'Laugh and the whole world laughs with you', expressed Keats poetically, and today researchers have discovered a scientific explanation as to why laughter is so contagious.
Sophie Scott at University College London measured the brain activity of 20 volunteers in a functional MRI scanner while she played them laughter, squeals of triumph and moans of fear and disgust.
She also played a neutral, artificial sound that would have no specific meaning to the subjects.
Inside the brain scanners, though, the subjects were not actually using these muscles.
To Scott, that indicates the brain is wired with "mirror circuits" that prime us to copy another's behaviour when we recognise their emotions.
The brain response was more pronounced for the sounds of laughter and triumph than the vocalisations of negative emotions, suggesting that the urge to copy is greatest when we hear another's delight or amusement.
That may explain how laughter is contagious, but the researchers wondered why should it be so.
One explanation stems from its evolutionary origins in rough-and-tumble play, where laughter sends out a clear message that the fighting is not for real.
"It might be important to have the whole group safely signal this so that a play fight does not turn ugly because someone 'didn't get the memo,'" New Scientist quoted Christian Hempelmann at Purdue University in West Lafayette, Indiana, as saying.
On the other hand, Scott believes that mirroring another's emotional state might ease communication and interaction.
Laughing at the same joke would help us to show affiliation with others, and this may be why it is especially contagious.
"Laughter is an incredibly binding thing," she said.
The study has been published in the Journal of Neuroscience.