"Of course, the idea of keeping up with the Joneses is an old one. But until recently, it's been hard to find proper evidence for it: if we see a group of neighbours spending more, how can we tell whether some are copying others or simply that the neighbourhood generally has enjoyed some good fortune? Perhaps the neatest evidence here comes from a study of the effects of the Dutch postcode lottery. Every week, this selects a postcode at random and gives a BMW to everyone in it who bought a ticket. Researchers have found that the neighbours of winners who didn't win themselves are significantly more likely to buy a new car. This is a clean sign of a network effect between consumers."
The whole article has an excellent overview and great links.
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