We've all heard of the six degrees of separation. We're all just six handshakes away from everyone on the planet. Nicholas Christakis talks about the three degrees of influence—our behaviors have impact on our friends, our friends' friends, and our friends' friends' friends.
It's technical at times but you'll get the idea—the powerful influence of relational networks. Movements instinctively know how to harness the power of contagious relationships.