Sunday, September 09, 2007

Facial attractiveness and correlation vs. experiment   posted by agnostic @ 9/09/2007 08:14:00 PM

The topic of facial attractiveness came up at Cognitive Daily, and it presents a good opportunity to contrast the two main approaches in psychology -- correlational and experimental. I'll start with an informal chat, and then proceed to look at a published study. Since the literature is vast, this will just touch on a few key points. And a warning to our mostly male readership: I'm only going to focus on what makes male faces attractive, both for "equal time" and because it's more mysterious.

Do girly eyes, lips, and skin make a guy less attractive? Hardly -- just look at any of those "teen hearthrob" magazines. In one response to a PNAS study, a photo of various dream guys features Billie Joe Armstrong next to the qualities "music" and "looks." What makes him look better than Bear Grylls and Kurt Vonnegut? Well, he has large girly lips and eyes, and tighter skin. This is typical of "pretty boys": other examples are Johnny Depp, Ryan Phillipe, and so on. However, the skeletal morphology (jaw, cheekbones, chin, brow) is masculine. It's only the non-boney parts that are girly.

So, for guys, we've already found two principal components of facial attractiveness: a manly skull and girly soft parts that fill it out. But this brings up a very important point in asking such questions. That is, cognitive scientists often hew to the experimental approach -- let's keep two faces exactly the same, but change one feature, and see which is more attractive. This game of Mr. Potato Head purportedly avoids the entangled mess of confounding factors that would turn up in a correlational study, such as one using principal components.

But there are good reasons to believe that there are non-trivial statistical interactions between facial features, so that isolating one and varying it misses the point: whether girly eyes are attractive depends on what the rest of the face looks like. It's just hard to judge the attractiveness of facial features out of context since facial perception is a pretty gestalt process.

It's clear that part of the variation in facial appearance is due to genetic differences between individuals. Whatever these variational genes may be, their effects are pretty fundamental. There are probably significant epistatic (or gene-gene interaction) effects in skull morphology just because so many different parts have to coordinate their work to make the face look right. (Here is a study showing epistatic effects on the symmetry of mice teeth.) Throughout development, a single gene could have pleiotropic effects on various parts of the face, and ditto for a single circulating hormone.

The point is, if we keep all of those the same and vary just one piece, we've lost the correlation structure. We could get wacko results for that reason alone. Imagine if a very ugly guy had his photo manipulated so that he had large girly eyes -- they would look very out of place, unsettling, perhaps jarring. We'd cringe from the bizarro effect alone. (For female faces, consider that very tight skin is attractive -- unless you take an old woman's head and give her a tight facelift, resulting in that extraterrestrial transvestite look.) In reality, though, girly eyes probably go along with other features that make them neutral or attractive.

With that in mind, let's turn to a study on girly facial features in guys. Here is a free PDF, so if you comment, at least look at the pictures and read the graphs. What the experiments show is that, keeping everything else the same, increasing the luminance contrast between the (darker) eyes and lips vs. the (lighter) rest of the face made a female face increasingly more attractive, but a male face increasingly less attractive. The interpretation is that female-typical traits are attractive in females but ugly in males.

If you look at the male pictures, though, it's clear why the feminized photo scored so low: he looks like a damned weirdo. Later in the paper, there are pictures of his entire head -- you can see that he has a very unattractive, schlubby male face. We react by cringing at his picture with high-contrast eyes and lips because it's so incongruous. Again, in real life, guys with more female-typical eyes and lips look like pretty boys, so it's not unsettling but rather attractive. I've already mentioned some mechanisitic reasons why that may be, but there could also be correlational selection on the skeletal and soft traits, or cross-assortative mating between males with manly skulls and females with doe eyes, pouty lips, and taut skin.

As a reality check on the unsexiness of high-contrast eyes and lips for male faces, in the picture of Billie Joe Armstrong that The Intersection chose to showcase his good looks, he is wearing heavy mascara -- just as Johnny Depp did for the Pirates of the Caribbean movies. I'll bet that's also part of the hunk appeal when attractive football or baseball players are photographed with that black stuff under their eyes (functionally unnecessary for having your picture taken).

Also, have a look at another study which created "average faces" from many real faces (these studies always show that the composite is most attractive). Their characteristics of beautiful faces shows a protypical sexy and ugly male face. First, note how similar the sexy non-skeletal features are for both sexes. It's hard to tell which face show a higher luminance contrast between the eyes and lips vs. rest of the face, since the sexy guy has darker skin. To me at least, the sexy guy has more pop-out-of-the-background eyes and lips, while the ugly guy has more uniformly drab features. But maybe a more sophisticated instrument than my eye will say that the sexy guy has lower-contrast features than the ugly guy. If so, the conclusion would be more believable since it did not come from a Mr. Potato Head experiment.

So, on a constructive note, the way these studies of facial attractiveness should be done is to do something like PCA on a huge dataset of certified dreamy and drab guys. Just "by inspection," it's clear that "manly skull" and "girly soft features (eyes, lips, skin)" are two. Symmetry is another one, although as I mentioned at Cognitive Daily, symmetry is not heritable, so don't think that says anything about "good genes" sexual selection using "fitness indicators."

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