Six degrees of separation
Six degrees of separation is the idea that all people are six or fewer social connections away from each other. As a result, a chain of "friend of a friend" statements can be made to connect any two people in a maximum of six steps. It is also known as the six handshakes rule. Mathematically it means that a person shaking hands with 30 people, and then those 30 shaking hands with 30 other people, would after repeating this six times allow every person in a population as large as the United States to have shaken hands.
The concept was originally set out in a 1929 short story by Frigyes Karinthy, in which a group of people play a game of trying to connect any person in the world to themselves by a chain of five others. It was popularized in John Guare's 1990 play Six Degrees of Separation.
The idea is sometimes generalized to the average social distance being logarithmic in the size of the population.
Early conceptions
Shrinking world
Theories on optimal design of cities, city traffic flows, neighborhoods, and demographics were in vogue after World War I. These conjectures were expanded in 1929 by Hungarian author Frigyes Karinthy, who published a volume of short stories titled Everything Is Different. One of these pieces was titled Chains or Chain-Links. The story investigated—in abstract, conceptual, and fictional terms—many of the problems that captivated future generations of mathematicians, sociologists, and physicists within the field of network theory.Technological advances in communications and travel enabled friendship networks to grow larger and span greater distances. In particular, Karinthy believed that the modern world was "shrinking" from this ever-increasing connectedness of human beings. He posited that despite great physical distances between the globe's individuals, the growing density of human networks made the actual social distance far smaller.
As a result of this hypothesis, Karinthy's characters believed that any two individuals could be connected through at most five acquaintances. In his story, the characters create a game out of this notion. He wrote:
A fascinating game grew out of this discussion. One of us suggested performing the following experiment to prove that the population of the Earth is closer together now than they have ever been before. We should select any person from the 1.5 billion inhabitants of the Earth—anyone, anywhere at all. He bet us that, using no more than five individuals, one of whom is a personal acquaintance, he could contact the selected individual using nothing except the network of personal acquaintances.
This idea influenced a great deal of early thought on social networks, both directly and indirectly. Karinthy has been regarded as the originator of the notion of six degrees of separation. A related theory deals with the quality of connections, rather than their existence. The theory of three degrees of influence was created by Nicholas Christakis and James H. Fowler.
Small world
Michael Gurevitch conducted seminal work in his empirical study of the structure of social networks in his 1961 Massachusetts Institute of Technology PhD dissertation under Ithiel de Sola Pool. Mathematician Manfred Kochen, an Austrian who had been involved in urban design, extrapolated these empirical results in a mathematical manuscript, Contacts and Influences, concluding that in a U.S.-sized population without social structure, "it is practically certain that any two individuals can contact one another by means of at most two intermediaries. In a structured population it is less likely but still seems probable. And perhaps for the whole world's population, probably only one more bridging individual should be needed." They subsequently constructed Monte Carlo simulations based on Gurevitch's data, which recognized that both weak and strong acquaintance links are needed to model social structure. The simulations, which were carried out on the relatively limited computers of 1973, were nonetheless able to predict that a more realistic three degrees of separation existed across the U.S. population, foreshadowing the findings of American psychologist Stanley Milgram.Milgram continued Gurevitch's experiments in acquaintanceship networks at Harvard University. Kochen and de Sola Pool's manuscript, Contacts and Influences, was conceived while both were working at the University of Paris in the early 1950s, during a time when Milgram visited and collaborated in their research. Their unpublished manuscript circulated among academics for over 20 years before publication in 1978. It formally articulated the mechanics of social networks, and explored the mathematical consequences of these. The manuscript left many significant questions about networks unresolved, and one of these was the number of degrees of separation in actual social networks. Milgram took up the challenge on his return from Paris, leading to the experiments reported in The Small World Problem in popular science journal Psychology Today, with a more rigorous version of the paper appearing in Sociometry two years later.
