Moral relativism at its best: your behavior is heavily influenced on how you feel in a particular moment, as revealed by this paper published in the Journal of Economic Behaviour
. The abstract says that those in a bad mood tend to reciprocate (you are good to me, I'll be too, you are bad, I fight back) while those in a good mood tend to be generous (happy, happy, joy, joy, I'll kiss and hug you no matter what).
The paper is also available from one of the author's home page
, and has been commented by Xenophon, another blog