It's a perennial philosophical question. I think it was Aristotle who first asked why all men love a good dirty joke, in a long lost book about comedy. Science, it seems, has finally delivered the answer to the mystery of humour, and it is apparently for the same reason people like cocaine, money and pretty young things. Neuroscientists at Stanford University have found that the brain's rewards circuits, especially a region called the nucleus accumbens, lit up like a Christmas tree when 16 of their volunteers were shown very funny cartoons. The research group used a technique called magnetic resonance imaging to look inside their subjects' heads, and the results have been published in the latest issue of Neuron in an eight-page paper with the decidedly unfunny title 'Humour modulates the mesolimbic reward centres'. The accumbens, you see, is awash with the feel-good chemical dopamine, released when people take drugs, have sex or make a killing with a winning bet. Humour addiction therapy, anyone?