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If you say you have never lied, you are almost certainly lying. Only little lies, you say? Well, it’s long been thought that’s where the seeds of dishonesty are usually sown. Even convicted swindler Bernie Madoff thought so. According to his secretary, he said: “Well, you know what happens, it starts out with you taking a little bit . . . and before you know it, it snowballs into something big.”

Now, for the first time, there’s evidence that confirms this escalation of dishonesty. Once we start lying, the extent of our dishonesty increases over time. And when that happens, our brain begins to tamp down its emotional response to our own immoral behavior, signaling in effect that lying is no big deal. That makes it easier to lie again … and again . . . and again. (Perhaps that explains some of what happened on the campaign trail this year.)

These provocative results came from a clever study designed by a team of experimental psychologists at Tali Sharot’s lab at University College London and published this month in Nature Neuroscience. According to PhD student Neil Garrett, the lead author, they started with the well-established fact that the brain adapts to sensory information. That’s why walking into a smoky room is most off-putting when you first arrive. Stick around and you don’t notice it as much. The researchers wondered if the same principle holds true for emotional decisions we initially avoid such as cheating, infidelity, and lying.

To find out, they brought participants into the lab in pairs to play a guessing game with monetary rewards. One became the Estimator, and had to look at photographs of jars of pennies and estimate the number of coins. The other was the Advisor, who had better information (larger, higher resolution photographs of the jars) and provided advice to the Estimator. It was the Advisors’ behavior that mattered. The Estimators were secretly in on the study.

There were a number of different conditions that affected how motivated the Advisor was to be accurate and honest. Sometimes accuracy benefited both people equally, sometimes accuracy benefited one more than the other. In other words, the Advisor could help himself or his partner by lying. (You can read the details here.)  A baseline condition was used to establish how many pennies the Advisor thought there were and the same photographs were used throughout, so, like Santa Claus, the researchers could tell if the Advisor was being naughty or nice.

Sure enough, self-serving dishonesty increased over time. There is some good news, though: “People were much more likely to lie when it also benefited someone else not just themselves,” wrote the researchers.

In addition to studying participants’ behavior, Garrett, who has since moved to Princeton University, and his colleagues looked inside their brains using functional magnetic resonance imaging. “A network of brain regions associated with emotion responded strongly when participants lied initially,” they said in describing the study. “But as time went on, it would respond less and less to the same amount of lying. The greater the drop in sensitivity, the more a person increased their lying the next opportunity they got.” There were individual differences. Some people lied more readily and rapidly.

The part of the brain they were looking at centers on the amygdala, which has been long been known to play a critical role in emotion and in the formation of habits. Of course, fMRI doesn’t show the amygdala recognizing a lie. What it shows is changes in the blood oxygenation level—representing emotional response—that were higher with the first lie and lessened as the trials went on and the lies mounted. It was not brain activity that lessened. It was sensitivity to lying that lessened.

Predictably, this study has been getting a lot of attention. In an accompanying analysis in Nature Neuroscience, psychologist Jan Engelmann and behavioral economist Ernst Fehr write that “the results . . . open up exciting venues for future research.”

But the rest of us don’t need to wait for the next set of experiments to recognize what this work means: Little lies have a way of turning into big lies and your brain will stop noticing if you let it. Far better to tell the truth.