system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why is smiling an instinct for humans when we are happy, but an act of aggression for apes/monkeys?
gpt
It's not an act of aggression for most apes, it's a signal of nonaggression or submission, or fear, stress and nervousness, depending on which type of smile it is.   

The first type is the appeasement grin, used as a signal of submission to a higher-status member of their family group.  It indicates peacable intent, acknowledgment of their lower status to the higher-status ape.

The second type is the fear grimace, which is a signal of fear, uncertainty, or stress.