Psychological research suggests chin shape may be a physical signal of the quality of a mate. For example, women may prefer men with broad chins because it’s sign that a man has good genes; likewise, a woman’s narrow chin may correlate with high levels of estrogen. Zaneta Thayer, a graduate student at Northwestern University, and Seth Dobson, a biological anthropologist at Dartmouth, examined the sexual selection hypothesis by measuring the chin shape of nearly 200 skulls in a museum collection, representing people from all over the world. The pair discovered that there is a small but distinct difference in chin shape between the sexes, with men having a taller, more pronounced chin. They argued in the American Journal of Physical Anthropology in 2010 that this difference is evidence against explanations that the chin evolved to resist mechanical stress. If chins evolved in response to eating or talking, then there should be no difference in chin shape between the sexes because, presumably, men and women eat and talk the same way.
I’m no Creationist, lord knows, but when I read popular accounts of the latest insights in evolutionary psychology, I almost always feel like I’m reading something that will be quoted by future writers with embarrassment or contempt–like we regard Phrenology today, or more aptly, Social Darwinism or Eugenics.
Dobson and Thayer, I’m sure, are as serious-minded as any scientists should be, but the underlying presumption of the Smithsonian article–that a human anatomical feature, if uniquely evolved, must have its own unique just-so story to explain it–strikes me as being sort of silly.
Even if you could prove that chiseled chins correlate with “good genes” (which genes?), why would our primeval forebears have looked to chins for evidence of those genes and not to something more proximate and unambiguous–big muscles say, for strength, sleekness for food-finding prowess, or overt signs of sexual interest (flirtatious behavior, visible arousal) to signal sexual availability? And why would a feature have to be unique to one sex for the other sex to find it attractive? Wouldn’t it be enough that it’s attached to a member of the opposite sex? Mightn’t a heterosexual man be indifferent to the shape of Steve Tyler or Mick Jagger’s lips but find the same shaped lips appealing when they’re tacked onto a woman’s face? Isn’t it possible that a heterosexual woman might be indifferent to a wispy mustache on a female’s upper lip but find it adorable on a hunky male?
What about all the sexually attractive characteristics that are completely outside the realm of reproduction and hence evolution–ie, some of the features that gay men and lesbians find attractive on their respective objects of desire? I can think of all sorts of sexually-attractive characteristics that are so culturally specific that they’re probably not evolved at all. Some people, for example, find tattoos to be real turn-ons or lacy underwear or spiked heels. Or think of those obese neolithic goddess figures. Evolutionary psychology says we must have chins because our ancestors liked them and that they liked them because they signaled something beneficial. Maybe we like our chins just because we have them–it’s not so much that Kirk Douglass’s is the ideal physiognomy, but that a really weak or recessive chin looks somehow less than human.
The real key, it seems to me, is to find the correlate. If the gene that controls chin shape is also involved in estrogen production and estrogen production confers a genetic advantage, then you’d be hitting closer to the mark. But correlation, as they say in social science, isn’t causation–and evolution isn’t teleology. The fact that we have a feature doesn’t mean that it’s the best possible feature–maybe it’s a crude adaption, a vestige, or a meaningless concomitant of some other beneficial mutation.