Retaliation for unfairness - depends on serotonin

Nature highlights an article by Crockett et al. showing that serotonin modulates our reaction to unfairness. The experimenters:

...temporarily lowered serotonin (5-HT) levels in 20 volunteers and had them play the part of responder in the 'ultimatum game'. The responder can either accept the division of a sum of money offered by the game's proposer, in which case they both get their share, or reject it and deprive both players of the amounts proposed.

Although mood, fairness judgments, basic reward processing, or response inhibition, remained unchanged when players' serotonin levels were lowered, they were more likely to reject unfair and very unfair offers, defined as 30% and 20% of the stake, respectively.