Low serotonin levels affect decision making

6 June 2008

The neurotransmitter serotonin, which acts as a chemical messenger between nerve cells, plays a critical role in regulating emotions such as aggression during social decision making, according to research published today in the journal ‘Science’.

Serotonin has long been associated with social behaviour, but its precise involvement in impulsive aggression has until now been unclear. Though many have hypothesised the link between serotonin and impulsivity, a new study carried out by Professor Trevor Robbins's group at the University of Cambridge is one of the first to show a causal link between the two.

The research highlights why some people become combative or aggressive when they are hungry. The essential amino acid necessary for the body to create serotonin can only be obtained through diet, so our serotonin levels naturally decline when we don't eat.

The research, funded by the Wellcome Trust and the Medical Research Council, also provides insight into clinical disorders characterised by low serotonin levels, such as depression and obsessive compulsive disorder, and may help explain some of the social difficulties associated with these disorders. It also suggests that patients with depression and anxiety disorders may benefit from therapies that teach them strategies for regulating emotions during decision making, particularly in social scenarios.

The researchers were able reduce brain serotonin levels in healthy volunteers for a short time by manipulating their diet. They used a situation known as the 'Ultimatum Game' to investigate how individuals with low serotonin react to what they perceive as unfair behaviour. In this game one player proposes a way to split a sum of money with a partner. If the partner accepts, both players are paid accordingly. But if he rejects the offer, neither player is paid.

Normally, people tend to reject about half of all offers less than 20-30 per cent of the total stake, despite the fact that this means they receive nothing. However, rejection rates increased to more than 80 per cent after serotonin reductions. Other measures showed that the volunteers with serotonin depletion were not simply depressed or hypersensitive to lost rewards.

Image: Illustration showing the action of neurotransmitters such as serotonin and noradrenaline in the synaptic cleft; Arran Lewis, Wellcome Images

Reference

Crockett, MJ et al. Serotonin modulates behavioural reactions to unfairness. Published online in Science Express, 6 June 2008.