From Nature:
Laughter is indeed infectious, according to a new study. Researchers have shown that the mere sound of giggles tickles the same area of the listener’s brain that is activated when smiling. The brain’s response helps to prepare the facial muscles for a good hearty laugh. “It really seems to be true: ‘Laugh and the whole world laughs with you’,” says study co-author Sophie Scott, a neuroscientist at University College London in the United Kingdom.
The team of played pleasant sounds, such as laughter or cheering, and unpleasant sounds, such as screaming or retching, to volunteers. They then monitored their brains with functional magnetic resonance imaging (MRI). All the sounds triggered neural responses in the premotor cortex of the brain — an area known to prepare groups of facial muscles to respond accordingly. When a person in the study actually smiled or laughed, the neural activity moved to a primary motor cortical region.
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