Friday Weird Science: Female Orgasm, Evolutionary Byproduct? Or not?

Dec 03 2010 Published by under Friday Weird Science

Today's post comes to you courtesy of Denim and Tweed, where Jeremy brought up this question, and cited his works very well! And after I saw all that stuff on clitorises, I had cover it myself. 🙂

Poor female orgasm. It's always been up for debate. From issues over the G Spot to issues of what it's FOR, the female orgasm has had a rough go of it. Evolutionary biologists have debated the "purpose" of the female orgasm for a good long while. They have debated WHAT the purpose is, to be sure, but even more so, they have debated whether there IS one. Is the female orgasm just an evolution byproduct? No one can deny the importance of the male orgasm in keeping the human species going through the generations. For the good of the species, man must cum. Even with the newfound glories of IVF, some guy, somewhere, had to shoot a wad to get the whole thing going.

But while the clitoris is anatomically analogous to the male penis...what about the orgasm? A man's orgasm must serve a function, but female? Well, it's not required, and many people take this, and the fact that a lot of women do not generally orgasm during vaginal intercourse, to mean that female orgasm may not really mean much. And so some people believe that it's just an evolutionary byproduct, something like the appendix (well, we used to think that, but apparently the appendix may help with things like gut flora), only more fun. Lynch. "Clitoral and penile size variability are not significantly different: lack of evidence for the byproduct theory of the female orgasm" Evolution and Development, 2008.

But others disagree. After all, female orgasm releases a lot of important chemicals, including things like oxytocin and dopamine, among others. Oxytocin has been implicated in pair bond formation, which could be a useful function to make sure a woman doesn't go haring off which some other guy. Other scientists have hypothesized that female orgasm produces some sort of "suction" which pulls semen up into the uterus, but those findings are up for debate. There are even hypotheses that the female orgasm can be used to assess the quality if your mate (well, at least we know whether he's quality in bed...).

But if the female orgasm served a would you TELL? What do you look for biologically to figure it out? I mean, with male orgasm, the function, or at the least the results, are rather obvious. But female? Not all females ejaculate (in fact, most don't), so that's hard to look for. And how would you tell that female orgasm is not just an evolutionary byproduct?

These scientists looked to the clitoris. The idea was that, if female orgasm (in this case I guess they just mean clitoral orgasm) was REALLY an evolutionary byproduct, then the size of the clitoris would not be important, as it would just be a somewhat analogous structure to the male. So then, it would not be subject to natural selection. And if it's not subject to natural selection, they hypothesized that the size of the clitoris would be more VARIABLE than the size of the penis. If the clitoris wasn't necessary, some women might have large ones, but others might have extremely small ones, and it wouldn't make a difference.

Now you could just measure length of the penis, and length of the clitoris, and measure variability. But is that really accurate? These authors didn't think so, and so resolved to look at both length and width, and thus to calculate volume. Previous studies had looked at just length, and determined that the length of the clitoris infect varied more than that of the penis, and determined that, therefore, the whole thing, including the orgasm, was a vestigial organ. The author of this paper disagreed, and turned his attention to volume. And got this:

What we're interested in here is the "pCv" the coefficient of variation. Basically, this author, as far as I can tell, didn't take any data of his own, or at least there are no methods (and one would REALLY have to assume you need big patient consent forms for this sort of thing). Instead, he took data from OTHER studies studying penile and clitoral length and volume. He determined (see the p values) that while clitoral length is more variable than penile length (p=0.02), the variability of the VOLUMES is not significantly different (p=0.17).

From this he concludes that, since the variabilities are not different, the clitoris must be subject to evolutionary pressure, and thus orgasm must have a function.

Um. W.T.F.

So first off, I'm ALL FOR female orgasm having a place and function in life. I mean, it would make the orgasm so...hollow...if it didn't have a purpose.

But this paper doesn't convince me of anything.

