The theory that every person is connected to every other person through a maximum of six relationships goes back to a short story published by the Hungarian author Frigyes Karinthy in 1929. In one of his many stories, he described that all people are inexplicably connected to each other via only five links. At the time, this notion was still considered science fiction, and only few believed that this idea might actually be true.
The Small World Experiment of Stanley Milgram
However, the young US psychologist Stanley Milgram found himself fascinated by this interesting idea, and so in 1967 he started an easy but illuminating experiment. He invited 60 participants from the distant towns of Omaha and Wichita to come to Harvard University in Boston and gave them each a small package. The participants were instructed to go back home and send the package back to a specific target person in Boston. The challenge was that the participants were not allowed to send the package directly to the target person, but only to friends and acquaintances. These included only those persons who they were allowed to address by their first name and who in turn were only allowed to forward the parcel to their friends and relatives. The objective of the exercise was to find a friend or relative who knew the target person and could finally deliver the parcel.
Read More: The Rosenhan Experiment
The surprising result: although only three parcels actually ever arrived with the target person, they only needed an average of 5.5 intermediaries. While it was clear that this result was not representative, the rumor quickly spread that each American was connected to any other person through only six relationships. In a second experiment with 217 packages, 64 of which reached their target, the number six was confirmed again. At the same time, however, ethnic differences were discovered, as well: only 13 percent of the packages reached their destination if the starting person was an African American. This number rose to 33 percent if the starting person was white.
To this day, there is disagreement about the results of the experiment, because the success rate in further attempts was always so low that no clear conclusion could be drawn about the packages that had not arrived. So maybe only a very small part of humanity is connected to all other people through six more people, while all others need more contacts.
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The Kevin Bacon Number
Time and again, Milgram’s experiment has found numerous imitators. Movie fans with a passion for science, for example, developed the Bacon number, named after the actor Kevin Bacon. The number describes the length of the shortest link of any actor to Kevin Bacon. For this, each intermediate person must have appeared with another actor in a film. The last intermediary must have been in a movie with Kevin Bacon to close the chain. Kevin Bacon therefore naturally has the bacon number zero, as no intermediary is necessary. Kevin Costner has a bacon number of one because he starred in “JFK” with Kevin Bacon. And Dennis Hopper, who has never been on screen together with Kevin Bacon, but who joined Kevin Costner in the cast of Waterworld, has a bacon number of two.
The largest bacon number ever found was a ten and applies to a total of seven actors listed on the IMDB film portal. The vast majority of all actors, however, have a bacon number of five or less, which is why scientists believe that Milgram’s findings must apply to most people.