Six Degrees of Separation: A Party Game That Changed Science

The stockbroker's name was Charles Cobb, and on a warm October day in 1967, he received a letter from a complete stranger. This wouldn't have been remarkable in itself—Cobb, who worked in the financial district of Boston, surely received dozens of letters every week. But this particular letter had traveled an extraordinary distance to reach him. It had passed through the hands of five intermediaries, each one a friend or acquaintance of the previous sender, before finally landing on Cobb's desk. No one had known where Cobb lived or worked. No one had looked him up in a phone book or searched for him online—because in 1967, there was no online. And yet, somehow, a complete stranger in Nebraska had managed to reach him through nothing more than a chain of personal connections.

The letter itself was unremarkable: a standard research communication, polite and businesslike. But what it represented was nothing short of revolutionary. Here was proof, gathered through a carefully designed experiment, that the world was far smaller than anyone had imagined. The stranger who sent that letter—a random person plucked from the cornfields of Nebraska—had managed to connect to a complete stranger in Boston through just five intermediate friends. Six steps. That was all it took to bridge the vast distance between a farmer in the Midwest and a stockbroker on the East Coast.

This is the story of that experiment, and of the idea it spawned: the concept of "six degrees of separation" that would eventually become one of the most influential theories in all of social science. It's a story about connections—about the invisible threads that link every person on Earth to every other person, about why your friend's cousin's college roommate might actually be closer to you than you think, and about what these connections mean in an age when we can video-chat with someone on the other side of the planet in an instant.

But perhaps most importantly, this is a story about how a simple party trick—a game that people played at gatherings to amuse themselves—evolved into a scientific theory that would reshape our understanding of human society, influence everything from how we market products to how we track the spread of diseases, and eventually give rise to one of the most viral ideas in the history of popular culture.

To understand how this happened, we need to go back to a small office in Harvard Square in the early 1960s, where a young psychologist named Stanley Milgram was thinking about an old question in a new way.

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Stanley Milgram was not an obvious candidate to become one of the most famous psychologists of the twentieth century. Born in 1933 in New York City to parents who had immigrated from Eastern Europe, Milgram grew up in Queens and showed early promise as a student, eventually earning a place at Queens College, where he studied political science before pivoting to psychology. He went on to complete his PhD at Harvard, where he became fascinated by the question of how ordinary people could be led to commit extraordinary acts of evil—as evidenced by the recent trial of Adolf Eichmann, the Nazi architect of the Holocaust who had insisted, famously, that he was just following orders.

Milgram's most famous experiment, conducted in 1961, tested how far ordinary people would go in administering what they believed were dangerous electric shocks to a screaming victim, simply because an authority figure told them to continue. The results were shocking—65 percent of participants continued all the way to the highest voltage, despite the victim's apparent distress. This experiment, which showed the terrifying power of authority to override personal conscience, made Milgram famous almost overnight. It also established him as a researcher willing to challenge conventional wisdom and push the boundaries of what was acceptable in the name of scientific understanding.

But Milgram was not content to rest on this success. He was a restless intellect, always looking for the next big question. And in the early 1960s, he became intrigued by a different kind of puzzle: the nature of social networks and the way information, ideas, and influence spread through human populations.

The question that fascinated him was deceptively simple: just how connected are we, really? How many steps does it take, on average, to get from any one person to any other person in the world? Is the world divided into isolated groups that rarely intersect, or are we all part of a single, intertwined web?

These questions might seem abstract, but Milgram understood their practical implications. In an era of increasing social fragmentation, of suburbanization and nuclear families replacing extended communities, of mass media reshaping how people communicated—was it still possible for information to flow freely through society? Could a rumor or a social movement spread from one corner of the country to another? Or had modern life created barriers that prevented people from connecting across social divides?

The idea for an experiment came to Milgram, appropriately enough, at a party. Social gatherings have long been occasions for the kind of indirect connections that would later fascinate researchers. You meet someone, and they know someone who knows someone famous or influential. You trade stories about overlapping acquaintances. "Small world," people say, when they discover an unexpected connection. Milgram decided to turn this party observation into a scientific experiment.

The basic design was elegant in its simplicity. Milgram and his collaborators would select random individuals from two different populations—people living in Nebraska and Kansas, representing the "heartland" of America, and people living in Boston, representing the urban East Coast. The participants from the heartland would be asked to try to get a letter to a specific target person: a stockbroker working in Boston. The catch was that they couldn't send the letter directly to the stockbroker—they could only send it to someone they knew on a first-name basis, who they thought might be closer to the target. That person would then forward it to someone they knew, and so on, until the letter eventually reached its destination.

The question was simple: would the letters ever arrive at all? And if they did, how many steps would the successful chains contain?

Milgram published his initial findings in 1967 in Psychology Today magazine, under the title "The Small World." The article was titled almost as an afterthought—it was a popular publication, after all, not an academic journal. But what Milgram reported in those pages would change the way people thought about human society forever.

The results were startling. Of the 296 letters that eventually reached their target, the average chain length was just 5.2 steps. In other words, it took roughly five intermediate acquaintances to connect a random person from the Nebraska plains to a stockbroker in Boston. Milgram called this the "small world" phenomenon, and he was Quick to emphasize what it implied: despite the apparent vastness of American society, with its geographic distances and social divisions, people were far more interconnected than anyone had suspected.

"We are all operating," Milgram wrote, "with a model of the world in our heads that is far more fragmented than the actual world. We tend to see our own corner of society as isolated, disconnected from other regions and other groups. But the chains that connect us to strangers are short—remarkably short."

This was a profound insight, and it resonated with readers in ways that purely academic findings rarely do. The idea that you might be connected to a complete stranger through just a handful of mutual acquaintances touched something deep in the human psyche—something about the nature of community, about the bonds that tie us together, about the possibility that even in an age of increasing anonymity, there are still invisibl