UW-Madison researchers: Types of smiles send different messages in social situations

A smile, like a picture, is worth a thousand words.

Although most commonly associated with happiness, smiles can indicate nervousness, embarrassment and even misery. To add to their mystique and versatility, smiles can express sophisticated messages that influence the behavior of others in social situations.

Due to their surprisingly complicated nature, scientists have long struggled to classify smiles, but a study published last week by the University of Wisconsin-Madison and collaborators has now categorized them into three groups.

Rather than reflecting emotions, the three types of smiles were defined by the social functions they serve. Reward smiles signal approval, affiliative smiles promote bonding and dominance smiles establish social status.

Researchers also determined the specific muscle movements used in each smile, which had not been done before.

“When distinguishing among smiles, both scientists and lay people have tended to focus on true and false smiles,” said Paula Niedenthal, a professor of psychology at UW-Madison involved in the study. A true smile indicated a state of happiness and was identified only by the formation of crow's feet around the eyes.

“But people smile in many different circumstances and during many emotional states. So asserting that only smiles that result from states of happiness are ‘true’ smiles limits our understanding of this important facial expression,” she said.

The study began by deconstructing the muscle movements performed during each smile. Researchers used a facial-expression generator to make 2,400 facial animations. The combination of facial movements was random, but each animation incorporated an action from the “smile muscle,” the zygomaticus major, that pulls up the corners of the mouth.

Study participants were then asked to sort the animations — all 2,400 of them — as reward, affiliative, dominance or neutral/other.

A second round checked if new participants could distinguish between the three types of smiles, and a third group ranked how well a smile accomplished each of the three functions.

The results showed that recipients felt positive when they saw a reward smile, which was defined by a symmetric, sharp pull of the lips that revealed teeth and by a bit of eyebrow lift. A reward smile communicates enjoyment and pleasure and is “the kind of smile you would use with a baby, so he will smile back or do things you like,” said Niedenthal.

An affiliative smile was also symmetric but thinner. The lips were pressed together to hide the teeth, signaling a lack of aggression. The acknowledgment and approachability communicated by affiliative smiles produced positive feelings in recipients, as well.

Dominance smiles were sharply different from the other two. In this case, the zygomaticus major produced a lopsided sneerlike smile that is often seen in athletics and politics when someone feels like they have won.

Clues provided by the other components of a dominance smile helped to explain the negative reaction of recipients upon seeing it. Wrinkling of the nose and raising of the upper lip are associated with anger and disgust. Raising of the eyelids to show more whites of the eyes is also found in expressions of anger, fear and surprise.

While participants could easily identify the dominance smile, they found it more challenging to distinguish between the reward and affiliative smiles.

Niedenthal suspected this result was partly due to the lack of common and concise labels for smiles. “People know exactly what that smile is when they see it and they respond to it appropriately. But it’s hard to talk about it in words,” she said. “People are not used to thinking about smiles this way.”

The results from the study may help with intercultural communication. Americans smile so much that visitors from other countries are taught to smile more.

However, only teaching them one smile can cause confusion. “Simply teaching people about the existence of different types of ‘true’ smiles can help people pay more attention and avoid some of those misunderstandings,” said Niedenthal.

Niedenthal’s collaborators are studying how affiliative and dominance smiles affect the results of games that require collaboration, trust and risk.

Niedenthal will use this information to advise surgeons who reconstruct and repair facial muscles and bones.

"They may have to make choices that will affect a patient’s expression for the rest of their life,” she said. “It’s useful for them to know how different kinds of smiles are used in the world and which muscles are involved in making them.”