Red genitalia study: Testing the sexually salient hypothesis

What Does the Color of Our Genitals Have To Do With Evolution?

What Does the Color of Our Genitals Have To Do With Evolution?

The state of the universe.
April 16 2012 2:04 PM

Pretty in Pink

What does the color of our genitals have to do with evolution?

woman with red lips, red flower
Are humans really naturally attracted to red?

Olga Ekaterincheva/iStockphoto

Thankfully, it’s not 50 shades of gray—at least, not until well after menopause—but the human vulva does come in different shades of pink. This color has hundreds of officially-recognized variations, and I suppose a busy gynecologist or promiscuous lover might eventually stumble upon a smoky mauve specimen, or one that's watermelon or even salsa. But according to a recent study by the anthropologists Sarah Johns, Lucy Hargrave, and Nicholas Newton-Fisher of the University of Kent, the traditional view of how this natural color gradient inspires lust in males of the species—namely, the redder the better—is deeply flawed.

Their research, published earlier this month in the journal PLoS ONE, tested a theory of preferred labial hue that's known as the "sexually salient hypothesis." According to many evolutionary biologists—and perhaps some makeup and costume designers, too—men find the color red especially stimulating, as it reminds them on some deep and species-wide level of the way that a woman's genitals might become flushed at the moment of maximum sexual receptivity, perhaps just in advance of ovulation. Among many Old World primate species, the brightly-colored anogenital swellings of females serve as blinking marquees that signal peak fertility, and while such swellings may have deflated over evolutionary time in our own species—just imagine how difficult it would be to fit into a pair of tight-fitting jeans with your bottom erupting every so often like a baboon’s rump—scientists have proposed that the effects of seeing red still linger in the brain of modern man. As far back as 1966, the pioneering sexologists William Masters and Virginia Johnson noted the existence of "red-light" districts, red Valentine's Day cards, red lingerie, red lipstick and rouge, and wondered if the appeal of the color red in the human sexual response might be a consequence of the concealed ovulation (and, more often than not, the concealed labia) of human females.


The male preference for the color red is, indeed, a robust and demonstrable effect: Across cultures, men find women more attractive when they are bedecked in red or appear against a red backdrop, sometimes without even being aware of their bias. But there's (at least) one problem with this axiomatic account—namely, that no one had ever bothered to examine the male reaction to actual red genitalia. This was a peculiar oversight. Given that the sexually salient hypothesis rests on the assumption that red clothes and makeup are proxies for a real-life, engorged vulva, one might expect to see a preference for the supposedly receptive thing itself. Having identified this gap in our understanding, Johns and her colleagues decided to test this idea using heterosexual male college students. When presented with vulvae of different colors—nothing carnivalesque, mind you, just a few on the naturalistic spectrum of pinkish-reds—which one would they find most attractive?

The experiment may sound straightforward enough, but testing the sexually salient hypothesis under laboratory conditions proved more difficult than the researchers had anticipated. First, there was the issue of acquiring suitable images of vulvae for evaluation. A proper scientist can’t just trot out a line of willing ladies brandishing labia in a bouquet of genital pinks for visual inspection. Such research demands attention to the small detail of “experimental control,” which in this case meant a series of carefully selected, close-up photographs of representative pudenda. The use of such images allowed the investigators to rule out differences in vulva morphology, of which there is actually considerable (and understudied) variation. Yet the authors inform us that even these photos were hard to come by: “Explicit images of anatomically normal, un-retouched, non-pornographic, similarly oriented female genitals were surprisingly difficult to obtain.”

Eventually, they came across a website with the stated mission of “providing information about the female body … celebrating its beauty with pictures of the clitoris, vulva, labia and vagina”—a scientific treasure trove, to be sure. From its wide selection of user-submitted images, the researchers identified four special vulvae that met their criteria—essentially, these organs weren’t scabrous, pierced, tattooed, or in the process of being invaded by some plying appendage. The pubic region also had to be shaved, enabling a clear view and assessment of its precise genital pinkness.

Once these four key images had been chosen, and the investigators given email approval from the models for the use of their copyrighted “girls,” each one was digitally retouched to generate a set of ostensibly realistic vulval shades: pale pink, light pink, dark pink, and red. (You can scroll down to Figure 1 of the journal article to see how this color manipulation appeared. Perhaps I’m wrong, but to my untrained, homosexual eyes, the colors do indeed appear to capture a natural-looking variation—though the red does look a bit raw and painful.) That left the investigators with 16 pictures in all: four different vulvae, each rendered in four distinct shades of pinkish-red. Potential male subjects for the study were first “warned of the graphic nature of the vulva images before participating”—and yet, wouldn’t you know, every single one of these brave young men decided to proceed with the testing. They were also checked for relevant visual problems. It’s unclear how a scarlet vulva would appear to a man with red-green color blindness, for instance, but better safe than sorry.

The 40 men who passed screening were allowed to advance through the 16 photos at their own slow and steady pace, rating each vulva shown on a sliding scale from 0 (unattractive) to 100 (attractive). The results—I know you’ve been dying to hear them—came out against the sexually salient hypothesis. The men showed no particular fancy for darker shades of pink; in fact, they found all the standard pinks to be equally attractive—with an average score of about 40—while the red vulvae were less appealing to a statistically significant degree, scoring a mean attractiveness value of just 35.

What might explain this slight aversion to red genitals? It's possible that the darkest shade simply looked more artificial than the other colors. There’s also a rather odd contradictory caveat written in the original article: The authors of the study claim to have used "colors within the normal range expected for human genitalia," (and they mention having a certified nurse midwife and clinical specialist sign off on the colors for her professional opinion). But then they claim that they were never trying to capture the male response to a "natural variation in vulval morphology or color.” Instead, they were interested in whether the male bias for the color red applies in the general context of female genitalia. As a result of this peculiar hedge, it's fair to wonder what might happen if one of the shades happened to fall near the very edge of the presumed "normal range." It’s easy enough to imagine Mother Nature despoiling our body parts with a palette of yellow, green, and purple disease, so perhaps the men would have an aversion to exotic colors independent of everything else. After all, red, inflamed genitals might be symptomatic of sexual arousal, but they could also signify a vulvovaginal infection. That logic applies to men as well; I think I speak for most of my gay peers, and straight women, too, in saying that a surprisingly crimson scrotum isn’t a terribly inviting sight, either.

In any case, Johns and her co-authors propose that whatever special attraction men might have to women wearing red is caused by some other psychological mechanism than was previously thought. Since red has been found to be associated with perceptions of male dominance, they suggest that a woman with red accoutrements might function as a sort of “badge” to enhance a man’s social status among his peers. “Women may even use red (clothing, cosmetics),” write Johns and her colleagues, “to stimulate such competition as a means to select higher quality mates.” In other words, a man’s desire to be seen with a lady in red may be less about his being attracted to the mirrored state of her wanton genitalia than it is a reflection of his own social psychology.

Theirs was a modest, imperfect little study. Personally, I’ve come to distrust any measure of male arousal that does not involve phallometry, and it seems like the researchers ought to have asked their male subjects what it was about the reddest vulvae that turned them off. The men might not have been able to articulate their color preferences in clear terms, but a direct question would still have given us some useful insight into their thought processes. Factoring in such limitations, however, Johns and her colleagues have at least given us reason to believe that—after decades of our having swallowed it up as an evolutionary truth—the sexually salient hypothesis might be a lot of bunk.