If you’ve ever found yourself laughing simply because someone else got the giggles, you know there’s truth to the expression "laugh and the whole world laughs with you." But what is it about laughter that's so contagious?

It all comes down to how your brain responds to the sound of giggles, chuckles and guffaws.

In 2006, researchers at the University College London used an fMRI scanner to measure how volunteers' brains responded to a variety of sounds. In addition to positive sounds like laughter and victorious shouts, they also played negative ones, such as screaming and retching.

All the sounds elicited a reaction in the premotor cortical region of the participants’ brains, the area that preps facial muscles to move in response to sound. However, responses were highest for the positive sounds, which could explain why we find ourselves laughing along with others, even if we missed the joke.

"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,” University College London neuroscientist Sophie Scott told LiveScience. "Now we've shown that the same appears to apply to laughter too."

The contagious nature of laughter may have played an important role in early social interactions. Scientists think laughter may have been a precursor to language and that our ancestors may have laughed to show they were friendly and meant no harm to other groups.

The ability for one person’s laughter to trigger another’s is also why the laugh track — that canned laughter that plays after sitcom punchlines — has stuck around so long. It works.

A 1974 study found that people were more likely to find jokes funny when they were followed by recorded laughter.

It’s the same reason why you laugh more watching a comedy in a crowded theater than you do watching the same movie alone at home. According to neuroscientist Robert Provine, laughter is 30 times more frequent when people are in groups than in solitary settings.

Want to see a contagious laugh in action? Check out the video below.

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