Tribes.

We are so tribal. We gather in tribes. All of us. It's a human thing. Even if some of us aspire to be in the tribe of no tribe, we end up in a tribe, by default. 

A social division. Shared customs, ideas, etc. We identify ourselves by dividing ourselves from the herd. This can be a strength. And a weakness. 

And if the division is too deep, too severe, much pain & bloodshed, and madness can ensue; one tribe beating and battering another...