The social psychologist Stanley Milgram is best known for conducting his infamous shock experiments (or perhaps to some for identifying the "six degrees of separation" that govern human connections), but he also produced a lot of insights about the peculiarities of city life. One thing Milgram noticed was the way that city residents will often encounter the same stranger time and again — the "familiar stranger," he called this person in an essay from 1972:
Nothing is more characteristic of urban life than the fact that we often gain extreme familiarity with the faces of a number of persons, yet never interact with them. At my railroad station, for example, I have stood at a commuter station for several years, often in the company of people whom I have never gotten to know. The faces and the people are treated as part of the environment, equivalent to the scenery, rather than persons with whom one talks, exchanges greetings.
A few years before Milgram wrote that essay, his students did a rudimentary study to see just how common the "familiar stranger" phenomenon was. They photographed people at a commuter station one morning, returned the following week to hand out the photographs, and asked people to pick out the faces they recognized. About 90 percent of people had at least one "familiar stranger" in their lives, and the average person had about four.
Fast-forward some forty years and that feeling of being alone together with others in the city crowd rings as true as it did in Milgram's day. What's changed is the technology at our disposal for studying the familiar stranger. In a recent study (reportedly accepted for publication in Proceedings of the National Academy of Science) a group of researchers from the Future Cities Laboratory in Singapore observed these encounters for the "first time" on a large, citywide scale.
They not only confirmed that city residents do indeed have their own "familiar strangers" but found an "imperceptible" community of strangers around us — a sort of hidden urban social network:
Viewed as a whole, the empirical encounter network we illustrate here is a well-connected small-world graph, in which individuals are no longer confined to local encounters in one vehicle, but interact strongly with increasing number of people across the whole city from day to day.
The researchers drew their conclusion by giving Milgram's old study a modern twist. They analyzed the time and location data trapped on transit fare cards from roughly 20 million bus trips taken across Singapore by about 3 million individuals during a week's time. By crunching the numbers they were able to pinpoint when any two card-holders rode the same bus at the same time.
Most of the encounters they discovered occurred around the same time of day, typically in the morning. There's a logical explanation for this finding: commuting behavior is habitual, particularly on the way to work. And sure enough, when the researchers isolated certain individuals, they found that the more rigid a person's routine, the higher that person's chance of encountering a familiar stranger.
In other words, these chance encounters weren't entirely left to chance.
The work has its drawbacks. The researchers couldn't say for sure that the familiar strangers actually saw one another on the bus. Nor, for that matter, could they know that the two people are not, in fact, friends.
Still, the study confirms the existence of a hidden social network stretching across cities and raises a number of other questions about collective behavior. Might knowledge of such a network help predict the spread of disease? Can familiar strangers help researchers establish a threshold for "social contagion" — the notion that our behaviors rub off on those around us? And, perhaps most intriguing, what would happen to our worlds if we started actually talking to these people we see all the time but ignore?
We'd probably have to recalculate those six degrees of separation, for one thing.