Louis Armstrong, Duke Ellington, Billie Holiday, Frank Sinatra, Nat King Cole and Judy Garland all sang about it.
A smile on another person's face actually triggers a smile on yours too, according to a new review paper published Thursday in the journal Trends in Cognitive Sciences.
If someone smiles at you, without even knowing it, you might smile back. What's happening, researchers say, is that your brain automatically tells your facial muscles to mimic the other person's expression. And this could help you understand how the other person is feeling.
"It's kind of like an extreme version of putting yourself in someone else's shoes," study lead author Adrienne Wood tells The Christian Science Monitor. "You're putting yourself in someone else's facial expression."
How does it work? The feeling of smiling, or frowning, reminds you of past experiences making that same facial expression. You might have smiled because you were happy to see a friend. Mimicking another person's smile reminds you of that happy feeling.
But you might not even realize this is happening. "It all happens completely below your awareness," Ms. Wood says, "And rapidly, within a few hundred milliseconds."
If our eyes are working fine, why do we need to feel the expression in our own faces to understand it?
Facial expressions use 42 different muscles. "They can produce hundreds of combinations," Wood says. "People's facial expressions in every day life tend to be incredibly subtle and fleeting."
So it might not be so simple as seeing a smile or frown on someone's face.
Wood likened it to the way other senses work together. For example, she says, most times you see a truck drive by, you also hear it. Both sight and hearing feed into your observation of the truck. So later, when you hear the same sound, you are able to identify it as a truck more easily.
Ursula Hess, a psychologist at Humboldt University in Berlin, who was not an author of the paper, cautions that there might not be always be a link between facial mimicry and understanding others' emotions in every scenario. It's complex.
Kevin Ochsner, director of the social cognitive neuroscience laboratory at Columbia University, who was not part of the paper, explains that facial expressions are very dynamic and might not always give clear clues. People may display their emotions differently in different contexts. For example, he says, people may make their faces somber at a funeral, so mimicry might not tell you how people are really feeling.
"Facial mimicry certainly plays some role, and perhaps a key role, in understanding other people's emotional states," Dr. Ochsner tells the Monitor. But "it may not be sufficient by itself to carry a full understanding of what other people are feeling in every context."
But it might not be just about your own understanding of others. Dr. Hess, whose own research focuses on the impact on the person initially making the expression rather than the perceiver, adds, "Even if it may not help understanding in all situations, people feel more understood if the other person mimics." And that could help the interaction go more smoothly too.
So should we go around smiling at everyone to make the world a happier place?
"Definitely yes," Wood says. Because of emotion contagion – the idea that emotions spread throughout social groups – smiling can alter other people's emotional states.
But not all contexts are appropriate for smiles, Hess says. "If somebody makes a sexist joke, that is a perfect occasion to look displeased."
It doesn't have to be all smiles, Wood says. "If you're trying to get people to feel a certain way, just make the expression yourself and it will probably spread to them – unless you're their enemy, in which case your smile means something very different to them."