Network science
Six degrees of separation
Six degrees of separation is the idea that any two people are connected by about six acquaintance steps — rooted in Stanley Milgram's small-world experiments.
Six degrees of separation is the claim that you can reach anyone on Earth through a short chain of acquaintances — your contact knows someone, who knows someone, and within about six hops the chain arrives. The phrase entered pop culture through John Guare's 1990 play, but the underlying science is the 'small-world problem', studied empirically by psychologist Stanley Milgram in the late 1960s.
In the canonical experiment, published by Jeffrey Travers and Milgram in Sociometry (1969), 296 volunteers in Nebraska and Boston were asked to move a folder toward a target person — a stockbroker living near Boston — by mailing it to a personal acquaintance who seemed closer to the target. Of the chains that were completed, 64 reached the stockbroker, with a mean of 5.2 intermediaries. Rounded up, six degrees was born.
Three decades later, Duncan Watts and Steven Strogatz gave the phenomenon its mathematics. Their 1998 Nature paper showed that networks need only a small fraction of random long-range links to combine high local clustering with short global paths — the 'small-world network'. That explains the everyday weirdness: your world feels clustered and parochial, yet a handful of bridge relationships make the whole planet a few handshakes wide.
Milgram's experiment — and its honest limitations
The Travers–Milgram study is both landmark and cautionary tale. The design was elegant: starters could only forward the folder to someone they knew on a first-name basis, so each completed chain traced a real path through the acquaintance network. The headline result — completed chains averaged 5.2 intermediaries — has held up remarkably well. The caveats matter, though. Most chains were never completed (64 of 296 in the main study; an earlier pilot had far lower completion), and chains that die can bias the average downward or upward in ways that are hard to correct. Starters were not a random sample of America, and the target was a high-status professional, likely easier to reach than average. The study also revealed 'funneling': a large share of completed chains converged on the same few final intermediaries — early evidence that a small number of super-connectors carry a disproportionate share of network traffic. So the right reading is not 'everyone is exactly six steps from everyone' but 'acquaintance paths are astonishingly short, and a few hub people make them so'.
Watts & Strogatz: why a few shortcuts shrink the world
The puzzle Milgram left open was mechanical: how can a network be highly clustered (your friends mostly know each other) and still have short paths to everyone? Watts and Strogatz answered it in 'Collective dynamics of small-world networks' (Nature 393, 440–442, 1998) with a simple model. Start from a regular lattice where everyone knows only their neighbors, then rewire a tiny fraction of links at random. Even one percent of random 'shortcut' edges collapses the average path length toward that of a random graph, while clustering stays almost untouched. Translated to real life: it takes very few people with long-range ties — the friend who moved to Singapore, the cousin in a different industry — to make an entire society small. Empirical follow-ups kept shrinking the number: Facebook's 2016 analysis of its own graph found an average of about 3.6 intermediaries between any two users. The world isn't just small; with digital ties, it has been getting smaller.
Using the small world: your second degree is the asset
The practical force of the small-world result is not trivia — it is that almost anyone you need to reach is closer than they appear, if you route through the right relationships rather than cold-contacting. Your first degree might be 150 people; their first degrees put tens of thousands within one warm introduction. Three habits convert that from theory into reach. Know your bridges: identify which contacts connect you to industries, cities or communities you otherwise can't touch. Ask for routes, not favors: 'do you know anyone close to X?' is an easy, answerable question. And keep the bridge ties warm before you need them — a path through the network only works if each hop is a relationship in good standing. This is one reason a personal CRM like Endearist encourages noting how you met people and which worlds they belong to: when you need a path, the map already exists.
Frequently asked questions
- Is six degrees of separation actually true?
- Roughly, yes — short acquaintance paths are real, though 'six' is a rounded average, not a law. Milgram's completed chains averaged 5.2 intermediaries, with serious caveats (most chains broke off; samples weren't random). Modern large-scale measurements find even shorter paths: Facebook reported about 3.6 intermediaries across its user graph in 2016. The robust finding is short paths, not a precise number.
- Where does the phrase 'six degrees of separation' come from?
- Not from Milgram — he spoke of the 'small world problem'. The chain idea was anticipated in Frigyes Karinthy's 1929 short story 'Chains', which imagined linking any two people through five intermediaries. The exact phrase was popularized by John Guare's 1990 Broadway play 'Six Degrees of Separation' and its 1993 film adaptation, after which it merged with Milgram's findings in the public mind.
- What is a small-world network?
- A network that combines two properties that seem contradictory: high clustering (your contacts tend to know each other) and short average path lengths between any two nodes. Watts and Strogatz showed in 1998 that adding a small fraction of random long-range links to a clustered network produces exactly this combination. Social networks, power grids and neural networks all show small-world structure.
Last updated: 2026-06-10
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