segregation

[zhul4nder]

Why do people do it?

I understand that it's much a learned tidbit of human nature...
As they say, monkey see monkey do..

But even so, children through my observations segregate even though they are not taught. From an early age as I remember, kids separate into these groups groups such as

athletes
Boys
Girls
rich kids
Poor kids

After these groups are made, hostility between these groups.

From an evolutionary stand point it doesn't make sense to hate their own race.
Why have we developed to hate each other, to eliminate competition?

Are we that selfish?