" It was not individual aggression which got out of hand, but devotion to the narrow social group with which the individual identified himself to the hostile exclusion of all other groups. It is the process we have discussed before: the integrative tendency, manifested in primitive forms of identification, serving as a vehicle for the aggressive self-assertiveness of the social holon.

To put it in a different way: to man, intra-specific differences have become more vital than intra-specific affinities; and the inhibitions which in other animals prevent intra-specific killing, work only within the group. In the rat it is the smell which decides who is friend or foe. In man, there is a terrifyingly wide range of criteria, from territorial possession through ethnic, cultural, religious, ideological differences, which decide who stinks and who does not. "

― Arthur Koestler , The Ghost in the Machine