Healthy people given the serotonin-enhancing antidepressant citalopram were willing to pay almost twice as much to prevent harm to themselves or others than those given placebo drugs in a moral decision-making experiment at the Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology. 

In contrast, the dopamine-boosting Parkinson’s drug levodopa made healthy people more selfish, eliminating an altruistic tendency to prefer harming themselves over others. The study was a double-blind randomised controlled trial and the results are published in Current Biology.

The research provides insight into the neural basis of clinical disorders characterized by a lack of concern for others, such as psychopathy. Serotonin and dopamine levels have both been linked to aggressive and antisocial behavior, and this study helps explain why.

The study, conducted by researchers from UCL and Oxford University and funded by the Wellcome Trust, compared how much pain people were willing to anonymously inflict on themselves or strangers in exchange for money. A total of 175 healthy adults took part, 89 randomised to receive citalopram or placebo and 86 randomised to receive levodopa or placebo.

A similar experiment conducted by the same team previously showed that people disliked harming others more than harming themselves, a behaviour known as ‘hyper-altruism’. This behaviour was seen again in this study, with most people more willing to harm themselves than others for profit.

On average, people given a placebo were prepared to pay approximately 35p per shock to prevent harm to themselves and 44p per shock to prevent harm to others. Those on citalopram were far more harm-averse, willing to pay an average 60p per shock to prevent harm to themselves and 73p per shock to prevent harm to others. Over the course of the experiment, this meant that people on citalopram delivered an average 30 fewer shocks to themselves and 35 fewer shocks to others than those on placebo.

People given levodopa, however, were not willing to pay any more to prevent shocks to others than to prevent shocks to themselves. On average, they were prepared to pay approximately 35p per shock to prevent harm to themselves or others. This meant that they delivered on average 10 more shocks to others during the experiment than the placebo group. They were also less hesitant to deliver shocks to others, making the decision faster than those on placebo.

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