Let me invite you to think of the last time you filed your income tax returns. Did you write off your car repair as a business expense? Did your beach trip or your trip to Europe suddenly become a work-related conference? Did you claim that some of the alcohol you bought was really for a work party? Did you refuse to report cash earnings or tips as income?

If so, welcome to the “fudge factor.” The fudge factor, eloquently articulated by Dan Ariely in his new book The (Honest) Truth About Dishonesty (HarperCollins Publishers), says that our behavior is driven by two opposing motivations. On one hand, we want to see ourselves (and be viewed by others) as honest, sincere and honorable. On the other hand, we want to gain as much (money, love, sex, power or advantage) as we can.

Clearly these two motivations are in conflict. How can we justify cheating—while at the same time still view ourselves as honest and trustworthy people who live our lives with honor and integrity?

Ariely’s answer is that we are master’s of the fudge factor, which says that as long as we cheat by just a bit, we can rationalize what we’re doing and still view ourselves as wonderful, honest and good people. He says most of us cheat—but only a little—not a lot. Cheating only a little still allows us to look in the mirror and feel good about ourselves. So if we bring home some office supplies from work and keep them for our own use, that’s OK in our minds, because it’s only a little, and nobody will ever notice. Most people won’t cheat too much, because it becomes harder to feel good about ourselves if we do.

To test this theory out, Ariely conducted a variety of experiments. He had one blind colleague and one sighted colleague take frequent taxi rides from the same starting point to the same end point. The taxi drivers took longer routes and charged more money to the sighted woman, and did so much less to the blind woman. Presumably, they would have felt bad about cheating a blind person, but they could easily justify to themselves cheating a sighted woman.

Ariely put a six-pack of coke and a plate of dollar bills in a shared fridge in a college dorm. When he returned, the coke was gone, but the dollar bills were all still there. Taking someone else’s coke was considered OK, while taking someone else’s money would have been more like stealing. He gave thousands of people a timed math test, with a money reward for more correct answers. When people turned in their answer sheets, they got, on average, 4 correct answers. When they were allowed to shred their answer sheets and self-report their answers, they reported 6 correct answers, and collected more money. They cheated a little, not a lot.

How likely are people you know to engage in the following behaviors? Stand in the express line with too many groceries; Try to board a plane before their group number is called; Inflate their business report; Tell their supervisor that progress has been made on a project when none has been made; Lie to an insurance company about the value of goods that were damaged; Buy a garment, wear it and then return it; Lie to their lover about the number of sex partners they have had.

Or how likely is it that the following lines are lies? I’m sorry I’m late. Traffic was horrible; I got straight A’s in school; It was good meeting you. Lets have lunch sometime; Sure, I’ll start working on that tonight; I thought I already sent that email out; The check is in the mail.

These experiments were done in the US, China, Israel, Italy, Turkey, Canada and England, with remarkably similar levels of cheating across the various cultures. Apparently almost everybody has the ability to be dishonest, but at the same time think of themselves as honest. It’s so easy to play with the flexible boundaries of our ethics, says Ariely.