Foul! Without serotonin, we'd find it harder to deal with others who treat us harshly. Credit: Punchstock

Controlling your anger and reacting sensibly when someone treats you badly can be a problem. And if you have low levels of serotonin, it can be even more of a problem, a new study has found.

Molly Crockett at the University of Cambridge, UK, and her colleagues gave volunteers a drink that temporarily lowered their levels of serotonin, a brain 'neurotransmitter' linked to happy mood. They then had them play ‘the Ultimatum Game’, which involves accepting or rejecting offers of money.

Those with lower serotonin levels showed increased retaliation to offers that they perceived to be unfair.

“We’ve suspected for years that there’s a link between serotonin and impulsive aggression and emotional regulation,” says Crockett. “Until this study it wasn’t clear whether serotonin was playing a causal role.”

It has long been known that low serotonin levels are associated with groups of people prone to impulsiveness and problems with emotional control, such as alcoholics, violent criminals and suicide attempters. Low serotonin is also found in clinical conditions such as depression and anxiety.

“We’ve known for 30 years that low serotonin is associated with impulsivity, inwardly directed aggression and outwardly directed aggression,” says David Nutt, head of the Psychopharmacology Unit at the University of Bristol’s Faculty of Medicine and Dentistry, who was not involved in the new study.

“What we are doing now is externally manipulating it. We need to study it in a more controlled environment.”

The Ultimatum Game

So how does the Ultimatum Game work? Well, imagine what you would do if someone offered you £6. But what if they offered you £6 out of a total of £13 that they’d been given to split with you?

Most people would take the money, especially if rejecting the money meant getting nothing.

But what if they offered you £6 out of £30 they’d been given to split with you? That is a clearly unfair split, but logically it is still better than nothing.

In the Ultimatum Game, you can punish the other player by rejecting the offer, meaning they get nothing. Of course, you also get nothing.

In Crockett’s study, detailed this week in Science1, 20 participants were given a number of attempts at the game, with fair offers, defined as 45% of the stake, unfair offers, defined as 30% of the stake, and very unfair offers, defined as 20%. Participants were randomized to get the serotonin-lowering treatment or a placebo.

While placebo participants rejected about 65% of very unfair offers, those with low serotonin rejected more than 80%.

Researchers also measured the mood, fairness judgement and reward processing of participants. They found these to be unaffected by lower serotonin, clearly implicating the neurotransmitter in the more aggressive response to injustice.

Our desire for fairness also seems to be a universal trait.

Crockett says even when the Ultimatum Game has been tested in poor countries with amounts of money equivalent to a week's or a month’s wage, people still reject unfair offers. “The motivation for fairness is a strong one,” she says.