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Small lies can spiral out of control. When this happens, some will admit they were deceiving but a select group will only feel they have to lie some more. Why is it so easy for some people to lie all the time? Scientists at the University College London think compulsive liars may have trained their brains to ignore emotional response until it becomes second nature. Free from guilt, repeated acts of dishonesty become as easy as breathing air.

“Whether it’s evading taxes, being unfaithful, doping in sports, making up data or committing financial fraud, deceivers often recall how small acts of dishonesty snowballed over time,” said Tali Sharot, an associate professor of cognitive neuroscience at University College London and a co-author of the study.

While there are many environmental and cultural factors that can explain why some are big liars, the researchers suspected an underlying biological mechanism has to be at place too. So, they designed an experiment in which participants were paired and asked to advise their partner about how many pennies were in a glass jar. Some of these participants were told they would get a reward if they overestimated the number of coins inside the jar, basically lying to their partners.

This incentive was enough to sway the participants to lie more often. The size of their lies, or rather the degree to which they overestimated, grew as the trials progressed.

Meanwhile, EEG caps scanned the brain of the participants to monitor their response, particularly in the amygdala — almond-shaped groups of nuclei in the brain which is linked to both fear responses and pleasure. It is in the amygdala that the ‘fly or fight’ response is triggered, and the researchers found that repeated lying was associated with a reduced amygdala response. In fact, the researchers say they could predict how large a person’s next lie would be just by looking at their brain scans.

The findings suggest that our brain undergoes emotional adaptation and repeated lying can numb the amygdala, essentially making a person less hesitant to deceive. This same mechanism might be involved in escalations like risk-taking and violent behavior, the researchers report in Nature Neuroscience.

“I think this study’s the first empirical evidence that dishonest behavior escalates when it’s repeated, when all else is held constant, and it ties this phenomenon to emotional adaptation,” study lead author Neil Garrett of University College London said. “The same mechanism may well underlie all sorts of other escalations, such as escalation of risk-taking or escalation of violent behavior.”

“I think it highlights the potential dangers of engaging in small acts of dishonesty on a regular basis,” Garrett added, “and also suggests that possible avenues for curbing dishonesty, such as finding ways to reproduce the negative emotional reaction that stops us from engaging in such acts.”