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updated 12/12/2006 8:56:07 PM ET 2006-12-13T01:56:07

If you see two people laughing at a joke you didn’t hear, chances are you will smile anyway — even if you don’t realize it.

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According to a new study, laughter truly is contagious: the brain responds to the sound of laughter and preps the muscles in the face to join in the mirth.

“It seems that it’s absolutely true that ‘laugh and the whole world laughs with you,’” said Sophie Scott, a neuroscientist at the University College London. “We’ve known for some time that when we are talking to someone, we often mirror their behavior, copying the words they use and mimicking their gestures. Now we’ve shown that the same appears to apply to laughter, too — at least at the level of the brain.”

The positive approach
Scott and her fellow researchers played a series of sounds to volunteers and measured the responses in their brain with an fMRI scanner. Some sounds, like laughter or a triumphant shout, were positive, while others, like screaming or retching, were negative.

All of the sounds triggered responses in the premotor cortical region of the brain, which prepares the muscles in the face to move in a way that corresponds to the sound.

The response was much higher for positive sounds, suggesting they are more contagious than negative sounds — which could explain our involuntary smiles when we see people laughing.

The team also tested the movement of facial muscles when the sounds were played and found that people tended to smile when they heard laughter, but didn’t make a gagging face when they heard retching sounds, said Scott. She attributes this response to the desire to avoid negative emotions and sounds.

Older than language?
The contagiousness of positive emotions could be an important social factor, according to Scott. Some scientists think human ancestors may have laughed in groups before they could speak and that laughter may have been a precursor to language.

“We usually encounter positive emotions, such as laughter or cheering, in group situations, whether watching a comedy program with family or a football game with friends,” Scott said. “This response in the brain, automatically priming us to smile or laugh, provides a way or mirroring the behavior of others, something which helps us interact socially. It could play an important role in building strong bonds between individuals in a group.”

Scott and her team will be studying these emotional responses in the brain in people with autism, who have “general failures of social and emotional processing” to better understand the disease and why those with it don’t mirror others emotions, she said.

© 2012 LiveScience.com. All rights reserved.

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