Overview

The amygdala has the hard-coded genetic stuff, and the cortex handles the "think it through" stuff. In between is a layer where trained experience can be stored and give canned responses to pattern matched stimuli. Rules like, "Don't put that in your mouth, it's dirty," "Don't say the N-word," or, "Red-Sox suck!"

It mostly comes from repeated exposure, with some degree of cortex-level analysis before adoption.

The cortex can and does override norm-based "quick responses," much as norms or the cortex can override genetic things like, "it's long and thin and black and red, it's a snake, run!"

Broadly speaking, those quick response rules are a hug part of how society works. They cover things like, "How are you today?" "I'm fine." A hug chunk of the social lubricant that enables humans to cooperate - in much larger groups than any other apex predator - comes from norms.

Social Reinforcement

Social reinforcement of norms is central to their propagation and reinforcement. This is a major part of how social groups adopt similar behavior patterns. Social media groups and filter bubbles have enabled norms to make the leap to the global scale.

In a particularly striking finding, the study found that 37 percent of all dog owners believed vaccines could cause their pets to develop cognitive problems, such as "canine/feline autism." - Ars Technica Article

Experience

Experientially, norms are the the thing that makes you go, "I know the answer," before your cortex gets involved and identifies the reasoning. This is how you can find yourself certain of the answer but scrambling to come up with a rationalization.