[SciShow intro plays]   Hank: What kind of day do you think this woman is having? And what’s going on here? And probably it isn’t hard to imagine how these two are feeling, right? That’s because most humans are really good at silently communicating and interpreting a whole range of emotions using only facial muscles. Whether it’s voluntary or involuntary, a simple curled lip, raised eyebrow, or crinkled nose says a lot.

In fact, many psychologists think that some of our basic facial expressions, like the ones that express anger, fear, happiness, surprise, sadness, and disgust are innate not learned, and are universal across cultures. That’s because these basic expressions probably started out as practical reactions to stimuli, and eventually became associated with emotions.

In his appropriately-named book The Expression of the Emotions in Man and Animals, published in 1872, Darwin helped popularize the idea of universality: that we’re born to express emotions in a certain way. He suggested that we humans inherited our emotional expressions from our ancestors, and that these expressions helped increase survival by facilitating communication in social groups. He also proposed that our expressions adapted to environmental stimuli, something that I will come back to in a minute.

Darwin’s book even came with olden-day photos of people demonstrating facial expressions, like these pictures of people trying their best to show what grief looks like. In the late 1960s, psychologist Paul Ekman began testing Darwin’s universality idea by traveling around and conducting lots of independent, cross-cultural studies. In New Guinea, for example, he encountered an isolated culture that had never seen outsiders before. He told subjects brief emotional stories like “old friends are coming to visit,” or “you just stepped on a rotten dead pig,” and he showed them a set of three different photographs of facial expressions.

Ekman found that they usually picked the expression that he expected them to associate with the emotion in the story, like a frown for sadness. So even though this culture was very isolated from ours, the population used the same basic facial expressions. Other studies have found that infants, as well as people who were born blind, also use the same facial expressions, without being taught which expressions go with which emotions.

By the 1970s, Ekman and other researchers had started working on a more objective way to measure these universal facial expressions, by documenting muscle activity on the face itself. They compiled what’s known as the Facial Action Coding System, or FACS. The system looks at muscle engagement, noting the intensity, duration, and asymmetry of an expression. By comparing the muscle movements on someone’s face with what an expression is supposed to look like, psychologists can use the FACS to help distinguish emotions. Like, telling real smiles from fake ones, to detect if someone is lying, or to identify certain emotions in people who can’t express themselves verbally.

So it seems like we’ve probably evolved with some basic facial expressions built-in, and a 2013 study out of Cornell University looked at why. The researchers had their subjects make neutral, scared, and disgusted expressions, and then measured how much light made it to their retinas with each expression. And it turns out that facial expressions may have first evolved to help us better react to our environments by controlling our sensory input, like the amount of light we let into our eyes.

Say you’re walking through the jungle, for example, and suddenly you hear a loud crash. It scares you, and as an expression of fear sweeps across your face, you widen your eyes. This immediately expands your field of vision, letting in more light, and heightening your visual sensitivity to help you locate any danger.

But if, say, you accidentally step on a mound of mystery poo in the park, you’ll do something very different. As you recoil in disgust, your eyes narrow, letting less light in as you sharpen your focus to examine your soiled foot. That classic disgusted nose-wrinkling also helps decrease the size of your nasal cavity and let less air flow through it, presumably so you don’t have to smell as much of whatever is giving you the nasties. This suggests that how you feel actually shapes your perception of reality, and how much light hits the back of your eye.

Some social communication may have evolved from these reactions to outside stimuli, which also supports Darwin’s idea that basic facial expressions are universal. Though there is definitely a learned component to our facial expressions, too. If you’ve ever nodded politely during a relative’s political rant, when really you felt like screaming inside, you know that we learn to control our faces and mask emotions in certain social situations. So, many facial expressions are probably innate. But managing them is a whole different story.

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