We are the sum of everything we think, say, and do. 

The brain gets desensitized to things it gets away with.

The study, published on Monday in the journal Nature Neuroscience, 80 volunteers played a game in which they estimated the value of pennies in a jar and sent their guess to an unseen partner. Sometimes participants were told they would secretly benefit at their partner’s expense if they overestimated the cash in the jar, incentivising them to lie.

Neil Garrett, also of UCL and a co-author, said: “We knew by how many British pounds they lied on each trial. The amount by which participants lied got larger and larger.”

At first, volunteers tended to alter the jar’s value by around £1, but this typically ramped up to about £8 by the end of the session.

Twenty-five of the volunteers played the game while having their brain activity monitored by an MRI scanner. This showed that the amygdala, a part of the brain linked with emotion, was most active when people told their first lie. But while the untruths escalated in magnitude, the amygdala’s response gradually declined - and larger drops in brain activity predicted bigger lies in future.

The researchers said this adaptation effect was similar to those seen in basic sensory experiences. A scent becomes less potent when smelt repeatedly, for instance.

“You can imagine cheating a bit like perfume,” said Sharot. The first time you cheat on your taxes you feel quite bad, she added, but if you get away with it and benefit this bad behaviour is reinforced and the next time it doesn’t feel quite so bad, leading you to cheat even more.

Professor Marcus Raichle, a neurologist of the Washington School of Medicine in St Louis who was not involved in the work, said that lying was probably a learnt behaviour. “It’s a nice study... It seems reasonable that if you develop a pattern of behaviour and it’s reinforced that you would return to that habit,” he said. “An interesting question is whether there would be interventions to un-train somebody. If you’re a chronic liar that’s really a problem for society.”