What does six degrees of separation mean?

-Karthik Gurumurthy

This is basically an idea that any person in the world is connected to any other through a surprisingly small number of intermediaries. This notion has a richer history than most people realize.

The concept traces back to 1929 when Hungarian writer Frigyes Karinthy first speculated that anyone could be connected to anyone else through a limited chain of acquaintances. It was a remarkably prescient observation for its time.

The first rigorous exploration came decades later from mathematician Manfred Kochen and political scientist Ithiel de Sola Pool. Their theoretical work attempted to provide a mathematical foundation for the problem. Working with a hypothetical model where individuals randomly chose 1,000 friends from a population of 100 million, they calculated that only two or three intermediaries (making three or four degrees of separation) would connect any two people. However, they recognized that people don’t choose friends randomly, suggesting the actual number should be higher, though they couldn’t solve this more complex version of the problem.

Stanley Milgram, the pioneering social psychologist, designed an ingenious real-world experiment in the late 1960s to test this hypothesis. He and his graduate student Jeffrey Travers gave 300 letters to people in Boston and Omaha with instructions to deliver them to a specific target (a stockbroker in Sharon, Massachusetts) by mailing the letter to an acquaintance they thought might be closer to the target. Each recipient would follow the same instructions, creating a chain of intermediaries. Milgram found that the completed chains averaged about six links – quite remarkable considering Karinthy’s earlier prediction.

Since then, the concept has become a cultural phenomenon, especially after playwright John Guare used the term “six degrees of separation” as the title of his 1990 play. Despite its popularity, until recently very little additional empirical research had been conducted beyond Milgram’s initial work, and the underlying mechanisms remained unexplained.

Recent theoretical work suggests that while the exact number may or may not be precisely 6, it is certainly small – definitely not anywhere near 100. The concept continues to intrigue researchers as we explore the surprising interconnectedness of our world.