CU Scientists Discover the Evolutionary Advantage of the Stink Eye

The next time you give the stink eye to that guy who spills beer on you at a Rockies game, you probably won't be thinking about the evolutionary advantage of your gaze. But according to a new study in an ongoing research program by University of Colorado Boulder scientist Daniel Lee, eye expressions actually serve advantageous evolutionary functions.

Notes Lee, "Our latest paper concludes that visual sensitivity for an expresser conveys mental sensitivity to a receiver" — which is essentially a science-y way of saying that when you give a stink eye or express some other emotional state through your eyes, "others perceive [you] as having mental states of sensitivity." The notion that you can glean someone's emotional state through their eyes may seem simple at first, but Lee's research is actually part of a series of rigorous studies with large scientific implications.

Lee's been conducting research since 2008, alongside mentor Adam Anderson. An early study they conducted looked at the correlations between fear and disgust and sensory functions. (The sight of their participants making scared or grossed-out faces while being measured in an optometrist's lab was probably entertaining enough to validate the entire project.) They concluded that when someone experiences fear, for instance, their eyes widen, which increases sensitivity to light, while on the opposite end, sensory apertures for light and smell sensitivity would close when experiencing disgust. Each of these reactions has an advantage: When you're fearful, greater sensitivity to light and other sensory information might provide useful data in a dangerous situation, while the stench of rotten tomatoes just might make you wish you'd never had a sense of smell to begin with.

A later study took this idea a step further and determined that human eyes had evolved in a unique way "to translate gaze signals," says Lee. Having a large area of white sclera (a trait that is unique among primates) in comparison to relatively small irises actually provides a distinguishing contrast that helps us signal to others. "We are capitalizing on the morphology of our eyes," says Lee. "If our eye whites were black, widening or squinting our eyes wouldn't convey a better signal." In other words, humans evolved unique methods of communicating simply through ocular expression.

Lee and Anderson's latest paper arrived at the conclusion that not only do eyes communicate signals, but they can convey complex mental states between an expresser and a receiver. When you make a "visually discriminant face" (one of suspicion or contempt — yes, the the stink eye), an observer can distinguish "mental discrimination," Lee says. That means we can communicate very intricate states of emotion through our eyes, often without even realizing it. So when your mother says she can tell when you're lying, you'd better believe it: She's probably looking at your eyes.

This research also helps explain why people can often recognize the emotional state of someone from a completely different culture or part of the world through facial expression, even though there are apparently 370,000,000,000,000,000 different facial expressions that people can make. "The fact that similarity [of expression] exists in this number suggests that there has to be some kind of reasonable origin that we can trace," says Lee.

In the future, Lee imagines that research like this may help those who have difficulty understanding social cues. If there is some guiding rule to facial expression, maybe we could teach a "grammar of the eyes," he says. There's also a possibility that companies might find a way to use computerized expression-reading to maximize consumer satisfaction and profit.

"A camera at Walmart might be able to read a customer's face and determine if they are pleased or displeased," Lee says, with a laugh.

Cue the stink eye.