[Q]

Originally posted by Krsqk:
By default, native populations have an anger decrease, while all enemy populations have an anger increase. They're not keeping their own characteristics, though; you could mod a special happiness trait for your race, and even standard alien races would follow your happiness type.

It is a little bit more complicated as explained in a other topic on happiness:
The standard negative value of happiness decrease for your races moves the happiness towards indifferent. The standard positive value of happiness decrease for foreign races moves their happiness toward extremes, either angry or jubiliant. You could call this a choleric behaviour.
And your are right if the two values of the natural decrease are identical the happiness of your race and foreign races you captured will be identical.