"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