Science doi:10.1126/science.1155577 (2008)

If someone treats you badly and you retaliate, blame serotonin. Lower levels of this neurotransmitter make people more likely to retaliate when they perceive others to have breached the maxim 'treat others as you wish to be treated', find Molly Crockett at the University of Cambridge, UK, and her co-workers.

The team temporarily lowered serotonin 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 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.