Through just five or six intermediaries, you could be linked to millions of others. It is the notion behind what has been dubbed the small world effect.

It has happed to most of us, and it's virtually guaranteed to happen at again - especially if you attend a social gathering. Whenever you have people eager to talk with one another, the chances are that some will find that they have friends and acquaintances in common. It is, as they say, a small world.

It may be a common enough experience, but the so-called small world effect is turning out to have some pretty big consequences. In the last 18 months it has become one of the hottest subjects in science. Now some believe it could revolutionize the way we think about everything from economic crashes to globalization.

The story of how an apparently trivial social phenomenon turned out to have far from trivial implications has its origins in a bizarre experiment carried out over 30 years ago by psychologist Stanley Milgram.

Milgram was trying to uncover the connections that lurk in our networks of friends and acquaintances, and hit upon a novel way of revealing them. He recruited people in various US states and sent each of them a package, together with some instructions.

These revealed that the packages were actually intended to two people picked by Milgram, who gave their names and some vague clues about where they lived, their occupation and age. What he did not give, however, was a precise postal address. The participants were then told to send the packages to whichever of their acquaintances they judged most likely to know the targets personally and be able to make the final delivery.

Keeping track of the postings, Milgram made the stunning discovery that the packages typically reached the two target people after passing through the hands of just five other people. Later experiments produced similar results, making the conclusion inevitable. It seems that, on average, everyone in America from arms dealer to zoo keeper can be connected to everyone else via a chain of just five or six intermediaries.

It is a result that becomes more bizarre the more you think about it. Sociologists estimate that we each typically have around 300 or so acquaintances - people we're on first-name terms with. That suggests we're just one hand-shake (or email) away from 300 people, two away from 90,000, three away from 27 million and so on.

Viewed this way, the real surprise about Milgram's research is that it takes as many as five or six handshakes to connect every American to every other. An average of just four should suffice to connect up to 250 millions inhabitants of the US.

But there is a big assumption in this quick calculation: that our 300 friends are randomly spread throughout the population so that every American is likely to know, say, Alan Greenspan as Al at the corner store. But the fact is that our friends tend to fall into cliques: people who have similar levels of education, interests and opinions.

This, however, just makes Milgram's findings even more baffling: for if all our friends were confined to such rigid cliques, we would hardly ever discover we have friends in common. Each American, for example, would then be separated by an average of almost one million handshakes?250 million divided by 300 from each other. By that reckoning, Milgram should have died long before any of his packages reached their targets.

There is clearly something odd going on here. Our networks of friends are not randomly spread across society. Yet they still allow us to be linked to each other via few intermediaries, so that we often end up discovering "It's a small world". How do they do it?

It was this that intrigued Duncan Watts, in 1996 still a graduate at Cornell University. Watts had been working on a nice, solid doctorate about the chirps of lovelorn crickets. But he had run into a problem: how do the crickets fall into step so quickly? Was each listening to all his fellow crickets, or just to his closest neighbors?

Then Watts remembered a funny bit of folklore that his father had told him: that every American is just a few handshakes away from knowing the president of the United States. Watts wondered if there was a connection between this apparent bit of folklore and the problem he was trying to solve and perhaps many others too.

Watts expected his idea would be ridiculed by his advisor, Steve Strogatz at Cornell's department of theoretical and applied mechanics. Instead, Stogatz also fell under the sp

[1] [2] [3] NextPage