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).

A favorite explanation for the big phallus is female mate choice, that females selectively make babies with males who have larger and, presumably, more pleasurable semen delivery devices. This is backed up by studies. When life size projections of naked men are shown to female subjects, they say they find the ones with bigger ones to be more attractive. [This is exactly how mate choice works where I live, how about you?]

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Other explanations include male competition. If you can deliver your package to the front yard but the other guy can deliver to the front door, his is more likely to be carried inside the house first. Or, if he can steal away what you just delivered, then, again, his package has yours beat. Thanks to his big penis he’s more likely to pass on his winning penis genes than you are to pass on your loser penis genes. Loser.

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.

But doesn’t it make sense that for a penis to be somewhat useful it has to be somewhat correlated to vagina size?

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…

Because walking upright made the vagina conspicuous and males thought a bigger vagina was better. Because big vaginas outcompete small ones at catching sperm. Because of male pleasure from coitus with a big vagina. Because of heat dissipation or thermoregulation. Because of a tradeoff with brain size.

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.

Point is, the literature rages on with the special explanations for the big penis with nary a big vagina in sight.

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!

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

Published On: December 3, 2015

Holly Dunsworth

Holly Dunsworth

Holly Dunsworth is Associate Professor of Anthropology at the University of Rhode Island where she teaches courses on human origins, evolution, and variation. She performs paleontological work at the early Miocene sites on Rusinga Island, Kenya where some of the most ancient fossil apes are preserved. She also studies living primates, particularly when it comes to their energy use, reproduction, and life history.

43 Comments

  • Eva says:

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

    • ButWait says:

      I feel like this comment is ambiguous and lazy in its claims and written from a really useless point of view

    • Chris says:

      Rhode Island may ultimately be named after the Dodecanese Island of Rodos, but the Ocean State is still an ocean away from any part of Europe, and that same ocean and most of a continent away from Rodos. Criticisms of Eurocentricity regarding things happening in Europe and being done by Europeans are arguably pointless, but when applied to the doings of non-Europeans who are not in Europe, they’re objectively false.

  • 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.

  • 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.

          • Elia says:

            Isn’t there a abnormally high childbirth mortality rate among humans (unaided by mordern medicine) in comparison to other primates?
            If we assume males had more ability to select mates, which you kind of imply was the case and which certainly seems to have been the case since marriage was implemented until the women’s lib movement, couldn’t an explanation for the lag in vagina size relative to baby size then be that men sexually selected for women with small vaginas relative to their penis?

  • 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:

        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 spark conversations.

  • 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.
    https://www.psychologytoday.com/blog/hive-mind/201307/the-lady-vanishes
    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.
    https://i.ytimg.com/vi/rhBa_P3A6gU/hqdefault.jpg

  • 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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  • Chris says:

    It’s a fascinating article. I expect that the lack of “hard” data on human vaginal size is a matter of medical ethics. The dimensions of the resting vagina in an unaroused and unpenetrated state are well known by now, but the only way to determine the practical limits of vaginal elasticity involves significant risk of injury. Similarly, the only way to determine how much air a balloon can hold involves popping many of them, and we can’t go doing that with the sensitive parts of people, simply in the interest of science, so it will and should remain an open question. As to the size of penises, I don’t think that we can say that the subject has been exhausted until human penis size is expressed in volumetric terms, in cubic centimeters or cubic inches. Using a tape measure may be interesting, but even for those who remember grade-school math, penises are not more than only roughly cylindrical. No one has yet undertaken the simple and safe process of using a water displacement test on any number of adult male humans. Penises are, after all, irregularly-shaped, three-dimensional appendages. Talking about linear inches or centimeters of penis length or circumference is about as precise as talking about bra cup size in letters, for which there is no standard in the garment industry. My points is that the surface of the subject of our consideration of human penis size and vagina size, and the directions and causes in movement of the sizes of these organs now and over time has been barely scratched in scientific terms, and that this article offers some very interesting ideas.

  • The two most measured organs of man’s body throughout history have been the nose and the phallus. The nose, placed right in the middle of the face, is the first thing that stands out in a face, but what is the reason for obsessing over the length of the penis when it is not in sight because it is hidden among the clothes?

    Since time immemorial man has covered his nakedness, initially the purpose of doing so has been to protect us from the cold, but also to protect us from external threats. The human skin seems to have evolved to feel and perceive pleasant sensations such as caresses or hugs, hundreds of nerve endings have provided this capacity of our skin. If we compare ourselves with other animals with exoskeletons, or covered with a thick fur our body is defenseless against any aggression whether climatic or by another living being. This is the main reason why we avoid nudity, and yet we are concerned about the size of the male limb, it will be interesting to look for the explanation and we will try to find it by immersing ourselves in different disciplines such as anthropology, physiology, biology, ethology, sociology and psychology. All these branches of knowledge can provide an interesting vision and perhaps we will find a common nexus and a cause-consequence that will answer our question.

    We can stop at one of the most important animal instincts, the reproduction or survival of the species, this instinct would determine the behavior of the first hominids.

