The vagina is not the homologue to the penis. Why do we keep treating it as such?

As an historian of sex, I surely must open this post with the following delightful quotation from Tom Laqueur’s book Making Sex: Body and Gender From the Greeks to Freud:

In 1559 ... Columbus—not Christopher but Renaldus—claims to have discovered the clitoris. He tells his 'most gentle reader' that this is 'preeminently the seat of woman’s delight.' Like a penis, 'if you touch it, you will find it rendered a little harder and oblong to such a degree that it shows itself as a sort of male member.' Conquistador of an unknown land, Columbus stakes his claim: 'Since no one has discerned these projections and their workings, if it is permissible to give names to things discovered by me, it should be called the love or sweetness of Venus.' Like Adam, he felt himself entitled to name what he found in nature: a female penis.

I am happy to report that finally—450 years later—sex researchers rediscovered the clitoris. What am I talking about?

For years now, we’ve been hearing that men on average are sexually target-specific, while women on average are not. In other words, if you show men various kinds of pornography while having a little measuring device strapped to their penises, those penises don’t get hard to every type of pornography; instead, they seem to evince a distinct preference for either men or for women as sexual “targets.” By contrast, if you insert a blood-flow measuring device into women’s vaginas and show them various kinds of porn, well, they appear to become aroused to just about everything sexual—men, women, monkeys, you name it.

This kind of research has been carried out in the lab of my Northwestern University colleague and friend, Michael Bailey, beginning with the dissertation research of Meredith Chivers. Bailey and Chivers and colleagues have subsequently done related studies seemingly confirming these sex-specific findings. Bailey has gone so far as to raise the possibility that women don’t have a sexual orientation, if we understand (as he reasonably does) sexual orientation to mean orientation toward that which arouses us.

Notably, life history studies, like those done by Lisa Diamond, seem to provide real-life support to these laboratory findings: In terms of sexual orientation, women appear to be more sexually fluid over their life courses—and to understand themselves as more sexually fluid—than men.

The vagina comes from different embryological tissue altogether, so why should we expect it to behave in a way that is comparable to the penis?

Nevertheless, the problem I’ve long had with the arousal studies is this: The vagina is not the homologue to the penis. The penis's homologue is the clitoris. The vagina comes from different embryological tissue altogether, so why should we expect it to behave in a way that is comparable to the penis? The reason the clitoris gets an erection when a woman is sexually excited, the reason most women don't reach orgasm via their vaginas, is because the clitoris is the organ that corresponds to the penis.

So why study the vaginal response and not the clitoral response when doing a study comparing the arousal patterns of males and females? When I have asked sex researchers this, they have given me two reasons.

First, they say they haven’t had a device for studying clitoral response, and they’ve had one for studying vaginal response. This answer always reminds me of the joke about the drunk who is found looking for his keys under the street lamp not because that is where he dropped them, but because that is where the light is.

The second reason they’ve given me is that the vagina is the route through which women conceive, and so if you want an evolutionary understanding of sex (which I do, as they do), then you have to look at what leads to babies.

I’m not so sure about that logic either. See, here’s the thing: It’s entirely possible that women’s vaginas lube up at the slightest provocation. More than one sex researcher has suggested to me this might be the case because women have historically had to deal with sexual assault, and automatic vaginal lubrication might protect them, to some degree, from injury that would ultimately interfere with reproduction.

OK, fine—let’s say that just-so story is true. That doesn’t mean that automatic vaginal lubrication indicates arousal. It might just indicate a primal sort of fear. That’s why I’ve wanted to know what clitorises might tell us. They might, as the female homologues to penises, indicate more choosy mothers.

By way of analogy, a choosy mother might find her mouth automatically salivating to both coffee and peanut butter, but given a more exacting test of her tastes, we’ll find she really would rather have Jif than Joe. As it is, it seems as if sex researchers have been doing the equivalent of comparing women’s salivary responses to various foods to men’s gastric responses to those same foods.

Well, good news. Years ago, an international group of sex researchers working out of the Netherlands managed to cleverly redesign the usual laboratory vaginal measuring device to also measure the response of the clitoris. (Their paper is referenced here.) They even managed to design one that did not require lab personnel to hold it to the subject; this is one the subject can insert herself, alone in a laboratory test room. And what did they find when they strapped women up to this elaborate little lap puppy?

The clitoral measurement approach “is a valuable additional tool, providing data that the vaginal photoplethysmograph cannot: it proved sensitive to inhibition of the sexual response, in contrast to the [vaginal device].” Meaning? By measuring the clitoris, the researchers were able to observe the women subjects’ downturn of arousal in response to being startled out of the moment. This was not so observable with the vaginal device. (The researchers staged the cold-shower moment by arranging a “problem” announcement over the intercom to suddenly interrupt the porn-viewing moment.) This difference seems important, no?

Moreover: “[T]he inverse relationship between VPA [the vaginal response] and CBV [the clitoral response] at moments of high sexual arousal suggests that VPA may be a more automatic, preparatory response rather than a measure of genital arousal per se.” In other words, their results suggest that, sure enough, women’s arousal patterns may be a lot more specific—more like men’s—than the vaginal measurements reveal. A woman’s vagina may indeed lubricate to sexual signals from both Jif and Joe, and even Jif's monkey and Joe's dog, but her clitoris might reveal that she is, in fact, much more aroused by Jif than any other option. And if she’s in an environment that allows her a choice of sex partner, that differential arousal may well matter to evolutionary history.

So is it true, as we've been told, that men are on average more sexually target-specific than women? Maybe not in terms of arousal. Then how could we explain the life histories that seem to show that women are on average more sexually fluid than men? Maybe the truth is that women choose sex partners—especially long-term partnerships from which surviving children may be more likely to result—based less on immediate arousal than on other considerations. Like what? Oh, you know, like a consideration of who can be trusted to find their clitorises.

A version of this post originally appeared on the author's personal site.