“Ingång / Entrance,” Mikael Tigerström, via Flickr.

We are obsessed with penis and testicle size. Yet, we can barely say “vagina” and when we do we’re usually talking about the vulva.

Everyone’s come across some article somewhere on-line that is thrilled to share how big human penises really are, for primates, and to explain why they evolved to be so big. It’s not really the length, but the girth. Alan Dixson is your go-to on this. He’s conservative in his assessment of the literature on penis size and even he concedes that human penis “circumference is unusual when compared to the penes of other hominoids (apes)” (p. 65 in Sexual Selection and the Origins of Human Mating Systems).


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All this is just terribly fun to write about and I’m not even going nuts (gah) like they do. And they do. They really do. And all over the Internet they do: “Evolution of human penis” gets 53,000 hits just on scholar.google alone, and about 832,000 on Google.


I’m talking about all penises in the universe and all vaginas too. Sure there’s variation, but a penis can’t be too wide. It helps to be long, probably, but it can’t be too long.

So neither pleasure nor psychology need matter at all, just function associated with some sort of fit. Pleasure and psychology are never invoked to explain penis morphology in other animals. If anything, it’s the cornucopia of horrifying, not pleasing, animal penises that begs for evolutionary explanations.

DOI: 10.1371/journal.pone.0000418

Examples of genital covariation in waterfowl. Stars are male parts, arrows show female parts. Figure from “Coevolution of Male and Female Genital Morphology in Waterfowl” DOI: 10.1371/journal.pone.0000418

Wouldn’t you explain the size and shape of the key by the size and shape of the lock? So wouldn’t it be a little more scientifically sound to hypothesize that the human penis is sized and shaped like that because it fits well into the human vagina?

Sure, it gets chicken-and-eggy or turtles-all-the-way-downy, but c’mon. Isn’t it a bit obvious that the privates that fit inside the other privates are probably correlated? You’d think that even the people who have never had intercourse would default to this explanation for the evolution of the human penis.

But we’re rarely, if ever, told that human penises are relatively girthy because human vaginas are. It’s always about male competition or female preference.

Sure, we may be a little weird compared to our close relatives for not having a baculum (penis bone), and maybe that’s the sort of thing you want to explain for whatever reason, but does human penis size and shape need a uniquely human story?

Assuming it’s correlated to the vagina like it probably is in many other species,* then no it doesn’t… unless the size and shape of the human vagina has an exceptional story.

Does it? We wouldn’t know. There are zero (look!) articles titled “Why is the human vagina so big?”

Until right now.

Here we go. If we were going to answer it the same way we’ve long explained the human penis, and other animal penis shapes, then we’ve got a few ideas…


And of course, we’d need to demonstrate that the human vagina is in fact larger, relative to body size, than the vaginas of other primates. Regardless, a sound answer to the question of vagina size and shape focuses on childbirth, wouldn’t you say? She’s got to be big enough to push out a baby and, for humans, it’s a great big baby.

Comparison of orangutan (Pongo), gorilla, chimpanzee (Pan), and humans. These are essentially departures from a primate-wide regression analysis. Figure from Dunsworth et al. 2012. PNAS 109(38): 15212-15216.

So if there’s an exceptionally human story for the great big human penis, that exceptional story originates not in a woman’s orgasms, not in her pornographic thoughts or her lustful eyes, but in her decidedly unsexy “birth canal.”

And I dug up a nice little note to explain this to us all written by Dr. Bowman, a gynecologist, back in 2008 for the Archives of Sexual Behavior which is magnificent. It starts out giving the only vagina-size-based, not to mention childbirth-based, explanation for human penises that I can find in the literature (which is thankfully cited by Dixson in his book mentioned above). But it still manages to bring the explanation beyond the vagina and onto another proud triumph: “In sum, man’s larger penis is a consequence of his larger brain.”

After you clean up the coffee you just spat onto your computer screen, you can read it all for yourself by clicking on the link up there (or emailing me for the pdf).

Guess who didn’t read it? That study in PNAS, mentioned above, that showed women naked penises, got a high attractive score for the big ones, and thinks that’s evidence for mate choice now, today, let alone back when (I’m going to speculate that) women had a tiny bit less of it.


But you heard it here, at least.

Childbirth is why the human vagina is so big and, consequently, why the male penis is so big. It’s pretty straightforward. Yet we’re still left scratching our heads as to why the penis question endures.

Is evolutionary science averse to big vaginas?

Does nobody love a big vagina?

Because that’s just ridiculous. Everybody came from one.


