An urban tribe is a group of friends with the following characteristics:

High clustering coefficient

In graph theory, clustering coefficient is a measure of how cliquey (that’s the technical term, I’m not making this up) a network is: the relationship between two members is similar to the relationship between any two other members. The most striking consequence of this is that urban tribes can contain pairs of members who don’t really like each other but still invite each other to tribe events.

Provides family functions

Traditionally, families fulfill a number of social functions besides raising children: entertainment, emotional intimacy, etc. Young adults used to live with their families until getting married. Now they move to cities while still single and are discouraged from marrying immediately. Urban tribes exist so single young adults can receive the benefit of families without getting married or living with their parents.

Homeostatic

Cybernetics generalized the concept of biological homeostasis to describe systems that adapt to maintain their integrity. Watters’ research begins with an attempt to explain the low marriage rates in his cohort, his striking conclusion is that urban tribes discourage long-term romantic relationships, especially with people outside the tribe. This is because members with a “real” family will have no need for the tribe. This isn’t a conscious action of individual members, but an emergent behaviour of the tribe attempting to survive.

Watters says most tribes have around 8 members, but it’s not clear if there are minimum or maximum sizes.