Six degrees of separation (also referred to as the "Human Web") refers to the idea that, if a person is one step away from each person they know and two steps away from each person who is known by one of the people they know, then everyone is at most six steps away from any other person on Earth.

|six degrees of separation |
|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. |