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.