It starts with a little lie—that haircut is perfect for you!—but before you know it, you’re bragging about your Olympic gold medal in curling. Now scientists at University College London and Duke University say they’ve figured out why we so naturally progress from little white lies to whoppers. They published their findings [PDF] in the journal Nature Neuroscience.

Researchers recruited 80 people between the ages of 18 and 65 and brought them into the lab to play a game. Each participant was introduced to their “partner” (actually a researcher), and then some of them were hooked up to an MRI scanner before they started playing. The premise was simple. A participant was shown a clear image of a jar of pennies. They were told that they were responsible for reporting the number of pennies to their partner via a microphone, and that their partner would pass on that information to the researchers. Both participants would then be given a certain amount of money. All participants had reason to believe that their imaginary partners were oblivious and would trust whatever they said. In some scenarios, the participants were told that the more accurate and truthful their guess, the more money they’d make. In others, they were told that they’d make more money if their partners guessed wrong; in other words, they were encouraged to lie.

The tests were set up to create four situations: those in which lying benefited both the participant and their partner; those in which it benefited only the partner; those in which it benefited only the participant; and those in which lying would only hurt them both.

The researchers noticed two clear, if unsurprising trends. First, they saw that participants’ willingness to lie increased as the game went on. Fudging a number and increasing or decreasing the estimate by a few pennies turned into a few more pennies, then a few more. Second, the tests showed that lying only increased for the two situations that benefited the participants, whether with or without their partners.

Reviewing the brain scans, the researchers could actually watch as participants became accustomed to lying. As the initial fib was taking place, the participants’ brains showed activation in the amygdala and other regions associated with strong emotional responses. It’s as if their brains were saying, “This is not a good idea. Let’s not do this.” But the next lie induced less amygdala activation, and the one after that, less still. It was as though they’d built up a tolerance to dishonesty.

Study co-author Tali Sharot compared the experience of lying to wearing a new perfume. At first, she said, the new scent is overpowering. The second time you wear it, it’s simply strong. But “two months from now when you put on the perfume,” she said in a press conference, “you can’t even smell it yourself, so you feel you have to put quite a lot on, and other people turn away. And that’s because the neurons in your olfactory bulb adapt.”

Like our sense of smell, the authors say, each person’s lying profile was different. Some participants lied more than others, and some people’s lies escalated more quickly.

The researchers have not conclusively proven that reduced amygdala activation reduces our pangs of guilt, thereby greasing the slippery slope, but they think it’s pretty likely. “This is in line with suggestions that our amygdala signals aversion to acts that we consider wrong or immoral,” said co-author Neil Garrett. “We only tested dishonesty in this experiment, but the same principle may also apply to escalations in other actions such as risk taking or violent behavior.”
 
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