Milgram's article described his 1967 set of experiments to investigate de Sola Pool and Kochen's "small world problem." Mathematician Benoit Mandelbrot, born in Warsaw, growing up in Poland then France, was aware of the Statist rule of thumb, and was also a colleague of de Sola Pool, Kochen and Milgram at the University of Paris during the early 1950s. This circle of researchers was fascinated by the interconnectedness and "social capital" of human networks. Milgram's results showed that people in the United States seemed to be connected by approximately three friendship links, on average, without speculating on global linkages; he never actually used the term "six degrees of separation". Since the Psychology Today article gave the experiments wide publicity, Milgram, Kochen, and Karinthy all had been incorrectly credited as the origin of the notion of six degrees; the most likely popularizer of the term "six degrees of separation" was John Guare, who attributed the concept of six degrees to Marconi.
Continued research: Small World Project
In 2003, Columbia University conducted an analogous experiment on social connectedness amongst Internet email users. Their effort was named the Columbia Small World Project, and included 24,163 e-mail chains aimed at 18 targets from 13 countries. Almost 100,000 people registered, but only 384 reached the final target. Amongst the successful chains, while shorter lengths were more common, some reached their target after only 7, 8, 9, or 10 steps. Dodds et al. noted that participants were strongly biased towards existing models of Internet users and that connectedness based on professional ties was much stronger than those within families or friendships. The authors cite "lack of interest" as the predominating factor in the high attrition rate, a finding consistent with earlier studies.Research
Several studies, such as Milgram's small-world experiment, have been conducted to measure this connectedness empirically. The phrase "six degrees of separation" is often used as a synonym for the idea of the "small world" phenomenon.However, detractors argue that Milgram's experiment did not demonstrate such a link, and the "six degrees" claim has been decried as an "academic urban myth".
Computer networks
In 2001, Duncan Watts, a professor at Columbia University, attempted to recreate Milgram's experiment on the Internet, using an e-mail message as the "package" that needed to be delivered, with 48,000 senders and 19 targets. Watts found that the average number of intermediaries was around six.A 2007 study by Jure Leskovec and Eric Horvitz examined a data set of instant messages composed of 30 billion conversations among 240 million people. They found the average path length among Microsoft Messenger users to be 6.
It has been suggested by some commentators that interlocking networks of computer-mediated lateral communication could diffuse single messages to all interested users worldwide as per the six degrees of separation principle via information routing groups, which are networks specifically designed to exploit this principle and lateral diffusion.
An optimal algorithm to calculate degrees of separation in social networks
Bakhshandeh et al. have addressed the search problem of identifying the degree of separation between two users in social networks. They introduced new search techniques to provide optimal or near optimal solutions. The experiments were performed on Twitter in 2011, and showed an improvement of several orders of magnitude over greedy approaches. Their optimal algorithm found an average degree of separation of 3.43 between 2 random Twitter users, requiring an average of only 67 requests for information. A near-optimal solution of length 3.88 could be found by making an average of 13.3 requests.Popularization
No longer limited strictly to academic or philosophical thinking, the notion of six degrees recently has become influential throughout popular culture. Further advances in communication technology—and particularly the Internet—have drawn great attention to social networks and human interconnectedness. As a result, many popular media sources have addressed the term. The following provide a brief outline of the ways such ideas have shaped popular culture.Popularization of offline practice
John Guare's ''Six Degrees of Separation''
American playwright John Guare wrote a play in 1990 and released a 1993 film that popularized it; it is Guare's most widely known work. The play ruminates upon the idea that any two individuals are connected by at most five others. As one of the characters states:
I read somewhere that everybody on this planet is separated by only six other people. Six degrees of separation between us and everyone else on this planet. The President of the United States, a gondolier in Venice, just fill in the names. I find it A) extremely comforting that we're so close, and B) like Chinese water torture that we're so close because you have to find the right six people to make the right connection... I am bound to everyone on this planet by a trail of six people.
Guare, in interviews, attributed his awareness of the "six degrees" to Marconi. Although this idea had been circulating in various forms for decades, it is Guare's piece that is most responsible for popularizing the phrase "six degrees of separation." Following Guare's lead, many future television and film sources later incorporated the notion into their stories.
J. J. Abrams, the executive producer of television series Six Degrees and Lost, played the role of Doug in the film adaptation of this play.