P.S. Unfortunately a few scholar.google searches led me to find no cross-species comparisons of mammalian vagina lengths or any vaginal measures. It may be out there, but I haven’ t found it. I found some measures for bitches… DOGS! And some heifers… COWS! So I’ve got to compile some data if I’m to do this properly. Baby size might be a way to do this.

P.P.S. p. 73 in Dixson has Figure 4.3 with nine primate species’ penile and vaginal lengths plotted. Thanks Patrick C. for reminding me where I’d seen something like this and where to point readers!

Published On: December 3, 2015

Holly Dunsworth

Holly Dunsworth



  • Eva says:

    I feel like this article is fairly tenuous in its claims and written from a certain Eurocentric view.

  • ButWait says:

    The conversation is penis-focused because they are the ones at the unfortunate end of the supply/demand spectrum. Any vagina seeking a penis finds one, the penis undergoes a more rigorous selection process

    • Harrow says:

      Which in turn drives the bigger head size, as the female needs a large brain to sort through all the lies presented by males seeking her vagina, and the male needs a large brain to figure out how to keep all the promises he made while navigating the female’s rigorous selection process.

  • cam says:

    Unsatisfied wife trying to make her marriage longer or compensating for her one is bigger

  • Robert Smart says:

    And we don’t need a pleasure related reason why a big penis is needed for a big vagina. One of the functions of our funny shaped male organ is to pull out sperm that is already there, to give ours the best chance. It needs to be.big enough to do that. Always pleasure follows good function.

  • MrE says:

    Parts is parts.

  • Mandingo says:

    This article does not explain why black guys have massive penises and why asian guys have such small ones.

  • Stooger says:

    People are so stupid.

  • John says:

    Has no one considered the vagina might be bigger because women on occasion push a human being through it? I mean, it’s just a thought.

    • John says:

      My bad, did not read the article closely enough.

      • Jason says:

        If you look at the mortality rate in childbirth (it was pretty high in pre-advanced medicine times), you could probably argue that the vagina isn’t even big enough, that the rate of evolution in the sizes of vaginas has not kept pace with the rate of change in head size, and that evolutionary process is likely being interfered with by caesarian sections, episiotomies, drugs, etc, while whatever process is selecting for bigger heads is not being impeded, and is possibly being sped up by the same, aforementioned actions. Plus, a vagina can accommodate a much larger penis than nature is currently supplying (girth-wise), so I don’t really see any correlation here between the data and the claims, aside from the obvious one about the decreased chance of insemination from throwing a hotdog down a hallway.

        • Holly Dunsworth says:

          Medical intervention that you’ve described is recent so you’d need to be writing from the future to backup your comment. The vagina is big enough, there are over 7 billion people on the planet, many more than that have been alive and the vast majority of all these people were not c-sected out of their mothers, but came through their mother’s perfectly big enough vagina.

  • P. Barrie says:

    Was the turtles comment a Carl Sagan reference? If so, you must have come through a huge birth canal, and my penis just got a little bigger.

  • wiiiii says:

    I watch curb your enthusiasm too! You oughta give deserved props tho!

  • Jesse Marczyk says:

    Humans have larger vaginas relative to other mammals due to the issues inherent with childbirth; that’s an argument I could buy, but there’s no evidence presented for it in the current article (“Unfortunately a few scholar.google searches led me to find no cross-species comparisons of mammalian vagina lengths or any vaginal measures”). I’d call it plausible nevertheless and I would be surprised if one found that vaginal girth was uncorrelated with birth-related factors.

    With that in mind, this would lead to the next matter: why would penis girth be expected to vary with vaginal size? The length correlation is understandable, as it positions the head of the penis closer to the cervix and could allow for a better placement of the ejaculate.

    But why girth? The author never really gets into the matter of what function the girth serves.

    • Holly Dunsworth says:

      Thanks for your comment. I was only following the rules of the genre as I made claims based on little evidence. As to “why girth”? The mind reels with possibilities. Perhaps if it’s not big enough to be detected on the inside, the female won’t put up with what the male’s doing on the outside. Or it’s just allometry and genetics. Speculations, unlike vaginas, are limitless…

      • Pioof says:

        I’m afraid I don’t buy this one. Any female vagina can contract to squeeze a teenie weenie penis. And basically childbirth puts constraints on width (actually rather, extensibility), not on length, wheras it’s usually the length which does impress ladies (no, actually, mostly men — ladies are not that interested in merely watching it).

