It is a mystery that has perplexed philosophers for centuries: why do people usually choose to do the right thing?
Now, using an experiment involving brain scans and electric shocks, scientists claim to have found the answer. Researchers at University College London discovered that, at a physical level, the brain finds decency far more satisfying than deception.
Their experiment involved 28 couples who were paired off anonymously and given the ability to give each other small electric shocks. They were given the option of receiving sums of money in return for a shock either for themselves or for their partner.
The research team scanned volunteers' brains as they made their decision and noticed that a region of the brain called the striatum, key to the understanding of value, was activated. MRI imaging found that this brain network was far more active when the participants gained money while inflicting pain on themselves than on another, suggesting they found it instinctively more worthwhile.
"When we make decisions, a network of brain regions calculates how valuable our options are," said Dr Molly Crockett, who led the research.
"Ill-gotten gains evoke weaker responses in this network, which may explain why most people would rather not profit from harming others.
"Our findings suggest the brain internalises the moral judgments of others, simulating how much others might blame us for potential wrongdoing, even when we know our actions are anonymous."
The scans also revealed that an area of the brain involved in making moral judgments, the lateral prefrontal cortex, was most active in trials where inflicting pain yielded minimal profit.
In an allied study, participants were asked to make moral judgments about decisions to harm others for profit. It showed that when people refused to profit from harming others, this region was communicating with the striatum.
The researchers believe this shows that moral rules are visible in the form of neurological signalling, and that these disrupt the value we might otherwise place on ill-gotten gains.
Senior author Professor Ray Dolan, from the Max Planck Centre for Computational Psychiatry and Ageing Research at UCL, said: "What we have shown here is how values that guide our decisions respond flexibly to moral consequences. An important goal for future research is understanding when and how this circuitry is disturbed in contexts such as antisocial behaviour."
The researchers' results, published in the journal Nature Neuroscience, stressed that the electric shocks administered to participants were carefully matched to each recipient's pain threshold to be "mildly but tolerably painful".
Previous research by UCL has suggested that generosity and altruism are governed by a specific region of the brain - the subgenual anterior cingulate cortex - and that it works naturally better in some people than in others.