In everyday life, we make a lot of decisions. Small decisions, like deciding to take the car to work today. And larger decisions, like what education to follow, or what job to pursue. Sometimes fairness is involved: for example, you might be offered a lower salary than expected for a job you applied for. That means you’ve got a decision to make: is the offer fair, and will you accept or reject it?

But let’s take a less extreme example, something we can measure and do science with: a game in which we have to divide 20 euros between the two of us. In this game, I make an offer and you decide whether you accept the offer, if you don’t accept the offer we both get nothing. We’ll play the game only once so reciprocity is not an issue.

So, here’s an offer: I get 17 euros, you get 3. Would you accept? And is this a fair offer? Research tells us that most people find that an unfair offer and would even reject it, while this means they won’t get any money (which is clearly less than 3 euros). The game I described is called the Ultimatum Game, and it has been used to investigate decision-making and sense of fairness. Recently, scientists made a fascinating discovery: there is a specific neurotransmitter that plays an important role in determining the sense of fairness: serotonin. This neurotransmitter has long been known to be involved in social behavior, but the precise mechanism remained unclear until now.

To find out more about the role of serotonin, recent research let participants play the Ultimatum Game, and measured serotonin levels afterwards. And guess what? People with low serotonin levels were more likely to reject an unfair offer than people with normal serotonin levels. While this is only correlational evidence, evidence for a causal role of serotonin has been provided as well: artificially reducing serotonin levels led to a higher probability of rejecting an offer.

This new understanding of the role serotonin plays in human behavior is very relevant. For instance, antidepressants are known to affect serotonin levels and so does MDMA (better known as ecstacy). We now understand how these drugs might also influence behavior. Another implication may be relevant in our daily life: our diet actually influences serotonin levels trough a precursor of serotonin called tryptophan. Examples of food with a high tryptophan level are spinach, egg and all kinds of soy. So if you’re afraid you’ll be accepting a potential job offer to quickly in the future, you might want to think well about what you eat in advance.