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Originally published January 25, 2011 at 12:11 PM | Page modified January 26, 2011 at 6:44 AM

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What's in a smile? More than you think

In a recent issue of the journal Behavioral and Brain Sciences, researchers argue that smiles are not simply the expression of an internal feeling. Smiles in fact are only the most visible part of an intimate melding between two minds.

The New York Times

In the middle of a phone call four years ago, Paula Niedenthal began to wonder what it really means to smile. The call came from a Russian reporter, who was interviewing Niedenthal about her research on facial expressions.

"At the end he said, 'So you are American?"' Niedenthal recalled.

Indeed, she is, although she was then living in France, where she had taken a post at Blaise Pascal University.

"So you know," the Russian reporter informed her, "that American smiles are all false, and French smiles are all true."

"Wow, it's so interesting that you say that," Niedenthal said diplomatically. Meanwhile, she was imagining what it would have been like to spend most of her life surrounded by fake smiles.

"I suddenly became interested in how people make these kinds of errors," Niedenthal said. But finding the source of the error would require knowing what smiles really are — where they come from and how people process them. And despite the fact that smiling is one of the most common things that we humans do, Niedenthal found science's explanation for it to be weak.

"I think it's pretty messed up," she said. "I think we don't know very much, actually, and it's something I want to take on."

To that end, Niedenthal and her colleagues have surveyed a wide range of studies, from brain scans to cultural observations, to build a new scientific model of the smile. They believe they can account not only for the source of smiles, but how people perceive them. In a recent issue of the journal Behavioral and Brain Sciences, they argue that smiles are not simply the expression of an internal feeling. Smiles in fact are only the most visible part of an intimate melding between two minds.

"It's an impressive, sophisticated analysis," said Adam Galinsky, a social psychologist at Northwestern University.

Psychologists have studied smiles carefully for decades, but mostly from the outside. When the zygomaticus major muscles in our cheeks contract, they draw up the corners of our mouths. But there's much more to a smile than that.

"A smile is not this floating thing, like a Cheshire Cat," said Niedenthal. "It's attached to a body." Sometimes the lips open to reveal teeth; sometimes they stay sealed. Sometimes the eyes crinkle. The chin rises with some smiles, and drops in others.

Cataloging these variations is an important first step, said Niedenthal, but it can't deliver an answer to the enigma of smiles. "People like to make dictionaries of the facial muscles to make a particular gesture, but there's no depth to that approach," she said.

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Some researchers have tried to move deeper, to understand the states of mind that produce smiles. We think of them as signifying happiness, and indeed, researchers do find that the more intensely people contract their zygomaticus major muscles, the happier they say they feel. But this is far from an iron law. The same muscles sometimes contract when people are feeling sadness or disgust, for example.

The link between feelings and faces is even more mysterious. Why should any feeling cause us to curl up our mouths, after all? This is a question that Darwin pondered for years. An important clue, he said, is found in the faces of apes, which draw up their mouths as well. These expressions, Darwin argued, were also smiles. In other words, Mona Lisa inherited her endlessly intriguing smile from the grinning common ancestor she shared with chimpanzees.

Primatologists have been able to sort smiles into a few categories, and Niedenthal thinks that human smiles should be classified in the same way. Chimpanzees sometimes smile from pleasure, as when baby chimps play with each other. But chimpanzees also smile when they're trying to strengthen a social bond with another chimpanzee.

Niedenthal thinks that some human smiles fall into these categories as well. What's more, they may be distinguished by certain expressions. An embarrassed smile is often accompanied by a lowered chin, for example, while a smile of greeting often comes with raised eyebrows.

Chimpanzees sometimes smile not for pleasure or for a social bond, but for power. A dominant chimpanzee will grin and show its teeth. Niedenthal argues that humans flash a power grin as well — often raising their chin so as to look down at others.

"'You're an idiot, I'm better than you' — that's what we mean by a dominant smile," said Niedenthal.

But making a particular facial expression is just the first step of a smile. Niedenthal argues that how another person interprets the smile is equally important. In her model, the brain can use three different means to distinguish a smile from some other expression.

