Moral behaviour is usually considered the province of philosophers and ethicists, but a new study suggests the way we treat others is actually tied to our brain chemistry.

Researchers from the University College London have found that commonly prescribed antidepressant medication can affect people's decision-making when choosing between whether to act selfishly or selflessly.

In an experiment involving 175 healthy adults where participants had the power to both administer and receive electric shocks, researchers sought to investigate how much pain people were willing to inflict on either themselves or others in exchange for money.

But it wasn't just a straight study to determine who would act selfishly and who wouldn't. Half the group were randomly selected to receive the serotonin-enhancing antidepressant citalopram, while the other half were given a dopamine-booster, levodopa, used in treating Parkinson's. Each group receiving medication was then compared against a group that received only a placebo.

The two medications had a marked impact on the dilemma facing the 'decider' group - the participants given control over the electric shocks - essentially, how many shocks should they inflict on themselves and another in order to maximise a cash reward, with more zaps equalling a better financial return.

The researchers found that, among the deciders in the zapping experiment, those receiving a one-off dose of citalopram were willing to pay almost twice as much to prevent harm to themselves or others when compared to a placebo group. In contrast, zappers on levodopa acted more selfishly, displaying a preference for harming others for the cash reward.

"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," said Molly Crockett, the lead author of the research. "We have shown that commonly-prescribed psychiatric drugs influence moral decisions in healthy people, raising important ethical questions about the use of such drugs."

The findings, published in Current Biology, follow on from other research showing that drugs can have the effect of making people more compassionate. Studies in this area could have important ramifications for how antidepressants and other kinds of mood-altering medications are tested, with researchers arguing for a broader model that incorporates looking at the kinds of real-world behaviour that medication users could demonstrate in the long term.

"Patients [taking these drugs] are tracked in terms of how their symptoms improve, but not necessarily in terms of how their behaviour changes," Crockett told Hannah Devlin at The Guardian.

"In the treatment of Parkinson's, some patients go on to develop compulsive gambling and compulsive sexual behaviour. The drugs have consequences that reach out into the world beyond the patient."