Natural Rationality | decision-making in the economy of nature


Ultimatum, Serotonin and Fairness

A new study links serotonin levels to Ultimatum decision: less serotonin is correlated with a higher rejection of unfair offers, suggesting that it "plays a critical role in regulating emotion during social decision-making"

[From ScienceNOW]

Deal or No Deal?

By Constance Holden
ScienceNOW Daily News
05 June 2008

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.


michael said...


It is a silly idea to think that a) there is a single correct play in the u-game which is prescribed by formal theory, and b) the red lights flashing in the brain as measured by the current fad justifies holding position a)

Ask some serious poker players to play the u-game as responders, and give them aspiration levels to get at least 50%

I can guarantee you that those who hold out for more will do so on cold, hard calculated grounds.

Benoit Hardy-Vallee said...


Thanks for your comment, but I never said that a) or b). This post only presented recent research on this subjects. My own view is developed in
How to Play the Ultimatum Game: An Engineering Approach to Metanormativity." Philosophical Psychology, 21(2), 173 - 192.

where I make a point similar to yours. So please refrain from calling me "silly" before knowing what I actually think.

michael said...


Trouble with the internet is that it is too easy to think that being hard on an idea is being hard on a person.

I apologize if you think I called you "silly".

I did however call an idea silly - having re-read your paper co-authored with Paul Thagard, I offer the following observations.

First, in most decision theory there is a lamentable tradition of conflating two different notions of solution:

a) a solution to a problem, being a recommendation that may or may not work out.

b) a solution being the result of a calculation or solution of equations.

It is rarely spelled out how a solution/recommendation is a solution/calculation.

Often the problem is simply translated into some formal calculus, a solution calculated, and it is then assumed without argumentation or illumination that this calculation has merit as a recommendation for the non formal problem.

This is rarely straightfoward.

Second, many of the experimental results tell us far more about what the experimenters thought the game was about that what the participants did - as the tradition in experimental economics is to try to remove all relevant clues or hints about what the point of the exercise is. So, for example, while many people argue about whether fair offers are being rejected in the U-game, we really have no good reason to think that it is fairness as opposed to some other focal point that is being considered.

Third, now that people know that if they are serontonin deprived it is likely that they will reject unfair offers, can they mimic this behaviour when the underlying chemical basis is not present? My guess is "yes", and it is this type of adaptation which makes it hard to the conclusion that rejecting unfair offers is simply an emotional and not reasoned response.

5. For different views, you might want to check out Tierny's lab today: