tribalism

Short definition from Wiktionary

Noun

1. 1
   The condition of being tribal.
2. 2
   A feeling of identity and loyalty to one's tribe.
3. 3
   The tendency among human beings to form socially cohesive groups within which there exists social pressure upon the individual to conform to either common behaviors, customs, habits, mores and/or norms, and which groups exhibit greater or lesser degrees of exclusivism.