Saturday, December 11, 2010
Once again, I'm going to ask you to close your eyes and picture this: your perfect mate. If you were to describe him/her to me how would you do so? Hair, eyes, and skin color? Height and build? Think about it while we go through a short sexual selection primer.
Natural selection produces changes in the genetic composition of a population from one generation to the next. These changes occur as traits become more or less common in a population due to effects on the survival and reproduction of the individuals within that population/species. There's all kinds of mechanisms and processes involved in natural selection, but we are going to focus on sexual selection. Sexual selection is a special case or adjunct to natural selection. This type of selection acts on an organism's ability to successfully attract a mate. One of the key words being "successfully". After all, you can't pass on a trait if you don't produce offspring. Sexual selection acts on the "attractiveness" of an individual to the opposite sex. I put that word in quotes because attractiveness is different in each species. In many species this type of selection leads to sexual dimorphism, where one sex looks different from the other, often as a result of ornamentation or primary sexual characteristics. In some cases a trait will go so extreme that natural selection acts upon it -- if your trait decreases your survival ability to the point where you do not live long enough to reproduce then that extreme trait gets removed from the population. The attractive trait doesn't necessarily have to be some type of bodily ornamentation. It can be courtship dances, nuptial gifts, building elaborate structures/nests, territoriality, combat skills, or any other of a host of things. The overall point is that you have to have or do something that attracts the opposite sex in such a way that it makes you the most attractive of all while still allowing you to survive to reproduce. As with most scientific theories, it gets much more complicated than that, but I think you get the point.
So now let's go back to that picture-this-scenario and add some information based off of what we know about sexual selection and the attractiveness of the human face. We know that facial features that increase a person's attractiveness serve as subconscious cues of biologically important variables such as health. We also know that human faces show marked sexual shape dimorphism, men's faces are different shapes than women's faces. Yeah, I know, a "duh" moment right. Well, just hang with me for this one. It has been found that attractiveness for female faces is related to signs of youth, symmetry, and averageness (an odd term, I know, but basically meaning 'not weird looking'), and that these features signal health, femininity, and fertility. Male faces are considered to be more attractive with increased symmetry and averageness. But as many women will tell you, greater masculinity does not always go hand-in-hand with greater attractiveness. In this instance, I'm using the word "attractive" to relate to facial features rather than an overall impression - thing pretty boy vs. tough guy. However, many women will also tell you that both the pretty boy and the tough guy can be attractive, just not necessarily in the same way.
The shapes of the human faces themselves are also important. Highly feminine faces tend to have relatively large eyes, smaller brow ridges, smaller jaws, and fuller lips. Attractive male faces tend to have longer and wider jaws, relatively smaller top halves and eyes, and more prominent brow ridges. Those descriptions I'm taking right from the article even though they tend to conjure up a rather funny looking person in my mind's eye. Anyway, its all related to genes and hormone levels during puberty. A topic better left for another post. For this particular study it is also important to note that humans show marked height dimorphism as well. Men, in general, are taller than women.
So far I've been relaying information (mostly) from a study I came across recently, published in the journal Evolutionary Psychology, about the evolutionary origin of the shape dimorphism in human faces and how that is related to height dimorphism. In layman's terms, does the angle or tilt at which you see someone's face make them more or less attractive?
Now, picture your perfect mate not just as a set of handsome/pretty characteristics but those characteristics on a person standing right in front of you. What do you see now?
This study had participants complete two tasks designed to measure the masculinity/femininity of a face as well as rate their attractiveness. They used a 3D face modeling program that manipulated the portrayed pitch of a model - untilted (straight), tilted slightly upwards, further upwards, slightly downwards, and further downwards - while using "examples of unattractive, real, attractive, and average" faces of the sexes.
They found that the pitch of the face directly influences its perceived masculinity/femininity and affects its perceived attractiveness. They found that an upward tilted face is judged to be more masculine (or less feminine in female faces) and downward faces judged to be more feminine (or less masculine in male faces). Sure, that makes sense, especially when you factor in the height dimorphism. Think about it: A male is taller than a female, the male viewing the female from above perceives her face as tilting down, the female viewing the male from below perceives his face as tilting up. Remember those funny sounding descriptions of the attractive faces (jaws, brow ridges, etc.)? Why those shaped features? Perhaps they are due to divergent sexual selection pressures that resulted in the selection for male and female faces that had these pitch perspective differences as part of their typical proportions. Or maybe they are more related to behavior. The authors draw a parallel between the dominance/appeasement displays of other species - stretching/rearing vs. crouching/bowing. Upward tilting faces are more dominant than downward tilting faces. I gotta say, the little feminist voice in my head cringes at that one.
So, was your picture-this perfect mate tilting their head upward or downward?
Guess maybe I should practice my coy look.
Here are your links:
Burke, Darren and Danielle Sulikowski (2010) A new viewpoint on the evolution of sexually dimorphic human faces. Evolutionary Psychology: 8(4), 573-585. (link)