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 (seven times for the whole world). 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 [socially] 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 (including the degree of connectedness). 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. (Kochen brought Mandelbrot to work at the Institute for Advanced Study and later IBM in the U.S.) 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.
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