system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do humans bare their teeth (smile) to show positive emotions when baring teeth is an almost universal signal of aggression or fear among other mammals?
gpt
In primates, showing teeth within a /closed/ mouth is a submissive gesture.  It shows that there is no aggression going on.  If a monkey bares fangs with their mouth open, then that is a dominant gesture.

I did behavioural research with monkeys for several years.