Why Is the Human Vagina So Big?

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).

每个人都见过,网上的一些文章在兴奋不已地告诉你,人类的阴茎在灵长类中有多大,为什么会进化成这么大。其实所谓的大,不是指长度,而是指茎围。说到这个话题,你去问Alan Dixson就准没错。他对有关阴茎大小的文献一贯持比较谨慎的态度,但连他也承认,人类的阴茎“周长和其他人猿的阳具相比是个异数” (见《性选择与人类繁衍系统起源》第65页)

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?]


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.


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.

当然,这会演变成一个鸡先蛋先,又或者是龟下有龟的问题【译注:”It’s turtles all the way down”来自一则古老的轶事,体现逻辑上的无限递归,后来成为一句玩笑话,用来表达表面立于不败之地,而实则回避逻辑问题的境况】。不过,拜托,互相契合的灵长类体征具有相关性,这不是很明显吗?你应该会同意,就算没有性经验的人,不需多想也会接受这个有关人类阴茎进化的理论吧。

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.


[水禽雌雄生殖器官协同变异的图例。标星型的为雄性生殖器,箭头指向的是女性生殖器。图片来自“水禽雌雄生殖形态的协同进化”。DOI编码: 10.1371/journal.pone.0000418]

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.



[红毛猩,大猩猩,黑猩猩与人类的对比。这些数值偏离了灵长类总体的回归分析。数据来自Dunsworth等人,2012年,PNAS第 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.S. 我在谷歌学术搜索了一下,可惜没有找到不同哺乳动物阴道长度或尺码的对比。也许有,但我找不到。我倒是找到了狗娘的尺码……是真的母狗啦!还有牛逼的尺码……也是真的母牛啦!所以如果要认真对比的话,我得收集整理一些数据才行。通过分析婴儿的大小可能也是一个办法。

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!

P.P.S. Dixson书中第73页的图表4.3,列出了九种灵长类动物的阴茎与阴道的长度。谢谢Patrick C.提醒我曾经看到过这样的图表,让我可以告诉读者上哪里找。



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