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Small World Theory


Explanations > Theories > Small World Theory

Description | Research | Example | So What? | See also | References 



We all have friends and they have further friends who have friends again and so on. In fact, this multiplication effect quickly leads to a huge number of connections with people within relatively few friendship 'hops'.

If I have 25 unique friends and each of those have 25 unique friends which are not friends with any of my unique friends (or me), and repeating this for six more hops, then over six billion people will have been reached by the final hop, which (at the time of writing) just about covers everyone in the world.

Of course statistical variation says it is not that simple and the reality of social links includes such patterns as groupings where people share multiple friends, and 'weak ties' where people have more distant, but less strongly connected, acquaintances.

A visible effect of the small world principle is that it is surprising how often you can meet a stranger and find you have acquaintances in common.


In 1967, Stanley Milgram (who is known more for his obedience studies) asked people in Nebraska and Kansas to get a letter to someone in Massachusetts with instructions to keep forwarding the letter it to someone who might know the target person or someone who might know someone who might know, etc. In repeating this experiment, he found that, for letters that arrived at their destination, it took an average of six hops to reach the target person. The principle is consequently also known as 'six degrees of separation'.

In 2003, Dodds, Muhamad, and Watts repeated the experiment on the internet and got similar results.


A person wants to send a package to a named person in Hong Kong but does not know their address. So they send it to a friend there, asking them just to forward it to anyone in the region who might know of someone who knows of the target person, with a request to keep forwarding the package until it reaches its destination. In a few weeks the originator hears from the target person that they have received the parcel.

So What?

Using it

When seeking to build trust with someone, see if you can find a common acquaintance. Or even an acquaintance of an acquaintance.

If you want to find someone or something about someone, ask your friends to ask their friends and so on.


When somebody asks if you know another person who you've never heard of, ask them why.

See also

Weak Ties Theory


Milgram (1967), Dodds, Muhamad and Watts (2003)


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