What if your friend had a large apple pie but gave you only a sliver? Would you throw the piece on the floor in protest? Maybe, depending on your brain chemistry. New research suggests that such emotional decisions can be influenced by a shortage of the neurotransmitter serotonin.
Researchers have linked low levels of serotonin in the brain to various mental states, including depression and impulsive, irrational behavior. A team headed by neuroscience Ph.D. student Molly Crockett of the University of Cambridge in the U.K. wondered whether the neurotransmitter would affect how people play the ultimatum game, an experiment used by economists that shows how people's economic decisions are sometimes irrational.
In the game, a "proposer" is given a sum of money, part of which he or she offers to share with a "responder." If a responder turns down the offer as too low, then neither player gets any money. What the ultimatum game reveals is that even though a responder would always gain by accepting the offered share, he will sometimes cut off his own nose to spite his face, as it were, punishing a proposer by rejecting an unfair offer.
In the current study, the researchers recruited 20 volunteers and asked them to fast the evening before the game. The next morning, some of the volunteers were given a drink chock-full of every amino acid the body needs to make protein, save tryptophan, an amino acid from which serotonin is manufactured. The result, says Crockett, is that the amino acids rush to the brain, "crowding out" any residual tryptophan and creating a temporary shortage of tryptophan and therefore serotonin. Control subjects were given drinks that contained tryptophan.
Both groups then played the ultimatum game as responders. The lack of tryptophan did not affect the subjects' general moods or their perceptions of the fairness of an offer, the team reports online today in Science. It did, however, appear to make people more likely to reject unfair offers. For example, when they knew that they were being offered only 20% of the pot, 82% of the acute tryptophan depletion group rejected the offer over multiple trials, whereas only 67% of the placebo group did.
The research bolsters the view that rejection of an unfair offer is "an emotionally driven impulse," says Crockett. To heed more rational monetary considerations in the face of an unfair offer, she says, requires that you "swallow your pride"--or the sliver of pie--which is a form of emotional control.
The new work is "a significant advance" in understanding the neural mechanisms of how emotions impact decision-making, says neuroscientist Michael Koenigs of the National Institute of Neurological Disorders and Stroke in Bethesda, Maryland. Psychologist Ernst Fehr of the University of Zürich, Switzerland, cautions, however, that the paper doesn't really address which behavior is rational or irrational. Rejecting low offers, he says, could be the result of a rational calculation about the value of fairness rather than an angry impulse.