How many times would you give your neighbor an electric shock to earn a few extra bucks? Your answer could be more malleable than you think. A new study finds that two common drugs—an antidepressant and a treatment for Parkinson’s disease—can influence moral decisions, a discovery that could help unravel specific mechanisms behind aggression and eventually help researchers design treatments for antisocial behavior.

Previous research has linked two neurotransmitters, the brain’s signaling molecules, to our willingness to inflict harm. Serotonin appears to help keep us civil; it’s reduced in the brains of violent offenders, for example. Dopamine, meanwhile, has been shown to prompt aggression in animals, and it’s elevated in a certain part of the brain in people with psychopathic behavior.

But measuring how these neurotransmitters contribute to moral decision-making is hard to do in the lab. Many studies rely on theoretical questions like the so-called trolley dilemma, which asks a person whether they would redirect an oncoming train to kill someone if it would save the lives of several others in its path. A person’s answer might not always reflect how they would behave in real life, however.

So neuroscientist Molly Crockett of the University of Oxford in the United Kingdom and her colleagues developed a lab test with real consequences. They asked subjects to make a series of decisions about how many moderately painful electric shocks to deliver to themselves or to others. Half the questions gave volunteers a chance to earn money by inflicting self-harm. (For example: “Would you rather endure seven shocks to earn $10 or 10 shocks to earn $15?”) The other half offered the same type of decision, except that someone else stood to be shocked. At the end of the experiment, one of these choices was randomly selected and carried out: The decision-maker got paid, and either they or another person—waiting in a different room—got a series of painful zings on the wrist. Any answer could be the one with real consequences, so “people have to sort of put their money where their mouth is,” Crockett says.

The researchers could then calculate the “exchange rate between money and pain”—how much extra cash a person must be paid to accept one additional shock. In previous research, Crockett’s team learned that the exchange rate varies depending on who gets hurt. On average, people are more reluctant to profit from someone else’s pain than their own—a phenomenon the researchers call “hyperaltruism.”

In the new study, the scientists tested whether drugs can shift that pain-to-money exchange rate. A few hours before the test, they gave the subjects either a placebo pill or one of two drugs: the serotonin-enhancing antidepressant drug citalopram or the Parkinson’s treatment levodopa, which increases dopamine levels.

On average, people receiving the placebo were willing to forfeit about 55 cents per shock to avoid harming themselves, and 69 cents to avoid harming others. Those amounts nearly doubled in people who took citalopram: They were generally more averse to causing harm, but still preferred profiting from their own pain over another’s, Crockett’s team reports online today in Current Biology. Levodopa had a different effect: It seemed to make people just as willing to shock others as themselves for profit.

The design of the study is “something that the field of aggression needed for a really long time,” says Joshua Buckholtz, an experimental psychologist at Harvard University. The results, he says, separate out two distinct components that drive our social behavior: the way we conceive of harm to others and our preference for enduring harm rather than inflicting it. The dopamine drug—but not the serotonin drug—seemed to change that preference.

Crockett says those effects could suggests multiple underlying mechanisms. For example, excess dopamine might make our brain’s reward system more responsive to the prospect of avoiding personal harm. Or it could tamp down our sense of uncertainty about what another person is experiencing, making us less hesitant to dole out pain. Serotonin, meanwhile, appeared to have a more general effect on aversion to harm, not just a heightened concern for another person. Such knowledge could eventually develop drugs that address disorders of social behavior, she says.

Still, the study did not measure levels of dopamine or serotonin in the brain to confirm that they were elevated while the subjects were taking the test, notes Jay Gingrich, a developmental neurobiologist at the Columbia University Medical Center. He questions whether a single dose of citalopram can reliably increase serotonin levels just a few hours later. “Maybe they’re onto something,” he says, “but my big concern was them over-interpreting what the pharmacology was doing.”

Crockett says she relied on past research to identify the point at which absorption of the drugs was highest, but she couldn’t confirm neurotransmitter levels for sure without a more invasive test, such as a PET scan.

She is also careful to point out that the two drugs—administered only to healthy volunteers in the study—are likely to have different effects on social behavior in people taking them to treat an illness. For example, in Parkinson’s patients who are deficient in dopamine, the drug may simply restore normal levels, not create an undesirable surplus, she says. “The last thing I want is for people who are taking these drugs for medical reasons to become concerned about the implications of this study for their own decision-making.”