Three degrees of separation

[principled]

I was watching an interview with an american professor (sorry forgot name) about the fact that an action (eg stopping smoking, slimming etc) by someone amongst one group of friends not only .
influences those friends, but their friends and their friends' friends. That is to say, the influence is felt by people who do not know the original person.
I didn't hear the theory or the research behind this, any ideas?
Also, did the exponential rise in the numbers taking part in the recent riots owe something to this three degrees of separation?
P
BTW. Don't have regular internet at the moment, but will respond when I can
P

[StuartG]

"Six degrees of separation"

"Six degrees of separation refers to the idea that everyone is on average approximately six steps away, by way of introduction, from any other person on Earth, so that a chain of, "a friend of a friend" statements can be made, on average, to connect any two people in six steps or fewer. It was originally set out by Frigyes Karinthy and popularized by a play written by John Guare. ..."

...

"Three Degrees Of Separation?"

"How does influence really work? According to a recent study at Harvard University: "although a person may be connected to other people by six degrees of separation, he or she is influenced only by those up to three degrees away." "

www.twistimage.com/blog/archives/three-degrees-of-separation/

...

"The Dynamics of Personal Influence"

by Nicholas A. Christakis

hbr.org/web/2009/hbr-list/dynamics-of-personal-influence

...

"Nicholas A. Christakis (Part 1) - Lecture at the Hellenic American Union, Athens "

www.youtube.com/watch?v=OiPlv91cNmY

StuartG