Happiness...driving me to dispair

[Q]

Did some tests about the "natural decrease" effect on happiness:
It seems indeed that a negative value drags the happiness to the indifferent state. If I plague a planet, the happiness level goes to angry but if I have a "natural decrease" of -200 it returns within one or two turns to indifferent and stays there. With a value of 0 however the happiness level remains angry.
What's odd is that I see no effect of a positive value for the "natural decrease" even at a value of 200.