One way people recognize smiles is comparing the geometry of a person's face to a standard smile. A second way is thinking about the situation in which someone is making an expression, judging if it's the sort where a smile would be expected.

But most importantly, Niedenthal argues, people recognize smiles by mimicking them. When a smiling person locks eyes with another person, the viewer unknowingly mimics a smile as well. In their new paper, Niedenthal and her colleagues point to a number of studies indicating that this imitation activates many of the same regions of the brain that are active in the smiler.

A happy smile, for example, is accompanied by activity in the brain's reward circuits, and looking at a happy smile can excite those circuits as well. Mimicking a friendly smile produces a different pattern of brain activity. It activates a region of the brain called the orbitofrontal cortex, which distinguishes feelings for people with whom we have a close relationship from others. The orbitofrontal cortex becomes active when parents see their own babies smile, for example, but not other babies.

If Niedenthal's model is correct, then studies of dominant smiles should reveal different patterns of brain activity. Certain regions associated with negative emotions should become active.

Embodying smiles not only lets people recognize smiles, Niedenthal argues. It also lets them recognize false smiles. When they unconsciously mimic a false smile, they don't experience the same brain activity as an authentic one. The mismatch lets them know something's wrong.

Other experts on facial expressions applaud Niedenthal's new model, but a number of them also think that parts of it require fine-tuning. "Her model fits really well along the horizontal dimension, but I have my doubts about the vertical," said Galinsky. He questions whether people observing a dominant smile would experience the feeling of power themselves. In fact, he points out, in such encounters, people tend to avoid eye contact, which Niedenthal says is central to her model.

Niedenthal herself is now testing the predictions of the model with her colleagues. In one study, she and her colleagues are testing the idea that mimicry lets people recognize authentic smiles. They showed pictures of smiling people to a group of students. Some of the smiles were genuine and others were fake. The students could readily tell the difference between them.

Then Niedenthal and her colleagues asked the students to place a pencil between their lips. This simple action engaged muscles that could otherwise produce a smile. Unable to mimic the faces they saw, the students had a much harder time telling which smiles were real and which were fake.

The scientists then ran a variation on the experiment on another group of students. They showed the same faces to the second group, but had them imagine the smiling faces belonged to salesclerks in a shoe store. In some cases the salesclerks had just sold the students a pair of shoes — in which they might well have a genuine smile of satisfaction. In other trials, they imagined that the salesclerks were trying to sell them a pair of shoes — in which case they might be trying to woo the customer with a fake smile.

In reality, the scientists use a combination of real and fake smiles for both groups of salesclerks. When the students were free to mimic the smiles, their judgments were not affected by what the salesclerk was doing.

But if the students put a pencil in their mouth, they could no longer rely on their mimicry. Instead, they tended to believe that the salesclerks who were trying to sell them shoes were faking their smiles — even when their smiles were genuine. Likewise, they tended to say that the salesclerks who had finished the sale were smiling for real, even when they weren't. In other words, they were forced to rely on the circumstances of the smile, rather than the smile itself.

Niedenthal and her colleagues have also been testing the importance of eye contact for smiles. They had students look at a series of portraits, like the "Laughing Cavalier" by the 17th-century artist Frans Hals. In some portraits the subject looked away from the viewer, while in others, the gaze was eye to eye. In some trials, the students looked at the paintings with bars masking the eyes.

The participants rated how emotional the impact of the painting was. Niedenthal and her colleagues found, as they had predicted, that people felt a bigger emotional impact when the eyes were unmasked than when they were masked. The smile was identical in each painting, but it was not enough on its own. What's more, the differences were greater when the portrait face was making direct eye contact with the viewer.

Niedenthal suspects that she and other psychologists are just starting to learn secrets about smiles that artists figured out centuries ago. It may even be possible someday to understand why Mona Lisa's smile is so powerful. "I would say the reason it was so successful is because you achieve eye contact with her," said Niedenthal, "and so the fact that the meaning of her smile is complicated is doubly communicated, because your own simulation of it is mysterious and difficult."

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