There's a ceremony that determines your entire identity.
Think the Choosing Ceremony in Divergent, the Ceremony of Twelve in The Giver, and the graduation ceremony in The City of Ember.
The ceremony tells you which thing you are, and nobody is ever more than one thing unless you’re the protagonist. As someone who takes online personality quizzes like it’s her job, I understand why this is a popular literary mechanic. We’re all directionless, amorphous human blobs just hoping someone will tell us who we are and what we should be doing, and it would be far easier if some higher power could sort us into arbitrary categories.