Why it’s written all over your (plain) face

Plain faces are more expressive than fancy ones, according to a new paper coauthored by a Howard University professor.

Our facial features give away our identity. But our facial expressions—an arched brow, a wink, a sneer—can give away our innermost thoughts and feelings. Those nonverbal cues play an important role in the formation of social groups: It promotes group cohesion, smooths communication during times of conflict, and facilitates bonding. And we humans are better at it than probably any other creature on the planet.

How did we come to be so good at expressing ourselves with our faces, and at reading the expressions of others? Rui Diogo, a professor in Howard University’s department of anatomy, and collaborators at the University of Washington and Dartmouth College hope to answer that question. To find clues, they’ve now turned their attention to our closest living relatives—non-human primates.

Some primates display a human-like penchant for facial expression. Indeed, the most gregarious primate species tend to be those capable of the widest variety of facial movements. Along many branches of primates’ evolutionary tree, however, more social species also tend to have colorfully patterned faces. The fancy mugs are thought to help members of a species identify one another, especially in situations where many different species may coexist in one geographical area.

Diogo and his coworkers wanted to find out if the complexity of a species’ facial features was linked its expressiveness. And it is—but maybe not in the way you’d expect.

The researchers looked at more than 20 species—from marmosets to macaques, from gibbons to gorillas. To quantify facial complexity, they tallied the number of uniquely colored areas on the animal’s face. To quantify expressiveness, they watched footage of each animal to see how frequently it, say, pulled the corner of its lip, stuck out its tongue, flared a nostril, or performed any of the more than 50 other movements catalogued in the Facial Coding Action System.

The researchers found that species with plainer faces tended to exhibit a wider range of facial movement. What’s more, an animal’s facial mobility could be predicted from the size of its facial nucleus, the part of the brain that controls the facial muscles. More evolved species tend to have larger facial nuclei, larger ranges of facial mobility, and fewer decorative embellishments.

Diogo and his colleagues think those results could point to an evolutionary tradeoff: Too much facial “clutter” may make an animal’s facial expressions hard for others to read; a simple face, on the other hand, is like a blank canvas on which to paint one’s thoughts and feelings. Another factor may be that smaller species tend to have smaller eyes, which limits their ability to discern subtle facial movements. For those animals, simply recognizing one another may take priority over reading each others’ thoughts. If nothing else, that might explain the diminutive marmoset’s haunting mug shot—and impressive poker face.