We know people have positive social behavior in part because of emotional reactions to real or imagined social harm  - we may not like seeing others slighted or we may not want to be perceived as the kind of person who does that sort of thing.

But some are a lot more sensitive than others and a new study says that the neurotransmitter serotonin can directly alter both moral judgment and behavior through increasing our aversion to personally harming others, rather than just controlling violent impulses or helping you sleep.

For their study they gave volunteers the anti-depressant citalopram (a selective serotonin reuptake inhibitor, or SSRIs) and compared its effects with a pharmacological control treatment (atomoxetine - a selective norepinephrine reuptake inhibitor) and a placebo regarding tests of moral judgment and behavior. 

The moral judgment involved dilemmas that weren't really dilemmas so the results would be more obvious, like saving five lives versus highly aversive harmful actions like killing an innocent person. The people with enhanced serotonin were more likely to judge the clearly harmful actions as forbidden and the harm-avoidant attitude was also evident in behavior during the ultimatum game, where they had to decide to accept or reject fair or unfair monetary offers from others with the qualifier than unfair offers enforce fairness norm but also harms the other player financially.

 Enhancing serotonin made them less likely to reject unfair offers but it also influenced empathy. Individuals high in trait empathy, measured using the Interpersonal Reactivity Index, which predicts prosocial helping behavior, showed stronger effects of citalopram on moral judgment and behavior than individuals low in trait empathy.  The researchers say this is evidence that serotonin could promote prosocial behavior by enhancing harm aversion, which affects both moral judgment and moral behavior.

Citation: Molly J. Crocketta, Luke Clark, Marc D. Hauser, and Trevor W. Robbins, 'Serotonin selectively influences moral judgment and behavior through effects on harm aversion', published online before print September 27, 2010, doi: 10.1073/pnas.1009396107 (open access!)