to create thousands of expressions.

But do these expressions look the same

and communicate the same meaning around the world,

regardless of culture?

Is one person's smile another's grimace?

Charles Darwin theorized that emotional expression was a common human feature.

But he was in the minority.

Until the mid-20th century,

many researchers believed that the specific ways we show emotion

were learned behaviors that varied across cultures.

Personality theorist Silvan Tomkins was one of the few to insist otherwise.

Tomkins claimed that certain affects—

emotional states and their associated facial expressions—

were universal.

In the 1960s, psychologist Paul Ekman set about testing this theory

by examining hundreds of hours of film footage

of remote tribes isolated from the modern world.

Ekman found the native peoples' expressions to be not only familiar,

but occurring in precisely the situations he would expect.

Conversely, he ran tests with tribes who had no prior exposure to Western culture.

They were able to correctly match photos of different facial expressions

with stories designed to trigger particular feelings.

Over the next few decades,

further research has corroborated Darwin's idea

that some of our most important emotional expressions are in fact universal.

The degrees of expression appropriate to a given situation can, however,

vary greatly across cultures.

For instance, researchers have studied facial expression in people who are born blind,

hypothesizing that if expressions are universal,

they would be displayed in the same way as sighted people.

In one study, both blind and sighted athletes

displayed the same expressions of emotion when winning or losing their matches.

Further evidence can be found in our evolutionary relatives.

Comparisons of facial expression between humans and non-human mammals

have found similarities in the structure and movement of facial muscles.

Chimpanzee laughter looks different from ours,

but uses some of the same muscle movements.

Back in the 60s, Ekman identified six core expressions.

Anger is accompanied by lowered eyebrows drawn together,

tense and narrowed eyes, and tight lips;

disgust, by the lips pulled up and the nose crinkling.

In fear, the upper white of the eyes are revealed as the eyebrows raise

and the mouth stretches open,

while surprise looks similar,

but with rounded eyebrows and relaxed lips.

Sadness is indicated by the inner corners of the eyebrows

being drawn inwards and upwards,

drooping eyes,

and a downturned mouth.

And of course there's happiness:

lips drawn up and back,

and raised cheeks causing wrinkling around the eyes.

More recently, researchers have proposed additional entries

such as contempt,

shame,

and disapproval,

but opinions vary on how distinct boundaries

between these categories can be drawn.

So if Ekman and other researchers are correct,

what makes certain expressions universal?

And why are they expressed in these particular ways?

Scientists have a lot of theories rooted in our evolutionary history.

One is that certain expressions are important for survival.

Fear and surprise could signal to others an immediate danger.

Studies of humans and some other primates

have found that we pay more attention to faces that signal threats over neutral faces,

particularly when we're already on high alert.

Expressions also could help improve group fitness

by communicating our internal states to those around us.

Sadness, for example, signals to the group that something's wrong.

There's some evidence that expressions

might be even more directly linked to our physiology.

The fear expression, for instance,

could directly improve survival in potentially dangerous situations

by letting our eyes absorb more light

and our lungs take in more air, preparing us to fight or flee.

There's still much research to be done in understanding emotional expression,

particularly as we learn more about the inner workings of the brain.

But if you ever find yourself among strangers in a strange land,

a friendly smile could go a long way.