First off, since when is clitoral volume a measure of female orgasm? Women with bigger clitorises do not have more orgasms, and men with smaller penises certainly have no trouble getting off. Not only that, but men with smaller penises actually aren't any evolutionarily worse off than men with large, and I have to assume the same is true for women (if not more so, esp since the size of the clitoris has never been shown to be related to female orgasm). So what does this study really SAY?! All it says to me is that the size of the clitoris and the size of the penis vary a good bit. But the variances are not different. OoooOOOooo. You know, you probably coulda told me the same thing about male and female nose size, ear size, finger volume, and the size of the friggin' appendix, and I would have nodded sagely and believed you. What does that MEAN?! At least with nose size, ear size, or finger volume, there's a definite correlation with function (the jury is still out on the appendix).

Secondly...dude, get your data. I don't believe the study of the guys you criticized...but I don't believe you, either. Get some data with clitoral volume (and heck, volume of the labia majora and minora while you're at it), and correlate the whole thing with the number of orgasms women have. Come back, show me THAT data. We might be getting somewhere. But what you'd REALLY have to do show me the function of female orgasm. I'd like to believe it has one, but I'm a good scientist and I ain't gonna call it until I have the data.

Lynch, V. (2008). Clitoral and penile size variability are not significantly different: lack of evidence for the byproduct theory of the female orgasm Evolution & Development, 10 (4), 396-397 DOI: 10.1111/j.1525-142X.2008.00248.x

34 responses so far

  • frances says:

    I find this kind of 'science' quite depressing for the number of assumptions it rests on. Firstly and most importantly is that the purpose of orgasm is tied to reproduction, and either by extension or directly through the trope of pair bonding, to monogamous heterosexuality.

    The opposite is also true, that the purpose of male orgasm is uncritically seen as serving the purpose of ensuring the future of the species.

    By starting from this assumption, the existence and 'function' of homosexuality and/or non-monogamous relationships are rendered void. Simultaneously, the idea of the need for a quantifiable function comes to dominate the research discourse.

    I'd suggest is speaks more of the researchers' inherent biases, as well as the continuing obsession with women's bodies as objects than it does with laying out any meaningful science.

    • scicurious says:

      I do not think that "the existence and ‘function’ of homosexuality and/or non-monogamous relationships are rendered void." by the assumptions that are taken in this paper. The reality is that a male MUST ejaculate, somehow, somewhere, in order to ensure the continuance of the species. This doesn't say anything about potential OTHER functions of both male and female orgasms. It's fully possible for something to have more than one function.

      I'd agree that this paper doesn't actually lay out much in the way of meaningful science (as I stated), but I'm not sure what you're driving at, here.

  • sexisgood says:

    Not to state the obvious, but as you say, orgasms are pleasurable for both sexes, and it seems pretty obvious that increases the chances of people having sex. So, if having sex more often is selective, that would be an evolutionary reason for more orgasms. But Frances has a great point. If you focus entirely on reproduction as the only goal of sex, you're just injecting a bias into your thinking. What about social organization?

    What about the effect of sexual release on the ability to think rationally? Jonathan Swift wrote a brilliant little essay called the Battle of the Books in which he attributed puritanical fanaticism largely to "humors" denied their natural outlet by sexual repression finding their way to the brain and inflaming it. There have indeed been some studies more recently of the emotional nature of political thinking in the modern world, but none that I know of investigating the relationship with sexual behavior. Though a more modern satirist, Colbert, has made entertaining bits from the connection between political views associated with homophobia and suppressed sexual feelings.

    Well, I only think of this connection because of a rather vivid piece of satire -- its a complicated question, but alas one where the "scientific issues" are likely to be defined by sociobiologist types who tend to proceed from assumptions like the evolutionary role of sexual pleasure must be entirely related to reproduction. Sniff.

    • scicurious says:

      I certainly do not think it is necessary that female orgasm have a function which is closely tied to reproduction, see my comments on possible involvement in pair bonding or merely assessment of mate quality. There are certainly other possibilities for its function other than purely physical enhancement of reproductive probability. This study didn't address the actual FUNCTION of female orgasm, merely the possible evolutionary pressures on it.

  • As always, I'm perplexed at the apparent need to find a biological function for something -- a reason it might have been "selected for" (rather than schlepped along on the evolutionary ride by engineering constraints) -- in order for us to feel like we can value it.

    It doesn't matter whether female orgasm conferred additional fitness on our foremothers on the savannah or was delivered to us by aliens. Does it need a function other than being enjoyable?

