Professor Sophie Scott - LOL! The science and art of laugher.mpg

Laughter is a strong, positive vocal expression of emotion, which is found throughout human cultures and also in many mammals. Although you might think of laughter as something people do when they hear jokes, in fact we laugh most often when we are talking with our friends. Indeed, for both rats and humans, laughter first appears in babies when they interact with their caregivers.

Laughter is a social emotion, and it is physically contagious. This can be detected in people's brains when they listen to laughter. Scientists have found that the brains of people who are good at distinguishing different kinds of laughs show a greater tendency to join in with the laughter they hear.