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." Playwright John Guare popularized the phrase when he chose it as the title for his 1990 play of the same name. Although Milgram's findings were discounted after it was discovered that he based his conclusion on a very small number of packages, six degrees of separation became an accepted notion in pop culture after Brett C. Tjaden published a computer game on the University of Virginia's Web site based on the small-world problem. Tjaden used the Internet Movie Database (IMDB) to document connections between different actors. Time Magazine called his site, The Oracle of Bacon at Virginia, one of the "Ten Best Web Sites of 1996."
In 2001, Duncan Watts, a professor at Columbia University, continued his own earlier research into the phenomenon and recreated Milgram's experiment on the Internet. Watts used an e-mail message as the "package" that needed to be delivered, and surprisingly, after reviewing the data collected by 48,000 senders and 19 targets (in 157 countries), Watts found that the average number of intermediaries was indeed, six. Watts' research, and the advent of the computer age, has opened up new areas of inquiry related to six degrees of separation in diverse areas of network theory such as as power grid analysis, disease transmission, graph theory, corporate communication, and computer circuitry.
A site called Six Degrees, launched in 1997, is considered to be the first social networking site and the precursor of sites like Facebook and Twitter which have effectively lowered the number of intermediaries in the chain, arguably to almost zero.