There’s a tired pop evolutionary psych idea that gets repeated by scientist and nonscientists alike—I think I heard it most recently in the movie “Hanna”—that red lipstick is meant to be a symbol of engorged female genitalia, and thus that lipstick is meant to signal sexual receptivity and possibly fertility. This idea apparently dates back to a chapter called “Sexual selection and human ornamentation” by one B. S. Low in a book called Evolutionary Biology and Human Social Behaviour. It’s called the “sexually salient” hypothesis—the idea that the colour red is attractive because of its resemblance to naughty bits. Supporting evidence (“evidence”?) for this idea included various studies indicating effects of the colour red on people’s perceived attractiveness, performance in sports, and perceived social dominance, as well as the fact that numerous primate species, especially baboons, have exaggerated, red female genitals and a corresponding male preference for red. Not surprisingly, the idea wasn’t properly tested but was picked up and repeated. Let me toss out a few objections to it before we even get to scientific hypothesis-testing. First, those primates with red genital displays have lots of body hair but hairless genitals and are quadrupedal. Humans are kind of the inverse, having pubic hair but little body hair, plus we walk on two legs, so chances are that female genitalia weren’t constantly on display in our australopithecine ancestors. Furthermore, redder lips could simply be a signal of better overall health or fertility, regardless of whether lip colour correlated with aroused genital colour.
If human females use red ornamentation to attract males by coopting a male preference for redder “sexual skin” as an indicator of fertility, we’d expect the following: male preference for redder female genital skin, male preference for redder lips (this relationship seems to be somewhat validated), and a correlation between female genital colour and health and/or fertility. The latter two predications would be tricky to measure properly—are we talking about a between-person correlation between a woman’s lip colour, labia colour, and health/fertility, or one that occurs within an individual woman over the course of her menstrual cycle? On top of that, we’d have to be careful to rule out a preference for red lips because they indicate health/fertility, as opposed to a preference for red lips because they look like more fertile genitalia. It would be easier to test—and easier to disprove—the first hypothesis, that men prefer redder female genitalia.
A new PLoS ONE study has done just that, using pictures of female genitalia digitally manipulated to be different shades of pink/red within the normal human range. They asked forty straight men to rate these pictures on a 1 to 100 scale of attractiveness. Interestingly, they rated the darkest red images as least attractive. Nor was there a correlation between attractiveness and the shade of pink. Hypothesis effectively rejected. Well, let me throw in some caveats about sample size (40 isn’t that many participants) and demographics (while the authors didn’t report anything about the age or race of their test subjects, perhaps their sample was too uniform to show anything*). Regardless, the preference for pinker genitals stands in sharp contrast to data on preference for red lipstick, clothes, and other ornamentation, making the “sexually salient” hypothesis highly unlikely.
This study does something I’m fully in favour of—testing a widely-repeated but unsupported idea—and disproves the hypothesis pretty soundly. It doesn’t, however, manage to break away from the assumption that a male mate preference must be universal and adaptive. It is entirely possible that strong evolutionary pressure shaped human male preference for a certain genital colour, and the authors list some quite plausible such pressures: menstrual blood (which indicates that a woman is not ovulating at that time) and diseases like yeast infections and trichomoniasis (which can cause reddening of the labia, and would put a mate at risk of infection). They don’t raise the point that a lower preference for red could also be cultural, though they do cite a study that found no change in the redness of Playboy centrefolds’ labia since the late fifties, suggesting at least that no recent cultural trend explains the preference for pink. But this preference doesn’t have to be adaptive at all—especially if vulva colour was never an important part of mate choice in our recent ancestors. Indeed, preference for red ornaments needn’t be adaptive or even genetically based either—just look through the archives of Sociological Images for examples of how the meaning attached to different colours changes over time and between cultures. Take-home message: test your hypotheses, but make sure you don’t limit your alternative hypotheses as well.
*But let me just say that, since many pop evo psych ideas come from observations of Western cultural expectations, if we can disprove it for a sample that fits those expectations, that’s a pretty strong sign.
Johns, S., Hargrave, L., & Newton-Fisher, N. (2012). Red Is Not a Proxy Signal for Female Genitalia in Humans PLoS ONE, 7 (4) DOI: 10.1371/journal.pone.0034669