It happens to almost everyone sooner or later. You meet someone new, far from home, and within minutes you discover a mutual friend, an old coworker or a cousin in common. Out comes the familiar line: "Well, it's a small world!"
Those moments feel like flukes, but science suggests they're not so unusual. Researchers have found that people are connected by surprisingly short chains of acquaintances. In the 1960s, psychologist Stanley Milgram's famous small-world experiment suggested that Americans are separated by about six people on average, the idea that later became known as six degrees of separation.
An Idea Born in a Short Story
The concept goes back further than you might think. In 1929, Hungarian writer Frigyes Karinthy published a short story called "Chains." In it, a character challenges others to name any person on Earth he couldn't reach through at most five other people. It's perhaps the earliest reference to the idea of six degrees of separation.
In the early 1950s, a mathematician and a political scientist working in Paris wrote a manuscript exploring the math of social networks. It circulated among scholars for more than 20 years before being published, and it left open a big question: how many steps really separate any two people?
Letters Across America
Milgram set out to answer it. He picked starting points in Omaha, Nebraska, and Wichita, Kansas, cities that seemed far from Boston both geographically and socially. Volunteers there got a packet describing a target person in Boston and were asked to pass it along to someone they knew personally who might be closer to that person. Knowing someone personally meant being on a first-name basis. Each person signed a roster and mailed a postcard to the researchers at Harvard so they could follow the chain.
Many chains simply broke when people didn't bother to forward the letter. In one case, 232 of 296 letters never arrived. Some packets arrived in just one or two hops, while others took nine or ten. In all, 64 reached the target, and the average chain was about five and a half to six steps long. Milgram published the results in 1967, though he never actually used the phrase six degrees of separation.
The Internet Era Shrinks It Further
Later studies found something similar on a much bigger scale. In 2011, Facebook's data team studied 721 million users with 69 billion friendship links. The average distance between any two users was just 4.74 steps, and 99.91 percent of users were connected in one giant network.
Scientists have also built mathematical models showing how this works. Most of us belong to tight local circles of family and friends, but a few connections reach far outside them. Those long-distance links act like shortcuts, shrinking the whole world.
Six Degrees of Kevin Bacon
The idea even became a party game. Three students at Albright College in Pennsylvania invented "Six Degrees of Kevin Bacon," in which players try to link any actor to Bacon through shared movies in six steps or fewer. They came up with it while watching Footloose.
Not everyone agrees on the exact numbers, and researchers have pointed out flaws in Milgram's methods. Still, the big picture holds up. So the next time you discover that a stranger knows your old neighbor, go ahead and say it. It really is a small world.
