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Think that’s not fair? Your serotonin transporters must be high.

The views expressed are those of the author and are not necessarily those of Scientific American.


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“That’s not FAIR!”

This is the line that rings through most houses with at least one kid. We all know when something’s not fair. That car that drove up the shoulder while you waited in traffic (rrrrr)? That’s a cheater, and that’s not fair. The person who cut in line at the grocery store instead of waiting? That’s not fair either.

We get a sense of what is fair or unfair at a pretty young age, and we also understand that we are allowed, and indeed encouraged in some cases, to punish unfair behavior. But we don’t all punish unfair behavior the same way, especially when punishment may be detrimental to us. What is responsible for this difference? What mediates our reactions to what is unfair?

The authors of this study think it might be serotonin, and that it may have as much to do with honesty as it does with a sense of what is fair.

Takahashi et al. “Honesty mediates the relationship between serotonin and reaction to unfairness” PNAS, 2012.

The basis for this paper was a game used in psychology testing, a very common game called the “Ultimatum game”. In this game, you play against one other player (or, often, against a computer). That other player has found some money, say $10. They want to divide the money with you, and you can choose whether you accept their offer, or reject the offer of splitting the money. For example, if they offer to split 50-50, you may take that offer and the $5, while if they offer to split 90-10, you may be insulted and reject it.

But wait. Think a minute. It’s a FOUND $10. The other player found it. They are offering to split it with you. Shouldn’t you accept all the offers? I mean, even if they offer to split 90-10, that’s $1 you didn’t have before!

But it turns out human psychology doesn’t work this way. We have a strong sense of what is a fair split, and we will reject “unfair” offers. Usually the line of “unfair” for a found $10 in the ultimatum game is somewhere around 70-30, if $3 or less is going to you, people tend to hold out for a better offer.

Of course, there’s a lot of variation in this. While some people are pretty flexible, others have a greater sense of what is “unfair” and will reject offers higher than 50%. And this is even when, as in the ultimatum game, the rejection is a disadvantage to the player.

The authors of this study wanted to examine the differences between people who rejected the most “unfair” offers, and those who accepted more of them. For this they looked to serotonin. Serotonin, a neurotransmitter that is involved in mood, sleep, memory, cognition, appetite, bowel issues, migraine, blood pressure, pain, nausea, premature ejaculation…I could go on. The authors of this study hypothesize that serotonin also plays a role in how we respond to unfair offers. In order to look at the serotonin system in humans, they conducted positron emission tomography (PET), which uses a radiolabeled tracer to look at activity in the body. In this case, the radiolabeled tracer was for serotonin. They could track the amount of serotonin in the brain by looking at concentrations of their labeled serotonin as it binds to the serotonin transporter, which recycles serotonin up from the synapse and back into the neuron. This allows you to see how much serotonin transporter is there, and thus whether the area has generally more or less serotonin.

So it was pretty easy to look at the levels of serotonin tansporters in human volunteers, and see how those levels correlated with their performance in the ultimatum game.

The yellow dot on the brain picture here is the raphe nucleus, the area where serotonin is produced. And on the right is the correlation between the serotonin transporter density in the raphe and the % of unfair offers that were rejected.

What they found here was a negative correlation. The MORE serotonin transporters you had, the less likely you were to reject unfair offers. The authors interpret this to mean that people with lower levels of serotonin transporter had a harsher sense of “fairness”, than those with higher levels of serotonin transporter, and were more inclined to reject unfair offers.

Why could this be the case? The authors looked at the personalities of the individuals. You might think that people with more aggressive personalities (or at least a tendency to get offended) might be more likely to reject unfair offers, but it turned out that this wasn’t the case. Instead, it was people with more peaceful personalities, but stronger measures of trust, were more likely to reject the unfair offers. The authors believe that the people with higher trustfulness had higher standards of behavior, and thus were more likely to reject unfair offers, even if the rejected ended up badly for them (as in, getting no money). Based on previous studies, the authors hypothesize that lower levels of serotonin correlate with trustfulness and an increase to reject unfair offers, and hypothesize that serotonin mediates this sense of fairness.

