clipped from www.masternewmedia.org

In 1967, American psychologist Stanley Milgram conducted the "small world experiment", in which he sent letters to sixty volunteers in Kansas and asked them to forward the envelopes to a specific person in Massachusetts—by hand and through friends or friends of friends.

The letters that reached the addressee were, on average, relayed by five to seven people. This is seen as an empirical proof that arbitrary people in our society are related to each other through friends and friends of friends. The small world hypothesis based on Milgram's findings states that the number of personal acquaintances needed to connect two random persons on the planet is small.

The hypothesis led to the expression "the six degrees of separation", meaning that any two random persons are associated with each other by a chain of about six individuals. The "six degrees of separation" is one of the underlying concepts of social networks on the Internet.