Scientists are using email to re-test the "six degrees of separation" experiment.
A team of sociologists at Columbia Universit in New York are researching what is known as the small world phenomenon - the idea everyone in the world can reach any other person through a chain of no more than six acquaintances.
The theory was formulated in 1967 by Harvard sociologist Stanley Milgram. He sent 300 letters to randomly-selected people in Omaha, Nebraska, with the instruction to get them to a single "target" person in Boston using only personal contacts.
The team at Columbia University is testing whether the original findings were reliable by replicating the experiment on a far wider scale.
They aim to persuade at least 100,000 people to take part.
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