(PhysOrg.com) -- The most common type of antidepressants, serotonin enhancers, alters peoples’ moral judgement and leads to a reduction in aggressive behaviour, a study published today in the journal Proceedings of the National Academy of Sciences (PNAS) has found.
The new research, by scientists at the University of Cambridge's Behavioural and Clinical Neuroscience Institute, discovered that healthy volunteers given drugs which increase their serotonin, selective serotonin reuptake inhibitors (SSRI), have an increased aversion to harming others, viewing such actions as morally forbidden.
Ms Molly Crockett of the Behavioural and Clinical Neuroscience Institute (a Medical Research Council and Wellcome Trust funded initiative) is the lead author of the paper. She said: "Our study suggests that these medications can affect people's sense of right and wrong, which influences the choices they make in everyday life.
"Interestingly, the drug's effects were strongest in people who were naturally high in empathy, suggesting that serotonin could enhance people's concern for others by making the prospect of harming them feel worse."
Antidepressants, which include SSRIs like the one used in the study, are among the most widely prescribed drugs worldwide. In the United States and the UK, an estimated ten per cent of the population take antidepressants for a range of psychiatric and medical conditions.
The Cambridge researchers measured the drug's effects on moral judgement by giving the participants 'dilemmas' which pitted beneficial outcomes (e.g., saving five lives) against highly unpleasant harmful actions (e.g., killing an innocent person). Individuals with increased serotonin were far more likely to judge the harmful actions as morally wrong even when such actions would result in significant benefits.
The participants' aversion to harming others, on the grounds that it was morally objectionable, was also apparent on a different test in which subjects decide whether to accept or reject fair or unfair splits of a sum of money from another player. People tend to reject unfair offers as an act of retaliation, because rejecting offers harms the other player financially. Subjects taking the SSRI were less likely to reject unfair offers, possibly because they were reluctant to hurt the other players financially.
A previous study by the same research group had found that decreasing serotonin made people more likely to retaliate against unfairness.
Explore further: Two definitions for chronic multisymptom illness afflicting Gulf War vets should guide treatment