The concept that no two humans are more than six acquaintances apart stems from an experiment using postal mail by Harvard sociologist Stanley Milgram in the 1960s.
In the 1990's the party game "Six Degrees of Kevin Bacon" challenged players to link random actors or directors to actor Kevin Bacon through a chain of films on which they worked. For example, actor Hal Holbrook's Bacon Number is 2, because he appeared in Julia (1977) with Maximilian Schell -- who, in turn, appeared in Telling Lies in American (1997) with Kevin Bacon.
In the last 1960's, colleagues invented the Erdos Number to measure the "collaborative distance" of other mathematicians from prolific author Hungarian mathematician Paul Erdos.
This property of networks is sometimes referred to as the Small Worlds concept.
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