    • scicurious says:

      It's a good question. My gut answer is no, but my gut's not a scientist.

      I think that people just really want to know WHY. The orgasm is such a massive deal to the human species that people really want to know why it happens, what it means, that sort of thing. Like the meaning of life. Who knows? We may get 42 or something.

      • figleaf says:

        I always like to put the shoe on the other foot. For instance you said in your lead paragraph that "No one can deny the importance of the male orgasm in keeping the human species going through the generations." But are orgasms really that functionally required in men? Ejaculation, yes. But orgasms? Especially our eye-rolling, bell-ringing ones?

        Like you I'm glad we've got them, and they're complex enough to sort of have to have been selected for. But...

        A biologist friend who worked with marine invertebrates questioned they hypothesis since an awful lot of the animal kingdom seems to get by just fine without them. It was a bit outside her field but she said that they don't seem like a very big deal for a lot of male mammals either. I think she said that in ungulates the effect looks a lot more like scratching an itch... which makes sense since most ungulates (antelopes, for instance) are prey animals and can't afford to zone out like that.

        Anyway, while I agree that reproductive urges are important, I just wouldn't take it as a given that there wouldn't be a human species without big male orgasms.

        Call me a rebel here but I'm pretty sure any study of women's orgasms that takes men's as a given probably isn't going to provide a credible answer. Or maybe put it another way, the evolutionary "mystery" of contemporary women's orgasms probably won't be solved without including the contemporary men's orgasms in the problem set.


    • Steven says:

      Do I need a reason to study the evolution of traits or behaviours other than the intellectual fulfillment I get from it? Female orgasm doesn't *need* to have a function. But pushing back the limits of our knowledge by finding out if it does or not is a worthy goal in itself.

      Besides, there is a *lot* of potentially interesting science here. The oxytocin link alone is worth exploring; there's work showing that administering oxytocin to humans manipulates "trust", though I'm not sure if oxytocin stimulated by sexual arousal is central or peripheral (it matters). Given oxytocin's roles in many other species as a knob for social behaviour, this would an extremely interesting line to follow.

  • drugmonkey says:

    sci, sci, sci....not a single word about REINFORCEMENT?????

    or to answer Dr. FreeRide, being "enjoyable" IS a function. It enhances the probability or frequency of a particular set of antecedent behaviors. In this case, the connection to enhanced reproductive fitness should be entirely obvious. trivially so.

    • scicurious says:

      Heh, I guess the reinforcement to me is the big "DUH" bit of it...obviously if you enjoy it you're going to do more of it.

      Didn't have enough sleep today I suppose. Should have mentioned it.

  • Yoder says:

    Omigosh. I am probably way too flattered to have provoked/inspired a Friday Weird Science post. Thanks!

    Re: adaptive function of female orgasm and monogamy, I think the presence of a pleasurable component of sex that isn't directly connected to reproduction is strong evidence that (for humans) sex itself isn't evolved to be purely reproductive. And if sex has nonreproductive functions, it seems far more likely to be employed in social interactions beyond monogamous pair bonding.

    • scicurious says:

      This "strong evidence that (for humans) sex itself isn’t evolved to be purely reproductive." I totally agree with this. I think a lot of the people researching it also agree, which is why people have looked for evidence of a role for female orgasm is things like bonding. Social interactions beyond monogamous pair bonding may be harder to get at in studies right now, though, simply because socially, our society (the one more likely to study it) has been monogamous for a very long time, I'm not sure we'd know what behaviors to look for or what it would look like when we saw it.

  • Christie says:

    So someone else might know this better than me... but do female bonobos have orgasms? Or do any female apes/monkeys enjoy sex? I mean, it might be hard to quantify these things, but I'm guessing you could check other female mammals for oxytocin levels/etc before, during, and after sex to see if they "cum" in some fashion. If it's a social thing, you'd expect that many social mammals might experience something akin to an orgasm. I'm just wondering if anyone has looked into it.

  • Jason Dick says:

    I've always sort of thought the most obvious purpose for female orgasm would just be to make sex more pleasurable for women, so that they'd want to do it, so that they'd reproduce more. Why isn't this talked about? Is it assumed that women have no choice in whether or not to have sex or something?

