This article originally appeared on Time.com.
Once a liar, always a liar, the old saying goes. Turns out there’s some scientific truth to that: researchers have tracked down how the brain makes lying easier as the untruths build up, providing some biological evidence for why small lies often balloon into ever larger ones.
In a study published in Nature Neuroscience, Tali Sharot from the department of experimental psychology at University College London and her colleagues devised a clever study to test people’s dishonest tendencies while scanning their brains in an fMRI machine. The 80 people in the study were shown pennies in a glass jar and given different incentives to guide whether they lied or told the truth to a fellow partner about how much money was contained in the jar. In some conditions, both the participant and the partner benefited if the participant lied; in others, just the participant benefited from his fib, or just the partner benefited (with no cost to either). In another set of scenarios, either the participant or partner benefited, but at the expense of the other if the participant lied. In each case, Sharot documented the changes in the people’s brains as they made their decisions.
They found that when people were dishonest, activity in a part of the brain called the amygdala—the hub of emotional processing and arousal—changed. With each scenario, the more dishonestly the participant advised his partner, the less activated the amygdala was on the fMRI. That may be because lying triggers emotional arousal and activates the amygdala, but with each additional lie, the arousal and conflict of telling an untruth diminishes, making it easier to lie.
Sharot also found that the amygdala became less active mostly when people lied to benefit themselves. In other words, self-interest seems to fuel dishonesty.
“Part of the emotional arousal we see when people lie is because of the conflict between how people see themselves and their actions,” Sharot said during a briefing discussing the results. “So I lie for self-benefit, but at the same time it doesn’t fit the way I want to view myself, which is as an honest person. It’s possible that we learn from the arousal signal…with less emotional arousal, perhaps I’m less likely to see the act as incongruent with my own self perception.”
The researchers were even able to map out how each lie led to less amygdala activation and found that the decrease could predict how much the person’s dishonesty would escalate in the next trial. Biology seems to back up the warnings parents give to their kids: that one lie just leads to another.