Six Degrees of Separation: How the Theory Works

Sometimes, even in faraway places, you might unexpectedly meet someone who was friends with your parents, sat next to one of your colleagues in school, or was the boss of your former boss. In such cases, people often say, “It’s a small world.” But just how far can this chain of mutual acquaintances stretch? Is it really possible to reach, say, the Queen of England through “friends of friends”?

The Origins of the Theory

The idea that any two people in the world can be connected through a sequence of personal contacts—usually no more than five intermediaries—was first proposed by Hungarian writer Frigyes Karinthy in 1929. In his story “Chains,” he described a mental game to prove that the world’s population was much more closely connected than people thought. The challenge was to name any person, famous or not, among the 1.5 billion people on Earth at the time, and build a chain of no more than five people connecting the player to that person.

Here’s a typical excerpt from the story: “Alright, Selma Lagerlöf,” said one of the players, “that’s easy.” Within seconds, he explained: “Selma Lagerlöf recently won the Nobel Prize in Literature, so she must know King Gustav of Sweden, who presented her the award. It’s well known that King Gustav loves tennis and plays in international tournaments. He’s played with Béla Kerling, so they must know each other. As it happens, I also know Kerling.” (The speaker was a decent tennis player himself.) “That took us two links out of five. It’s always easier to find someone who knows a celebrity than an ordinary person. Give me something harder!”

Today, this idea is known in the English-speaking world as the “six degrees of separation” theory.

Experiments That Support the Hypothesis

Without experimental evidence, this idea would remain just a thought experiment. But several experiments have been conducted. The first major test was by American psychologist Stanley Milgram in 1967, called the “Small World” experiment.

Where the Name “Six Degrees of Separation” Comes From

Playwright John Guare, author of the play “Six Degrees of Separation,” helped popularize the term. Interestingly, he was inspired not by Milgram’s research, but by Guglielmo Marconi, one of the inventors of radio. In his Nobel speech, Marconi said he managed to transmit a message over 1,551 miles and calculated that only six transmitters would be needed to cover all inhabited land on Earth. Guare used “six” as a symbol of something that spans the globe.

In Milgram’s experiment, 300 randomly selected residents of Omaha, Nebraska, and Wichita, Kansas, were asked to send a letter to a stockbroker in Boston. They didn’t know his address, but could forward the letter to someone they knew who might know the recipient, and so on, until the letter reached its destination. Each intermediary added their name to the letter, allowing researchers to track the chain. The results showed that the average chain length was five people (or six “degrees” of separation).

Similar experiments have been repeated over the years, confirming the hypothesis. For example, in 1998, Cornell University researchers Duncan Watts and Steven Strogatz created a mathematical model of the “small world” and repeated Milgram’s experiment on a larger scale. Tens of thousands of volunteers from around the world participated, with recipients living in different countries, cities, and backgrounds. This time, messages were sent via the internet. The average chain length was about six links, similar to Milgram’s result. The model also revealed that certain individuals, who belong to multiple communities, play a key role in global communication.

The largest study was conducted in 2006 by Jure Leskovec and Eric Horvitz at Microsoft. They analyzed logs from MSN Messenger—over 30 billion messages sent by 240 million people in 30 days. Without reading message content, they could see user data: gender, age, location, communication frequency, and connections. The average distance between two users was 6.6 links, slightly higher than Milgram’s result but still close.

With the rise of the internet, the principle of easy access to almost anyone has become obvious. Social networks and online communities like Facebook, VKontakte, Twitter, and even Wikipedia offer services to trace chains of mutual acquaintances, games based on “small world” principles, and research tools. There are even special online projects dedicated to exploring global communication.

“Small World” Games

Among movie fans, the game “Six Degrees of Kevin Bacon” is well known: the goal is to find a chain from any actor (from any era) to Kevin Bacon, using the rule “they acted together,” with no more than six links. The game originated after Bacon boasted in an interview that everyone he worked with had, in turn, worked with every other Hollywood actor. A similar game among mathematicians is called the “Erdős number,” which counts the number of collaborative links between a given scientist and the prolific Hungarian mathematician Paul Erdős.

Scientific Fact or Urban Myth?

Today, the “six degrees of separation” hypothesis is widespread. It’s popularized in movies, TV shows, and other aspects of pop culture. Social networks vividly illustrate the “small world” principle, and the idea of being able to reach anyone is very appealing. Who wouldn’t want to be “a friend of a friend of a friend” of the Queen of England, Bill Gates, Johnny Depp, or Fidel Castro?

However, the hypothesis is often misunderstood: people say, “I’m connected to anyone on Earth through six handshakes.” But “six” is just the average chain length; to reach an African pygmy, a Tibetan monk, or a Polynesian fisherman, it might take ten or fifteen links—if a chain exists at all.

Another Experiment by Stanley Milgram

Milgram is also known for his 1963 experiment on obedience to authority. In this study, three people participated: the “teacher” (the real subject), the “learner” (an actor), and the “authority” (the researcher). The subject was told the experiment was about memory. The “learner” was placed in another room, and the “teacher” gave them memory tasks. For each wrong answer, the “teacher” was instructed to administer an electric shock, increasing the voltage each time. In reality, there were no shocks, but the actor would scream, beg, and eventually fall silent, simulating pain or unconsciousness. The “authority” would calmly urge the subject to continue. Before the experiment, psychiatrists predicted that no more than 20% would go halfway up the scale, and almost no one would go to the end. In reality, all 40 subjects went well past halfway, and 65% went to the maximum voltage. The experiment has been repeated in various countries, with 61–66% willing to go all the way.

Common Misconceptions

Another common misconception is that after just one or two levels of acquaintances, we have access to a huge number of people. For example, if you have 100 friends, and each of them has 100 friends, and so on. In reality, people tend to form closed groups based on location, profession, interests, beliefs, education, and income. There are even caste systems with strict boundaries. If you actually count your “friends of friends,” you’ll soon hit the limits of your social group(s), and by the third level, you might reach only a few thousand or tens of thousands, not millions.

Besides misunderstandings, the “six degrees” theory has its own limitations. Karinthy himself noted that humanity hasn’t always been so interconnected. If Julius Caesar had tried to contact an Aztec or Mayan priest living in America at the same time, he would have failed—there was no way to build a chain, not in five or even three hundred links, since America was unknown to Europeans. Even today, the world isn’t as interconnected as we might think. Isolated groups still exist, and internet access is uneven worldwide. So, the “six degrees” result may apply to Europe, the US, parts of Russia, or major cities, but not the entire planet.

There are also issues with the experiments themselves. In 2006, Judith Kleinfeld, a psychology professor at the University of Alaska Fairbanks, pointed out that 95% of the letters in Milgram’s experiment never reached their destination—they were lost along the way. She found similar results in other studies. For example, in the Watts and Strogatz experiment, only 384 out of 24,000 letters arrived. “If 95–97 out of 100 letters don’t make it, can we really call this proof?” Kleinfeld asks. “The seductive idea that we live in a ‘small world’ where everyone is connected by at most six intermediaries is the academic equivalent of an urban legend.”

Conclusion

Is the “six degrees of separation” theory fact or myth? The truth is probably somewhere in between. Still, it’s an intriguing idea, and as the world’s population grows, the internet spreads, and cultures intermingle, people may indeed become even more closely connected.