system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is open-mouth breathing often associated to lower intelligence? Is there any biological or behavioral link between the two?
gpt
It's part of the "boorish brute" stereotype that's roughly as old as social classes are. Laborers who have to expend lots of energy throughout the day will end up panting heavily and loudly over the course of their work. More "refined" individuals accustomed to leisure are more likely to be able to placidly go about their time breathing through their noses. Lateral stereotypes stemming from this phenomenon includes colorism in Southeast Asia and grooming conventions around fingernails and hand calluses in the West and elsewhere.

Over time, rules of etiquette codified not breathing through your mouth, and the brutish stereotype of mouth breathing was further entrenched into what we see today.