The Man Who Started a Revolution: Stanley Milgram and the Small-World Problem

In the spring of 1967, Stanley Milgram, a young and ambitious psychologist at Harvard University, set in motion a deceptively simple experiment that would unravel the fabric of social connectedness and plant the seed for an entire scientific discipline. He mailed 160 identical packets to random individuals in Omaha, Nebraska, each containing a brief letter explaining the purpose and a list of contact information for the next person in the chain—a stockbroker living in Sharon, Massachusetts, a suburb of Boston. The instructions were clear: if you did not know the stockbroker personally, please forward the packet to someone you thought might know him, and so on, until it reached its destination. The goal was to measure how many intermediaries it would take for a message to traverse the vast geographical and social distance between the heartland of America and the financial corridors of the East Coast.

Milgram’s experiment was not born in a vacuum. The 1960s were a time of both existential dread and burgeoning curiosity about human relationships. The Cold War cast a long shadow, with fears of nuclear annihilation making the world feel both terrifyingly small and impossibly vast. Against this backdrop, Milgram, already famous for his controversial obedience studies, turned his attention to a more benign but equally profound question: How connected are we, really? His interest was piqued by a conversation with a colleague who mused that it might take only a few steps to link any two people. This notion, later popularized as “six degrees of separation,” was then just a provocative hypothesis. Milgram saw an opportunity to test it empirically, using the postal system as a giant sociological microscope. He was influenced by the work of social psychologists like Fritz Heider and the concept of “psychological distance,” but also by the practical question of how information spreads through a population. The experiment would become a landmark, not only for its findings but for its method—a creative, if crude, probe into the hidden architecture of society.

The Genesis of an Idea: Milgram’s Intellectual Context

Stanley Milgram was a figure of contradictions: a rigorous scientist with a flair for the dramatic, a man who probed the darkest corners of human behavior while seeking to illuminate the bonds that hold society together. By the mid-1960s, he had already shocked the world with his obedience experiments at Yale, demonstrating that ordinary people would administer what they believed were lethal electric shocks to others when instructed by an authority figure. That work had exposed the fragility of moral judgment under pressure, but Milgram’s curiosity ranged far beyond obedience. He was fascinated by the structure of social networks—how individuals are woven into a larger tapestry, and how that tapestry influences behavior, information flow, and social norms.

The specific idea for the “small-world experiment” emerged from a seminar discussion at Harvard. Milgram recalled a comment from a colleague, Jeffrey Travers, who later became a co-author on the seminal paper. Travers had suggested that it might be possible to measure the “social distance” between any two randomly chosen individuals. At the time, sociologists like Jacob Moreno were developing sociometry—the quantitative study of social relationships—but most work focused on small, bounded groups like classrooms or workplaces. Milgram asked: What about the entire society? Could one measure the degrees of separation between two people selected at random from a vast population? The question was both mathematical and deeply human. It touched on a primal intuition: that the world is simultaneously immense and intimate, that we are all linked by chains of acquaintance that somehow bridge continents and cultures.

Milgram framed the problem in terms of “acquaintance chains.” He posited that if you started with a random person in one part of the country and asked them to forward a letter to an acquaintance who might know a specific target person in another part, how many steps would it take? The experiment was designed to answer two intertwined questions: (1) What is the typical length of such a chain? and (2) What do these chains reveal about the underlying structure of social networks? The first question was empirical; the second was theoretical, pointing toward a new way of thinking about society as a network rather than as a collection of isolated individuals or homogeneous groups.

The choice of Omaha, Nebraska, and Sharon, Massachusetts, was deliberate. Omaha represented the American heartland—geographically remote from the East Coast, socially distinct from the financial and intellectual hubs. Sharon, a wealthy suburb of Boston, housed the target: a stockbroker, a profession symbolizing the economic and social elite. The distance between them was not just about miles (approximately 2,000 kilometers) but about presumed social distance: a middle-class or working-class person in Omaha was unlikely to move in the same circles as a Boston stockbroker. By spanning this chasm, Milgram could test whether social networks were truly “small” or whether vast gulfs separated different strata of society.

The stockbroker was selected for practical reasons: he had a stable address, a profession that suggested a wide range of contacts, and presumably, he was not an extreme outlier in terms of social connectivity. Milgram could have chosen any target—a professor, a politician, a celebrity—but a stockbroker represented a node with potentially diverse links across economic sectors. The experiment thus became a probe for the existence of “weak ties” (a concept later formalized by Mark Granovetter) that bridge social circles. Milgram did not use that terminology at the time, but his intuition was that acquaintances, rather than close friends, might be the key to long-range connections.

Designing the Experiment: From Theory to Practice

Translating the idea into a feasible experiment required ingenuity and pragmatism. Milgram and his team faced a daunting challenge: how to initiate chains from truly random starting points and track their progress across the country without modern communication tools. They settled on a postal-based approach, leveraging the reliability of the U.S. Postal Service and the ubiquity of addresses. The core protocol was elegantly simple:

1. Selection of starters: The team obtained a telephone directory for Omaha, Nebraska. From this directory, they randomly selected 160 names. Randomness was crucial: they did not choose names based on any perceived likelihood of participation. This random sampling aimed to approximate a random draw from the population of Omaha adults. However, the use of a telephone directory introduced a bias: it excluded people without phones (though in 1967, phone ownership was high in Omaha) and overrepresented households with stable addresses. Nonetheless, it was the best available method for approximate randomness.

The Execution: Chaos, Human Quirks, and the First Data

The elegance of Milgram’s design belied the messy reality of its execution. What began as a clean, theoretical probe into social architecture quickly became a grand, uncontrolled sociological expedition across the United States, subject to the vagaries of human behavior, postal delays, and sheer luck. The journey of each packet was a microcosm of societal functioning—a story of trust, social obligation, strategic calculation, and occasional indifference.

The Packet’s Journey: A First-Hand Account

Each starter packet contained:

• A cover letter from Milgram explaining the study’s purpose (“We are studying the channels of interpersonal communication in the population of the United States”).
• A detailed set of instructions for the recipient.
• A list of the target’s name, address, and occupation (“stockbroker in Sharon, Massachusetts”).
• A progress card to be filled out at each h