Basketball coach Frank McGuire speaks on the phone smiling while hie wife listens, in 1956


The next time you catch yourself smiling during a phone conversation, just because, ask the person on the other end of the line whether they’re smiling, too. According to a small study from cognitive-science researchers in Paris, there’s a strong possibility that one person smiled, and the other “heard” it, then mimicked the gesture.

In other words, not only do smiles have a sound, but it’s contagious.

A path to empathy

Smiles, we’ve long known, are a universal human signal. They are understood across cultures and “pre-programmed,” as a professor of psychology at Knox College in Illinois, once explained to Scientific American. People who are born blind smile in the same way as the sighted, and for the same reasons, he said.

We’ve also been aware of a smile’s catchiness for decades. Scientists have documented how the sight of a various facial gestures, including a genuine or “duchenne” smile, can trigger the same in its viewer. In fact, psychologists first theorized that facial mimicry was a key path to accessing another person’s inner state, and thus developing empathy, more than 100 years ago (pdf).

In 2008, scientists in the UK found that people don’t even need to see a smile to perceive it. We can pick out the sound of different types of smiles when merely listening to someone speak.

Now, this research suggests that not only can we identify what the study authors call the “spectral signature of phonation with stretched lips” or “the smile effect” in speech, but that it seems to register on an unconscious level. And—as with the visual cue—it inspires imitation.

To conduct their experiments, the Paris researchers first recreated the smile’s auditory signature digitally, creating software that adds a smile to any recorded voice. They then outfitted 35 participants with electrodes attached to their facial muscles to see whether they could detect the sound of a smile in recorded French sentences—some of which were manipulated to include the effect, others not.

Their results showed that not only could the listeners most often hear the enhancement, even when they consciously missed a smile, their zygomaticus-major muscles prepared to grin in response to it.

Admittedly, they acknowledge that they don’t know how the experiment would have turned out had its participants not been asked to listen specifically for a smiling voice. Nevertheless, they argue in the paper that “the cognition of smiles is not as deeply rooted in visual processing as previously believed.”

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