Chances are, you don¿t personally know any Australian policemen, Estonian archival inspectors or Norwegian army veterinarians. But you could probably get in touch with one of these distant individuals through a friend, or a friend of a friend, or a friend of your friend¿s friend. The notion that every person on the planet is separated from everyone else by a chain of about six people has been around for some time. A report published today in the journal Science provides experimental data from the Internet in support of that claim.
The proverbial "six degrees of separation" was first proposed in 1967 by sociologist Stanley Milgram. Milgram asked 96 randomly selected people around the country to send a piece of mail to an acquaintance, who would send the mail along to another acquaintance, in an attempt to reach a designated "target" person in Boston. The messages that actually made it to their destination passed through an average of six people. But Milgram¿s experiment was fairly small and has since been questioned by some sociologists. Peter Sheridan Dodds and his colleagues at Columbia University conducted a modern version of this study on the Internet, recruiting over 60,000 participants from 166 different countries for the experiment.
"If we tried to do this with paper, it would be completely unfeasible," says study co-author Duncan J. Watts. Study volunteers were given the identity of a target individual (one of 18 located around the world) and instructed to send an e-mail message to someone they thought could get the message closer to the target. The high number of participants turned out to be necessary for this global study, as only 384 e-mail chains (3 percent) actually found their way to the targets¿ inboxes. The average completed chain comprised four people¿an artificially low figure because the longer the chain, the less likely it was to reach its intended destination. By factoring in the rate of dropouts, the researchers calculated a median chain length of between five and seven people.
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The study of social networks tends to attract the interest of researchers from many disciplines. Scientists hope it will shed light on such questions as how diseases spread, how people find jobs and how criminals can be caught. In addition to strengthening the six-degrees claim, the Columbia study concluded that successful message transmission did not rely as much on "hubs"¿individuals with unusually high numbers of acquaintances¿as was previously suspected.
"If you¿re trying to send something to Siberia, you don¿t think, ¿Who do I know with a lot of friends?¿" Watts remarks. "You think, ¿Who do I know who is Russian?¿" The authors further observe that perception of the chance of success can affect the actual success rate. Those participants given targets who seemed relatively easy to reach (but who were not, in reality, much more "connected" than other targets) were far less likely to drop out of the experiment. This in turn led to a far higher success rate in a kind of self-fulfilling prophecy--a point worth noting by those networking for a new job. Dodds and his colleagues plan to conduct further network studies on the Internet. Those interested in participating can sign up at http://smallworld.columbia.edu.