Who came up with the theory of six degrees of separation?

The theory that everyone in the world is separated by at most five acquaintances was first proposed in a 1929 short story by the Hungarian writer Frigyes Karinthy. The story was called "Chains," and while the six degrees theory was a purely fictional conceit, the idea proved popular.

In 1967, psychologist Stanley Milgram tried to test the theory by sending several letters to random people in the Midwest. The letter featured the name, address, and occupation of a single person on the East Coast; participants were asked to forward the letters to the people who they thought were most likely to know the person. It took an average of five intermediaries to reach the target.

The experiment came into some scrutiny afterwards, but the results were published in Psychology Today and gave birth to the phrase "six degrees of separation." Playwright John Guare popularized the term with his play, which later became a film starring a then up-and-coming Will Smith.

But get this -- the original 1967 experiment was repeated in 2001 with email, and the same results came back!