If you lie once, you’re probably more likely to lie again. New research shows that the part of the brain that is activated during dishonesty responds less and less as we “get used” to cheating — and that could make us lie even more.

There are anecdotes about people cheating more over time. (People like Bernie Madoff didn't exactly begin with huge Ponzi schemes.) But there hasn’t been research confirming this biologically until now, says study author Neil Garret, a psychologist at University College London. For a study published today in Nature, Garret’s team had participants play a game where they would sometimes get more money for lying to their partner. Brain scans of the participants confirmed that lying can be a slippery slope: people did lie more over time. Their brains got desensitized to it, and how much it was desensitized could predict how much more someone would lie the next time.

When we deceive someone, the part of the brain that regulates emotion — called the amygdala — is activated, and we often feel shame or guilt. The amygdala also reacts when we see cute pictures of puppies or very sad photos. We already knew that when our brains see these cute or sad photos again and again, the amygdala reacts less and less every time. Garret and his team wanted to know if this was true for lying, too.

The team recruited adults to work with another person that they didn’t know. The participants had to look at a picture of a glass jar and then tell their partner — who was helping the researchers — how many pennies were in it. At the end, both would get paid, but sometimes the participants would get more money if they lied. (They could lie to help themselves, help their partners, help both, and so on.) As the participants played the game, the researchers did brain scans of some of them. These scans, called fMRIs, show which regions of the brain used more oxygen; this is an indicator of brain activity. The researchers saw that as the participants continued to lie, the amygdala reacted less.

Participants in the game also became more dishonest more quickly when it would benefit just them and not their partner. And the amygdala really did activate less as people lied to help themselves. The participants kept lying to help themselves even if lying didn’t lead to more money every single time. This means it’s likely that people keep lying not because of rational calculation, but because they become desensitized.

All of us get bolder with dishonesty

“The reduction in activity in the amygdala can predict how much people increase dishonesty subsequently,” adds Garret. Predicting future behavior didn’t work accurately for everyone, but the overall trend was there. (The researchers didn’t track demographic trends of what kind of person became more used to lying.)

There are some limitations. This study tested a specific game, so we don’t know exactly what would happen in other types of situations when dishonesty is involved. And while the fact that this was done in a lab meant that the researchers could control things like who the participants were working with and how the game worked, the downside is always that it’s less clear how much this will apply in the real world. Brain scans also have to be taken with a grain of salt, as sometimes they can be misleading. (One fMRI test showed that a dead fish had brain activity.) In the study, just because a part of the brain was less active doesn’t necessarily mean that people didn’t feel as bad (and the researchers couldn’t ask because then that would give away the experiment).

But Garret says it’s pretty likely that there really is a slippery slope situation happening. We feel guilty the first time we cheat on a test, but by the third time we’re used to it. Now we know how the mechanism works — not just for people like Madoff or serial plagiarists that become more and more bold with their dishonesty, but for all of us.