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," or, "Don't say the N-word."

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