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Peer-to-Peer by Andy Oram

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It’s a small, small world

In 1967, Harvard professor Stanley Milgram mailed 160 letters to a set of randomly chosen people living in Omaha, Nebraska. He asked them to participate in an unusual social experiment in which they were to try to pass these letters to a given target person, a stockbroker working in Boston, Massachusetts, using only intermediaries known to one another on a first-name basis. That is, each person would pass her letter to a friend whom she thought might bring the letter closest to the target; the friend would then pass it on to another friend, and so on until the letter reached someone who knew the target personally and could give it to him. For example, an engineer in Omaha, on receiving the letter, passed it to a transplanted New Englander living in Bellevue, Nebraska, who passed it to a math teacher in Littleton, Massachusetts, who passed it to a school principal in a Boston suburb, who passed it to a local storekeeper, who gave it to a surprised stockbroker.

In all, 42 letters made it through, via a median number of just 5.5 intermediaries. Such a surprisingly low number, compared to the then-U.S. population of 200 million, demonstrated concretely for the first time what has become popularly known as the small-world effect . This phenomenon is familiar to anyone who has exclaimed “Small world, isn’t it!” upon discovering a mutual acquaintance shared with a stranger.

Milgram’s experiment was designed to explore the properties of social networks : the interconnecting ...

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