    Based on the discipline of ethology, we found an interesting study of the American Ornithological Society Journals https://academic.oup.com/aosjournals/?code=coop-site shows that sexual competition between ducks determines the size of their penis, the study reveals that the length and morphology of the genitals of ducks changes according to their social environment. In the published article it is deduced that the males that suffer a great competition tend to develop bigger phalluses of size than those of the males that live in a less competitive environment, this responds to a clear strategic before the difficulty that supposes to have to compete between great quantity of males to copulate with the same female.

    If we study the anatomy of the human penis, we observe how two bodies are clearly differentiated, the shaft of the penis and the glans. The bulge of the glans and the perpendicular arrangement of the crown of the same is a characteristic of the male. According to a study by Gordon G. Gallup, Jr. “Semen Displacement as a Sperm Competition Strategy in Humans” https://journals.sagepub.com/doi/full/10.1177/147470490400200105 argue that the human penis has evolved in this way to displace the semen of rivals in sperm competition. Analyzing the study, it can be observed that the anatomy of the human penis has evolved to be introduced into the woman’s vagina and, during intercourse, the mission of the glans would be extracted from the vagina the semen of another male if there was one, leaving the entrance of the uterus free to later inseminate the woman with her own sperm.

    Humans have gone from being a nomadic and hunter group to promoting stable settlements with collection of crops to ensure food for their population. Some anthropological studies indicate that in many Amazonian Indian tribes it is thought that a woman needs to receive semen from different males in order to become pregnant. Women would seek to have relationships with several men so that their son could inherit the best of each: the best hunter, the strongest. The study is: Evolutionary history of partible paternity in lowland South America. https://www.pnas.org/content/107/45/19195 .

    According to the evolutionary anthropologist Kit Opie of the University of London “our ancestors, the first primates would be solitary males who preferred to live in isolation, uniting only for mating”. Homo sapiens evolved, became gregarious forming small settlements, created societies and enhanced monogamy, but the phylogenetic vestige of the importance of penis size in man remained.

    At the beginning of the first civilizations of western culture, a matriarchy spread over most of the Mediterranean. With the invasions of the peoples of the sea they brought the patriarchy, marrying the masculine gods with the ancient goddesses, relegating them to a second place. Where the ancient goddesses of fertility are exalted in the matriarchy with developed secondary sexual characteristics with large breasts and hips praising the magical power of women in reproduction. With the victory of the patriarchy, the importance of the penis, symbol of the power of the male, begins to be enhanced. Known with images of the god Priapus with a large erect phallus or the god Dionysius, satyrs etc … In the excavations of Pompeii and Herculaneum can be distinguished on the lintel of many houses the representation of a penis associated with fortune and prosperity. It has not been unusual to find in contests the use by the victors of violations over the vanquished in a demonstration of power and triumph.

    Sociologists, psychiatrists and psychologists have collected this deep interest in the phallus of man in multiple studies.

    Freud the father of psychoanalysis states in the theory of psychosexual development that during the phallic phase of the child is in the area of the urethra where his libidinal point resides. The theory suggests that the phallus becomes a major priority interest for both sexes, linking fundamental events in psychosexual development. These facts are later reflected in the Freudian theory of the Oedipus complex, where the child develops his first sexual impulses towards his mother. The girl realizes that she does not have a penis to have a heterosexual relationship with her mother and then wants it, this is described as the envy of the penis. Consequently, a sexual desire towards his father is stimulated.

    Subsequently from behavioral psychology framed with the identification of environmental variables that interact with man and produce a response. For years, sociologists have been asking the question: “Does size matter? On the Internet we can find studies of affirm and disprove, but it is true that sociological studies may be biased by the author to tilt the balance to one side or the other. For this reason we abandon sociological study to focus on Psychology.

    Several studies show that the visualization of a large penis influences the sexual response of the sexual partner. If we use the language of behavioral psychologists, an unconditioned stimulus (Ei) would generate an unconditioned physiological response (Ri), the greater that stimulus (larger penis), the greater the probability of a greater response (more desire). Subsequently, new stimuli that would initially be neutral would be associated with that response, for example a muscular body, wealth or power would become a conditioned stimulus (Ed) that would also provoke a response of sexual desire, this time differential for being of different stimuli (Rd).

    The male associates power, money or having a great failure to get desire answers in the woman, then occurs operating behaviors and will enhance all these attributes associated with profit and success. Similarly if we buy a suit and many people flatter us (social reinforcement) will increase the likelihood that we put on that suit again to be reinforced.

    We live in large urban centers where we must constantly compete with our neighbors to achieve our goals. There is a constant comparison, and just as the males fight to get to mate, sometimes the males compare with each other. This is the case of the locker room syndrome, https://www.andropenis.com/es/dismorfofobia-pene/ coined for the first time by the psychologist Fernando Molina-Campuzano, the man compares the member size of others with his own. In that comparison we find reaffirmation if the size is above the average or also a feeling of complexity if you see that the size is smaller.

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