Serotonin may play an important role in mood regulation, and a new study from the journal Science adds to our understanding of how this neurotransmitter, which has been linked to depression and anxiety, affects our daily behavior. Researchers from the University of Cambridge and the University of California at Los Angeles examined the influence of this crucial brain chemical on the social interactions of 20 healthy subjects. Their work indicates that when serotonin levels are low, we are more likely to react in an irritable or aggressive manner while making decisions that affect another person.

The amino acid tryptophan, which we take in through certain foods, like poultry and fish, must be present for the body to produce serotonin. Because of this, serotonin levels decrease when we haven’t had enough to eat, and the researchers were able to induce low serotonin levels by controlling their subjects' diets. The subjects then played a decision-making game where they were exposed to unfair circumstances. In “the Ultimatum Game,” a pair of players is given a sum of money and one player is asked to divide that amount between the two of them. Both players must agree on the offered solution, however, and if one player rejects the other’s suggestion, neither of them receives any money. At times throughout the game, some subjects were instructed to offer their partners 30 percent or less of the original sum, and in these unfair scenarios, the other player refused to take the money about half of the time. But when the subjects had low serotonin levels, their rate of refusal rose to over 80 percent. This rise may have been motivated by a desire to punish the other player, and the researchers suggest that low serotonin levels may have caused the subjects to act on their negative emotions more readily.

Serotonin is not the only factor influencing emotional response, but these results indicate that our daily behavior may be affected by this neurotransmitter in discernible ways, and an awareness of the situations that can lead to low serotonin levels may be helpful for some. In a press release, lead author Molly Crockett, a PhD student at Cambridge, explained further: "Our results suggest that serotonin plays a critical role in social decision-making by normally keeping aggressive social responses in check. Changes in diet and stress cause our serotonin levels to fluctuate naturally, so it's important to understand how this might affect our everyday decision-making."

Though the study was limited by its small size, it hints at forms of treatment that may be effective for conditions that have been associated with low serotonin levels. Individuals with depression and anxiety often have difficulty interacting with others, and therapies that help them better regulate their emotions in during social situations may be beneficial. Researchers should continue to study serotonin and its relationship to depression, anxiety, and other mental health conditions. With a clearer understanding of this neurotransmitter’s functions and roles, we may discover new ways to address these common and often debilitating conditions.