Serotonin selectively influences moral judgment and behavior through effects on harm aversion

In my chapter INSIDE THE BRAIN I look at the picture that is beginning to emerge of the way gene variants, the environment (particularly of childhood upbringing), and neurotransmitters affects our moral behaviour. I also deal with the way in which the moral decisions we make depend on the input of basic emotional signals like fear and disgust which are massaged into moral decisions by higher order parts of the brain's neo-cortex, especially the ventro-medial pre-frontal cortex (VMPC). An international team of scientists, lead author Molly Crockett from Cambridge, has just reported on the role of serotonin in these processes by its activity in these crucial components of the "social brain" - the amygdala, insula and VMPC. The abstract below tells you what you need to know and this paper is open access in PNAS.

"Aversive emotional reactions to real or imagined social harms infuse moral judgment and motivate prosocial behavior. Here, we show that the neurotransmitter serotonin directly alters both moral judgment and behavior through increasing subjects’ aversion to personally harming others. We enhanced serotonin in healthy volunteers with citalopram (a selective serotonin reuptake inhibitor) and contrasted its effects with both a pharmacological control treatment and a placebo on tests of moral judgment and behavior. We measured the drugs' effects on moral judgment in a set of moral 'dilemmas' pitting utilitarian outcomes (e.g., saving five lives) against highly aversive harmful actions (e.g., killing an innocent person). Enhancing serotonin made subjects more likely to judge harmful actions as forbidden, but only in cases where harms were emotionally salient. This harm-avoidant bias after citalopram was also evident in behavior during the ultimatum game, in which subjects decide to accept or reject fair or unfair monetary offers from another player. Rejecting unfair offers enforces a fairness norm but also harms the other player financially. Enhancing serotonin made subjects less likely to reject unfair offers. Furthermore, the prosocial effects of citalopram varied as a function of trait empathy. Individuals high in trait empathy showed stronger effects of citalopram on moral judgment and behavior than individuals low in trait empathy. Together, these findings provide unique evidence that serotonin could promote prosocial behavior by enhancing harm aversion, a prosocial sentiment that directly affects both moral judgment and moral behavior."