    There may be some correlation with mate choice, as obviously some guys are going to be better lovers than others, and one might potentially think that a better lover is more caring, more attentive and thus might be a better parent. This contradicts rather strongly with many stereotypes we have, but it is at least somewhat conceivable that it may be true for all but a few outliers.

    • Jon says:

      If female orgasm does have a specific function, it doesn't seem too likely to me that it would be solely to make sex desirable. After all, there are plenty of things that feel good and are therefore desirable, but which don't have a single peak moment. For instance, eating is another basic drive which can certainly be pleasurable. And I guess sometimes that first bite can taste really, really good, but I don't think I've ever experienced a food based "orgasm." (I assume that's normal, someone please tell me if I'm missing out.) Maybe that's part of it, but I'd be surprised if that was the entire role.

      Unrelated to that, I have an issue with the basic assumption of this study. If we assume that there is no selective pressure on female orgasm, and if we further grant the assumptions relating clitoris size to orgasm, this doesn't necessarily mean we'd expect a greater amount of variation in the size of a clitoris than in the penis. If the clitoris is also developmentally analogous to the penis (is it? I can't recall,) then maybe alleles affecting the development of the penis which are selected for because they favor male orgasm end up becoming more common and end up being passed on to daughters as well as sons, even though they only have their major effect in the sons. In this case I'd expect the variance in clitoris volume to be comparable to that in penis volume.

      Alternatively, if there's no selective pressure on female orgasm/clitoris size, this doesn't mean there would necessarily be more variance than in the (presumably) selected for penis size. Neutral alleles can drift to fixation, so that there would be fairly little variation in clitoris size just by accident. And since we don't know how this author's data was assembled, for all we know there's been some accidental mixture of different populations, which may have had different sets of alleles fixed, making the results even more murky.

      Anyway, I'm not convinced that a lack of function for female orgasm would necessarily result in a greater variance in the size of the clitoris, even if that is related to rate or quality of orgasm in some way. So I don't think this study tells us much at all.

      • scicurious says:

        Excellent points, and I agree, I really don't think clitoris size variation really has anything to do with female orgasm, or at least I wont' think that until I see the study.

  • razib says:

    great post. love the illustrative pictures.

    i'm curious when people will start wondering as to the bias of scientists who study the origin of sex focusing on dioecious species. perhaps imposing their hetero viewpoint that the original of hetero sex needs "explaining" 🙂

  • D. C. Sessions says:

    In the not-quite-totally-OT department:

    Aiding and abetting the "evolutionary incidental" hypothesis is the fact that some mammals (cats, notably, so there goes the "monogamy" thing) trigger ovulation by orgasm. Obviously not us, though, so the question isn't settled that easily.

    Per the "female bonobos" issue, well, they certainly engage in a lot of female-female erotic activity. That's not proof that they're getting off but kinda points in that direction.

  • James Kornell says:

    I'd wonder if female orgasm would lead to a longer quiescence (laying there grooving on the feeling) -- which in turn (I'd wonder) would increase chances of conception? But really, I think changing the word from "function" to "role" brings out that it doesn't have to be exactly and only one thing. It might seem like just a word, but framing it as "function of orgasm" as contrasted with "role of profound and specific pleasure" -- it seems to me I'd design different (and probably a lot more whole-person) research for the latter question than the former. Probably more likely to look for commonalty, too, with men's experience. After all, why take it for granted that men's orgasms "automatically" have to be pleasurable? Contra DrugMonkey there are a lot of things we're compelled to do that aren't pleasurable.

    • scicurious says:

      Good point about function vs. role, though I know in my field we accept multiple functions all the time, others may not.

  • Mike from Ottawa says:

    I'm tired, so I may have missed it, but has anyone yet looked at whether women who regularly experience orgams produce more children than those who don't?

  • Isabel says:

    Christie makes a good point, no? If we are looking at the evolution of a structure we should be looking at sister groups and outgroups of the organism as well.

    "the fact that a lot of women do not generally orgasm during vaginal intercourse"

    should this be *most* women *never* orgasm during intercourse?

