How happiness works? (answer is here)

[Suicide Junkie]

IIRC, natural decrease moves your mood towards indifferent.

So if your people are rioting, and there are no modifiers making them angry, they will eventually get bored and go back to work.
If they are super-happy, with no modifiers making htem happier, they will eventually settle back into the drudgery of day to day work.

The natural decrease for other races is of the opposite sign, so if they are happy, they get happier, and if they are angry, they get angrier.