Serotonin: Neurochemistry of perception of fairness

It has been known for a while that people who have taken (through nasal administration, for example) oxytocin are less fearful and more trusting.

Here's an interesting finding about the effect of serotonin:

In [the ultimatum game], a proposal is made to the subject to unevenly split a sum of money. If the subject accepts, both parties get paid; if the proposition is rejected, both walk away with nothing.

Typically, people reject lowball offers of 20 to 30 percent of the total sum -- but those with depleted serotonin turned down amounts as high as 80 percent of the total. Researchers note that their moods did not change -- only their behavior -- meaning their perception of "fairness" became severely skewed.