A single sentence was all it took to disable the BBC commentary box. The cricketer Ian Botham fell over his stumps in a freak dismissal, prompting the commentator Jonathan Agnew to exclaim: "He couldn't quite get his leg over."

The remark sent Agnew and his colleague, the late Brian Johnston, into paroxysms of mirth for several minutes before normal service was resumed. It demonstrated what everyone knows - that laughter is contagious.

Scientists studying how the brain responds to emotive sounds believe they understand why. Researchers have shown that positive sounds such as a giggle or a shout of triumph trigger an involuntary response in the brain that prepares facial muscles to join in, helping forge social bonds.

"It seems that it is absolutely true that 'laugh and the world laughs with you'," said Sophie Scott, of the Institute of Cognitive Neuroscience at University College London, who led the study published in The Journal of Neuroscience.

"We have known for some time that when we talk to someone we often mirror their behaviour, copying the words they use and mimicking their gestures. Now we have shown the same appears to apply to laughter too - at least at the level of the brain."

The study involved playing sounds to volunteers while their brain response was measured in an MRI scanner. The sounds were either positive, such as laughter or exclamations of triumph, or negative, such as screaming or retching.

All triggered a response in the area of the brain called the premotor cortical region, which prepares the muscles of the face to respond accordingly. But the scans showed the response was greater for positive sounds, suggesting these were more contagious than the negative ones. This could explain why we respond to laughter or cheering with an involuntary smile, the researchers said.

Professor Scott said the key lay in the group situations in which such emotions usually occurred, such as watching a comedy programme with the family.

"The response in the brain, automatically priming us to smile or laugh, provides a way of mirroring the behaviour of others, something which helps us interact socially," she said. "It could play an important role in building strong bonds between individuals."

The use of laughter as a therapy is growing. The University of Haifa in Israel is starting a degree in medical clowning, and students hope to be hired by hospitals as part of the salaried staff - proving laughter is the best medicine.

Best medicine?

* Children laugh about 400 times a day; adults just 14.

* Laughter protects the heart and reduces blood pressure.

* An occasional chuckle increases tolerance of pain, reduces allergic reactions and bolsters the immune system.

* A belly-laugh brings a rush of endorphins, the body's natural opiates, similar to that triggered by brisk exercise.

* "Laughter yoga" began in Mumbai in 1996. There are now 5,000 clubs.