      • Pioof says:


  • Robert King says:

    The author is only considering one causal factor at a time. One thing we do known is that the clitoris is large, mostly internal and composed of complex functional sensitive tissue.
    So–as the canal area expands to fit expanding (premature) then penises become engaged in an arms race to fit and interact with this. Evidence in favour? Well, none of that (costly) female senstive tissue has become vestigial. Quite the reverse. And do penises just deliver sperm ? They do not–they bend internally so as to interact with every single one of those sensitive areas.

  • Michelle says:

    Vagina’s aren’t “big” though, and if they are “big” to pass out babies they are not 35cm diameter big which is the average circumference of an newborns head. More critical to this is pelvic diameter vs head as the infant must negotiate the bony passage on the way out.

    Instead vaginas are about 7.5cm deep and more like a collapsed tube normally, maybe a bit over 2 cm in diameter. It’s that the vagina is a muscular tube which is elastic and has rugae which makes it capable of great expansion during childbirth and during sex.

    Walking upright wouldn’t have made the vagina less conspicuous rather than more because it is an internal structure not easily seen at any time and that doesn’t work for the vulva either which then is pretty much pointing at the ground, we aren’t like baboons who typical move on all fours where the female presents her bottom to the male but in fact humans even have concealed ovulation, where no one can really tell when a woman is doing it.

    This does really need someone to look at this more coherently, with an eye to anatomy and function as a whole. Maybe the male penis adapted along with female anatomy, and they can’t be separated out.

    • Holly Dunsworth says:

      Thanks for your comment. I’m sorry that I wasn’t able to get the satire (etc) across more obviously to every reader of this post. Perhaps it’s also a language issue. Perhaps it’s also a problem if readers haven’t seen the human penis evolution literature or the popular dissemination of it.

      • Holly Dunsworth says:

        P.S. the viral meme that humans have “concealed ovulation” deserves a satirical piece as well. I wish I had one ready to post right now.

  • Marco says:

    When you measure the penis and make comparisons, you leave the penis territory and you enter the phsllus area. And no matter your biological speculations, you are under castration complex. No measure , from now on, will satisfy your craving for a good answer. Only to grasp what kind of jouissance or enjoyment is the one, without any social approval, is the good one for you.

    • Tanitani says:

      Huh? I attribute pretty good reading comprehension to myself, but I have no idea what your pompous sounding writing is about.

  • Tanitani says:

    Thank you, Holly, for your … commentary as you put it. I wondered about this question myself, although I did not enter into research mode.

  • danielx says:

    This blog post is long on implausible speculation and short on actual data. A more serious exploration would consider the histological composition and functional properties of the vaginal tube. The wall is made up primarily of multiple layers of smooth muscle and like the uterus and the urinary bladder, has a capacity to stretch and expand far beyond its resting dimensions. This capacity to enlarge allows it to accommodate birth of the baby is enhanced by the unusual number of layers of epithelial cells, and the elongate dermal papillae that attach to the basal layers of the epithelial cells.

    • danielx says:

      As for your speculation that “walking upright made the vagina conspicuous…” I think someone needs an anatomy lesson….

      • Holly Dunsworth says:

        And I think someone needs to go back and read the piece more carefully and see it for the satirical commentary that it is.

        • Holly Dunsworth says:

          To be fair, perhaps it isn’t as widely known that bipedalism and its display of the penis is an actual hypothesis for why the human penis is so big. However, I carefully crafted the sentence heading that paragraph so that readers would be aware that those hypotheses were intentionally ridiculous or strange.

          • Helga Vierich says:

            The body size of the human infant – relative to the level of development – is unusual. other animals with such relatively large infants do have bigger vaginas and corresponding penises – I refer you to the fairly numerous on-line photos and videos of erections in stallions and bull elephants. But these tend to be species where the infant is on its feet and mobile within a few hours. The human evolutionary pattern went the other way – the relative infantile (and vaginal and penis) size produced a remarkably helpless creature requiring, literally, at least a decade and a half of parental care.

            And, unlike the horses and elephants, very like our close relatives the Bonobos, sex serves social functions a thousand times as frequently as it serves reproductive ones… in fact, I may have made a conservative estimate just now. Hence the vital importance of the enlarged clitoris and the famously high sex “drive” of human males, who seem – at least to me- to be completely undeterred by absence of physical signals of ovulation. The fact that sex is pleasurable is not just about incentives to fertilization. In humans, it may not even be primarily about reproduction at all, but rather about social bonding.

  • Albert Constantine, Jr. says:

    If the average vagina is 7.5 cm deep as is noted in the comments section, and the average penis is 5.1 inches in length (as noted in the linked article), then I estimate that adults in Rhode Island are experiencing a 39 mile shortfall of vagina.
    Is a stimulus package in order to help close this important gap in infrastructure?

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