Six degrees of separation is the theory that anyone on the planet can be connected to any other person on the planet through a chain of acquaintances that has no more than five intermediaries. The theory was first proposed in 1929 by the Hungarian writer Frigyes Karinthy in a short story called "Chains."
In the 1950's, Ithiel de Sola Pool (MIT) and Manfred Kochen (IBM) set out to prove the theory mathematically. Although they were able to phrase the question (given a set N of people, what is the probability that each member of N is connected to another member via k_1, k_2, k_3...k_n links?), after twenty years they were still unable to solve the problem to their own satisfaction. In 1967, American sociologist Stanley Milgram devised a new way to test the theory, which he called "the small-world problem." He randomly selected people in the mid-West to send packages to a stranger located in Massachusetts. The senders knew the recipient's name, occupation, and general location. They were instructed to send the package to a person they knew on a first-name basis who they thought was most likely, out of all their friends, to know the target personally. That person would do the same, and so on, until the package was personally delivered to its target recipient.
Although the participants expected the chain to include at least a hundred intermediaries, it only took (on average) between five and seven intermediaries to get each package delivered. Milgram's findings were published in Psychology Today and inspired the phrase "six degrees of separation."