Back in the 1960s, a scientist at Yale University called Stanley Milgram conducted one of the most famous psychological experiments of all time, revealing how people are perfectly capable of administering excruciating electric shocks to innocent victims when ordered to do so by a superior. More than half a century later, researchers have finally figured out how receiving commands from authority figures alters our brain activity, thereby enabling us to go against our moral scruples and inflict pain on others without feeling guilty.

To conduct their study, the researchers recruited 20 pairs of volunteers, with one member of each duo playing the role of ‘agent’ while the other took on the role of ‘victim’. Agents were placed in a magnetic resonance imaging (MRI) scanner so that their brain activity could be monitored while they made a series of decisions regarding whether or not to administer a mildly painful electric shock to the victim in exchange for a small monetary reward.

At times, agents were free to choose whether or not to administer the shock, while at other times the decision was taken out of their hands and they were ordered to do so by the study authors.

Results – which are published in the journal NeuroImage – revealed that the parts of the brain that enable us to feel empathy and guilt showed diminished activity when agents were ordered to act. As a consequence, agents were less able to identify with the pain experienced by victims when delivering an electric shock under orders.

In a statement, study author Valeria Gazzola explained that “we can measure that empathy in the brain, because we see that regions normally involved in feeling our own pain, including the anterior insula and the rostral cingulate cortex, become active when we witness the pain of others.”

When agents were instructed to administer electric shocks to victims, these empathy-related brain regions became less active than when they were acting freely. Neural signatures associated with feelings of guilt were also reduced when agents were commanded to shock their victims.

Unsurprisingly, therefore, agents typically administered fewer shocks when acting freely than when carrying out orders. Yet in an unexpected twist, agents also rated these shocks as less painful when coerced into administering them, despite having previously been told that the shocks would always be equal.

All in all, the researchers say that these alterations in brain activity explain how “obeying an order relaxes our aversion against harming others,” thereby revealing “how people's willingness to perform moral transgressions is altered in coerced situations.”