In the experiment, the "deciders" had the power to pay money from their allocated amount to prevent either themselves or their "receiver" from being shocked.
The researchers behind the new study, from University College London and Oxford University - both in the UK - tested to what extent people were willing to inflict pain on themselves or others in exchange for money.
The neurotransmitters serotonin and dopamine have been linked to aggressive and antisocial behavior in previous studies. As several commonly prescribed psychiatric drugs work by enhancing levels of these neurotransmitters, the researchers compared how the effect on moral decision-making of a drug that enhances serotonin levels (citalopram) differed to that of a drug that enhances dopamine levels (the Parkinson's drug levodopa).
For the study, the results of which are published in the journal Current Biology, the researchers recruited 175 healthy adults - 89 of which were randomized to receive citalopram or a placebo, while 86 received levodopa or a placebo.
In the experiment, the participants were also randomly assigned a role of "decider" or "receiver." The deciders and receivers were anonymously paired - the deciders did not know who their receiver was and vice versa.
At the start of the experiment, all participants were given "mildly painful" electric shocks that were matched to each participants' pain threshold.
The decisions made by the deciders related to shocks that they would receive themselves, as well as shocks that would be inflicted upon the receiver. The deciders and receivers were allocated to receive an equal number of shocks by the researchers, but the deciders had the power to pay money from their allocated amount to prevent either themselves or the receiver from being shocked.
The deciders were placed into rooms on their own with a computer terminal. Over 170 trials, the deciders were offered different amounts of electric shocks costing various amounts of money, up to a maximum of 20 shocks and about $30 per trial.
The deciders knew that any decisions they made would not be revealed to the receiver. As such, the deciders had the option of inflicting shocks upon the receivers without fear of judgement or retaliation.
Levodopa group 'most likely to deliver shocks to others'
In this experiment, the participants who received placebos paid an average of 55 cents per shock to prevent harm to themselves and 69 cents per shock on average to prevent harm to others.
However, participants who received citalopram were more considerate when it came to harm, showing willingness to pay an average of 94 cents per shock to avoid harming themselves and $1.14 per shock to prevent harming others.
The authors say that over the course of the study, people on citalopram gave themselves 30 fewer shocks and 35 fewer shocks to other people compared with the placebo group.
By contrast, the participants given levodopa were generally unwilling to pay any more to prevent shocks to others than to prevent shocks to themselves. They would pay an average of 55 cents per shock to prevent themselves and others from being harmed.
On average, the levodopa group delivered 10 more shocks to others during the study than the control group. The authors also report that the levodopa participants made decisions faster and were less hesitant to deliver shocks to receivers than participants who received a placebo.
In a previous experiment, the same research team had found that people were more averse when it came to hurting others than they were when it came to harming themselves. This behavior - known as "hyper-altruism" - was also seen in this study, as the participants were mostly more willing to take the electric shocks themselves than inflict them on another participant for profit.
"Our findings have implications for potential lines of treatment for antisocial behavior, as they help us to understand how serotonin and dopamine affect people's willingness to harm others for personal gain," says lead author Dr. Molly Crockett.
"We have shown that commonly-prescribed psychiatric drugs influence moral decisions in healthy people, raising important ethical questions about the use of such drugs."
However, Dr. Crockett cautions that these drugs may have different effects in psychiatric patients compared with healthy people. "More research is needed to determine whether these drugs affect moral decisions in people who take them for medical reasons," she concludes.