I think it’s pretty likely that serotonin plays a role in this trait (what doesn’t it play a role in, after all?), but remember, correlation is not causation. There are probably many more things mediating a response to an unfair offer than just serotonin transporter levels in the raphe of the brain. For one thing, most of this kind of higher level decision making takes place in higher structures such as the prefrontal cortex. I assume the authors did not find differences there (or presumably they would have mentioned it), why was there only this difference in the raphe? Perhaps because small changes make big behavioral differences, but I think it is just as likely that serotonin transporter levels are not the most important player here. But it’s still an interesting thought, to see so nicely demonstrated that our brain chemical correlate with our decisions. Hopefully more studies will be able to tell us why.

Takahashi, H., Takano, H., Camerer, C., Ideno, T., Okubo, S., Matsui, H., Tamari, Y., Takemura, K., Arakawa, R., Kodaka, F., Yamada, M., Eguchi, Y., Murai, T., Okubo, Y., Kato, M., Ito, H., & Suhara, T. (2012). Honesty mediates the relationship between serotonin and reaction to unfairness Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1118687109

Scicurious About the Author: Scicurious is a PhD in Physiology, and is currently a postdoc in biomedical research. She loves the brain. And so should you. Follow on Twitter @Scicurious.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. jtdwyer 1:46 pm 03/19/2012

    Thanks for your very excellent discussion of the conclusions of these researchers in the next-to-last paragraph (beginning with: “I think it’s pretty likely that serotonin plays a role in this trait (what doesn’t it play a role in, after all?), but remember, correlation is not causation…”)

    For example, an alternative hypothesis apparently supported by this research data might be: individuals who have been conditioned to accept unfair conditions without complaint may produce insufficient serotonin levels to alleviate the discomfort such conditions generally produce…

    Link to this
  2. 2. BradLead 5:55 pm 03/19/2012

    How do we explain the growing tendency of doctors to recommend SSRI antidepressants to treat P.E.? Why are they doing this with increasing frequency even though they are aware of the many potential side-effects associated with these preparations?

    The answer sadly is that they do not have the time to impart the information men need to deal with the problem naturally. When the options are half an hour of discussion or five minutes to write a prescription, the choice is easy. Even if some were willing to spend the time, most patients would choose the significantly lower cost for the shorter consultation.

    There are many possible side-effects of SSRIs. The commonest are fairly innocuous yet hardly desirable as part of a sexual experience. Nausea, headaches, diarrhea, rash, dry mouth, nervousness or agitation, weight gain and decreased sexual desire, But there is one that is far more serious. It is for this reason that the FDA requires that these potent medications carry a warning that young adults may be at increased risk of suicide. This, by the way, is the very population that most requests these drugs.

    In addition a study recently published has reported a decreased sperm count in men taking SSRIs. With infertility becoming a more frequent issue for couples trying to start a family, they do not need to add anything that will increase the odds against conception.

    What choice do doctors have if they want to help their patients? The answer is to follow the guidelines of The American Association of Family Physicians which states:: “Behavioral methods are helpful for more than 95 percent of men who have premature ejaculation”

    For the past 35years, following the pioneering work of Masters and Johnson, thousands of men have learned to control ejaculation using behavioral techniques they learn from books.. Books like “Lasting Longer: The Treatment Program for Premature Ejaculation” which has been available since1978. Written by: Dr. Sy Silverberg M.D., it is a virtual do-it-yourself manuals for overcoming P.E. Doctors can write a prescription for such a manual in the same time they write one for SSRIs that may have significant side-effects. And there is no ongoing cost beyond the ten bucks for the book.

    Resorting to these products remains an option for the small percentage of men who do not respond to the behavioural approach. In this way physicians can fulfill their mandate to, “First do no harm”. This is one of the principal precepts of medical ethics that all medical students are taught in medical school and is a fundamental principle for emergency medical services around the world. Another way to state it is that “given an existing problem, it may be better not to do something, or even to do nothing, than to risk causing more harm than good. (Definition from Wikipedia).

    Link to this
  3. 3. Travza 9:55 pm 03/19/2012

    @BradLead

    As interesting as that may be, lets try and keep this comments section on-topic. Granted, serotonin is discussed here, but the article does not relate to SSRI’s in even the most preliminary of senses.

    Link to this
  4. 4. JDahiya 3:54 am 03/20/2012

    I like your analyses even more than I like the way you describe such research so accessibly. Please continue the good work!

    Link to this

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