  • Isabel says:

    perhaps I should specify, as a direct result of vaginal intercourse, i.e. without additional stimulation of some type.

  • Alex says:

    It makes perfect sense that something as fundamental as orgasm would be present in both sexes -- it would take quite some chromosomal juggling to have it be entirely missing in one. Case in point: male nipples.

    All viewpoints described in this article seem overly fixated on clitoral orgasms. For one thing, the fact that many women do not orgasm from intercourse is based on vaginal intercourse alone *without* other stimulation (clitoral, etc.), making that irrelevant to questions regarding functions of the clitoris. For another, stimulation helps excite the vulva, which gets women's natural lubrication flowing, making intercourse both easier and more pleasurable for both parties... with or without orgasm(s). Case in point: K-Y Jelly.

    The clitoris is also analogous in function and development to the glans ("head") and upper shaft of the penis, rather than to the entire thing. In the clitoris and the male glans, the nerves are most densely concentrated, and nerve density is actually what creates pleasure. For both of these reasons, employing a comparison between clitoral and penile size seems ingenuous. As a side note, while the glans is directly stimulated during vaginal intercourse, the reverse is true for the clitoris in many positions (usually including missionary)... part of the reason for the male/female orgasm disparity.

    • scicurious says:

      Well, I think part of the problem with clitoral vs vaginal orgasms (I didn't really want to get into this in the post because people get very passionate in defense of their vaginal orgasms) is that some scientists believe that vaginal orgasms are in fact the same as clitoral, and that it's merely stimulation via the anterior wall of the vagina and possible of prostate tissue that produces the orgasm without direct clitoral stimulation.

      And the male nipples is a good point. But I imagine that a lot of women would be upset if it was found that their orgasms were "vestigial". Hence some of the pressure to find a role for it.

  • Vincent Lynch says:


    That's my paper, which a friend pointed out had been blogged here.

    I'm afraid that you have missed my point. This paper was written in reply to an earlier study in the same journal (Wallen and Llyod 2008), and the point wasn't that the clitoris (and by extension the female orgasm) must have a function. Rather, my main conclusion was that the rational and data Wallen and Llyod (2008) used can not support their conclusion that the female orgasm has *no* function and is thus a byproduct of the male orgasm.

    That also explains why I did not gather my own data, in order to show that Wallen and Llyod's conclusion was wrong I had to use the same exact data they used.

    I also never assumed a relationship between the size of the clitoris/penis and the ease of achieving an orgasm. In fact, my concluding paragraphs say just that:

    "More generally, the primary argument used to support the byproduct hypothesis for the origin of the female orgasm is its uncertain occurrence during sex, which is taken as evidence for reduced or absent stabilizing selection on female orgasm. Thus, Wallen and Lloyd (2008) assume that the clitoris, as the structure primarily responsible for generating the orgasm (Narjani 1924; Masters and Johnson 1966), must also be free from stabilizing selection and this will be reflected in variation in its size. This relationship is essential for their conclusions, however, there is no relationship between clitoral size and ability to achieve orgasm (Masters and Johnson 1966). For that matter, it seems unlikely that penis size is directly related to a males ability to reach orgasm during sex. Indeed, the ape penis is much smaller than the human penis with the average chimpanzee’s penis ~14 cm long, and the average gorilla and orangutan penis only ~3 cm long (Diamond 1992). Yet I doubt proponents of the byproduct hypothesis would conclude that males of these species experience orgasm during intercourse less frequently than human males."

    I think that good reading of my paper would have shown that I was calling BS on the Wallen and Lyod (2008) paper and did not come down on either side of the argument over whether the female orgasm was adaptive or a by-product. After all, my final words on the matter are:

    "...empirical data to test hypotheses, both functional [i.e. adaptation] and structural [i.e. by-product], are lacking. The recent conclusion of Wallen and Lloyd (2008) that the size of the clitoris is under weaker selection than the size of the penis is not supported by a more realistic metric for genital size. Indeed, the data presented here suggest similar selection pressure on these structures [if considering only their rational and data]. However, there appears to be no relationship between clitoral and penile size and ability to achieve orgasm from unassisted vaginal intercourse, calling into question the use of any metric for genital size as a proxy for selection on orgasm."

