them — even if not for the individuals in question!

  13. #13 Christy Stone
    October 17, 2009

    when speaking of homosexuality i think we have become to loose with the term! sex is supposed to be a cherished, sacred thing between a MAN and WOMAN, evolution is just a theory and if we went on every little thing we hear we are very simple minded and ignorant. LOVE? you call lust and cheating betweem two men LOVE? its all wrong and goes against the laws of nature!! its SICK!!!

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