The Dunbar number is the number of friends you can maintain.1 There are layers of 5 people who are very close, then 15, then 50, up to about 150. This is true for many human and primate groups (validated for humans against electronic messaging).
There is an inner-inner sanctum where men have one (either a romantic partner or a “drinking buddy”) and women have two (a romantic partner and a Best Friend Forever (BFF), usually another female. Close friends are likely to have:
- The same language (or dialect).
- Growing up in the same location.
- Having the same educational/career experiences.
- Have the same hobbies and interests.
- Have the same worldview (an amalgam of religious, moral/political views).
- Having the same sense of humour and musical tastes.