“The cheesy secret behind successful decision making” is a tempting headline in The Independent today. The newspaper analyses a claim that the ordinary cheese sandwich contains a vital ingredient that helps people to successfully negotiate pay rises. This conclusion originates from a study reporting that “people with high levels of the brain chemical serotonin are more likely to succeed in delicate negotiations affecting their own interests.”
The newspaper explains that serotonin is manufactured in the body from the amino acid tryptophan, which is present in several foods, and cheese is a particularly good source. Other newspapers also covered the story and linked big decisions to a full stomach, suggesting that it is best not to skip meals as this can lead to careless, impulsive behaviour.
In this study, 20 volunteers played a financial negotiation game after drinking either a tryptophan-enriched or tryptophan-free drink. Some differences were noted between the groups, but the claims that “food or cheese can increase your pay” are unsubstantiated. Overall, this study does not support the concept that either a full stomach or eating cheese could improve the quality of decision-making in real life.
Where did the story come from?
Molly J. Crockett from the Department of Experimental Psychology and other colleagues from the University of Cambridge, UK and the University of California, Los Angeles carried out the research. The study was funded by the Wellcome Trust charity and the Medical Research Council. The study was published in the peer-reviewed scientific journal: Science.
What kind of scientific study was this?
In this randomised controlled trial, the researchers investigated what effect manipulating serotonin levels had on behaviour in a financial negotiation game. To do this, they enrolled 20 healthy volunteers - 14 women and six men - with an average age of 25.6 years. The volunteers were paid to attend two sessions, which occurred at least a week apart. They were asked to fast the night before each session, whereupon they were each given a protein drink.
Tryptophan is an amino acid found in cheese and other foods, and ingesting it can boost serotonin levels. In alternate sessions the volunteers were either given a drink containing tryptophan, or a drink which reduced the blood level of tryptophan through a procedure called acute tryptophan depletion (ATD). The experiment was double-blinded, meaning that neither the researchers nor the volunteers knew what was in each drink, and it was randomised, meaning that the order that the volunteers were given the drinks was random. The drink containing tryptophan was used as the control drink, since the researchers were interested in the effect of reducing the level of this chemical.
In each of the two sessions, the participants gave a blood sample, drank either the placebo or ATD drink, then waited for five and a half hours to ensure stable and low tryptophan levels. They then gave a second blood sample, completed some psychological tests, and played the “Ultimatum Game”. This game involves two people, a “proposer” and a “responder”, who are presented with a sum of money to be split between them. The proposer decides how the money should be split, and the responder decides whether to accept the sum they have been allocated or not. However, should the responder not accept the proposal, neither person gets the money.
In this experiment, the volunteers played the part of the responder, and were shown a photograph of someone who had supposedly offered them a deal, comprising a share of money, and had to decide whether to accept that offer or not. The game was played 48 times on each day. The participants were told they would receive the financial outcomes from two randomly selected trials of the game.
Some of the offers were fair (40-50% of the stake), some unfair (27-33% of the stake) and some very unfair (18-22% of the stake). In different games, the same monetary amount could appear as a large percentage of the total stake and therefore “fair,” or as a small percentage of the total stake and therefore “unfair”. This design allowed the researchers to observe the independent effects of ATD on responding to different levels of fairness versus different levels of monetary reward.
What were the results of the study?
Since at least some reward was possible by accepting all offers, the researchers assumed that the ideal response would be to not reject any offers.
They found that more than 82% of volunteers with low serotonin rejected the “very unfair” offers. Those same people rejected the same offers 66% of the time when they had normal serotonin levels.
What interpretations did the researchers draw from these results?
The researchers say that they have shown “manipulating serotonin function can selectively alter reactions to unfairness in a laboratory model of self-regulation.” Lowering serotonin levels for a short while increased a person’s retaliation to perceived unfairness.
What does the NHS Knowledge Service make of this study?
This small experimental study has investigated some of the neural mechanisms and transmitters that could be associated with the rejection of unfair offers. There are some limitations to the study itself, and further limitations to the interpretations that the researchers and media might draw from this study:
- There was no formal assessment of blinding in the study, so it is not certain that the participants remained unaware of which drink they were taking. It is possible that they noticed the effects of tryptophan depletion, which could have unknowingly affected the results.
- The study was very small with only 20 volunteers, and it is possible that these results were chance findings only.
- The control group was those that received tryptophan. This study, therefore, theoretically shows the adverse effect of removing the amino-acid, such as by avoiding cheese, rather than of adding cheese to the diet, as implied by the newspapers.
- It is not clear if the participants were “hungry” or not, and what the normal variation of tryptophan levels through the day might be.
- The complex psychological and neurological mechanism involving brain transmitters, emotions and decision-making in healthy volunteers were examined in laboratory conditions, and therefore could be different from those found when complex decisions are made in real life. It is not clear how a normal diet intake and variation affects the neurotransmitters.
- The rejection of the other offers, those thought to be fair and unfair, was no different between the groups, suggesting a complex interaction is operating, which is not simply the effect of a depleted single amino-acid.
In association with other studies of the brain using MRI-scanning techniques, this may further the scientific community’s understanding of the parts of the brain and mechanisms involved in this type of activity. However, this study does not support the concept that either a full stomach or eating cheese could improve the quality of decision-making in real life.