    I am a good scientist too, lack of evidence for one theory is not not evidence in favor of alternative theories.

    • scicurious says:

      Hi Vinny,

      Thanks so much for commenting. I absolutely agree with you about calling out the previous paper (if I did not make it clear in this post I apologize), and I think you were totally right to do so, as their conclusions were...not particularly carefully drawn. And I agree with you that I don't feel that there's a relationship between genital size and selection for orgasm (though I'm surprised that only Masters and Johnson have done work on that subject). So I totally agree with your conclusions, but I feel like the discussion of the paper in various sections:

      "Indeed, the data presented here suggest similar selection pressure on these structures."

      This is the sentence I have an issue with, because I don't feel that these data suggest similar selection pressure on the structures, because I don't feel that female genital size needs to be under selection pressure to have any effect on orgasm. It was this upon which I based my final critique. I also wished to dispel the notion that many people have that orgasm must have a defined role based purely on the physical structures involved. While this may turn out to be the case, it has by no means been proven.

      But I do agree with you calling out the previous authors and their work. I do think that a study of any correlation between female genital structure and orgasm in women would be an excellent study, and I hope that you are currently undertaking those studies or are aware of studies like this occurring. I have no doubt that you are an excellent scientist, and I appreciate you coming by to comment on the blog.

      • Vinny Lynch says:

        No worries, I did not take personal offense.

        So, I said “Indeed, the data presented here suggest similar selection pressure on these structures.” for several reasons.

        The first was that my paper was a direct response to the Wallen and Llyod paper. In their paper they claim that they found evidence that the variability in the length of the clitoris was significantly greater than the variability in the length of the penis. Thus, according to them, the selection pressure acting on the length of the penis is greater than the selection pressure acting on the length of the clitoris. Then the erroneously assume there is some connection between length and orgasm (which is of course baseless). But, using the same data, albeit with a different take, I was able to show that the size of the variation in the size of clitoris was not different than the size of the penis. I put that statement in the paper as a direct jab at Wallen and Llyod, so it can't really be understood in the context of my paper alone. It really only assumes a meaning as a response to Wallen and Lyod, it's their context not mine.

        But, it does raise an interesting issue. Selection can either be a stabilizing force or a disruptive one (and I don't mean force in the physical sense, selection is more a a sieve than a force, but its a useful short hand so....). Wallen and Llyod base their argument on stabilizing selection. If there is an optimum size for a structure, selection with reduce the variance in the size of the structure (statistical variance I mean); imagine a normal distribution of the size of the penis, for example. If there is an optimum size, selection will push the tails of the distribution in and the shape of the distribution will be very narrow with small tails. If, however, selection on size is weak then it will allow a lot of variation in the size of the structure and the distribution will become very broad. By comparing the size of the distributions one can gauge how much variance is allowed and how strongly selection is acting on the structure. It ends up that the the variation in volume of the penis is the same as the variation in volume of the clitoris. So, the actually are under similar selection pressures for size.

        But, that is a "value-free" statement. It makes no assumption as why they are under similar selection pressure. It may be for orgasm, or it may be that they are developmentally linked in such a way that you cannot change the shape of the penis very much without changing the shape of the clitoris as well (or visa versa), thus selection is not acting on both individually but at the same time. Or it may be?????

        "I don’t feel that female genital size needs to be under selection pressure to have any effect on orgasm. "

        Me either, but again my statement was value-free. I never said that the selection pressure was for orgasm. Just that the variance in size between the penis and clitoris were not significantly different, so the selection pressures acting on volume are similar. I even cite Masters and Johnson as saying that there is no link between the size/shape of the clitoris (or the penis) and orgasm.

        I too tend toward pair-boding as a potential explanation. I can't remember who said it, so therefore will paraphrase it badly, but someone once said (maybe Roughgarden) that sex is not like soccer, hands are allowed. As are many other body parts for that matter. There is no need to restrict the discussion on the role of the clitoris in orgasm to vaginal intercourse, since humans do seem to restrict the ways we have sex to vaginal intercourse either. Hell, have you heads what bats are up to?

  • Isabel says:

    I mean "these are". Haha, sorry. Lots of mistakes lately.

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