University of Cambridge researchers have found that decreased levels of the neurotransmitter serotonin increases emotional response to a perceived unjust or unfair situation.

Volunteers who had their serotonin levels temporarily lowered were much more likely to reject unfair offers in the Ultimatum Game (one player divides the money, the other decides to reject or accept the money for both players). A less emotional response would accept something rather than retaliate and receive nothing.

Humans have a strong motivation for fairness. Even when the Ultimatum Game is tested in poor countries with amounts equal to a week’s or a month’s wage, people still reject unfair offers.