The authors of the book Mistakes Were Made (but not by me) argue that our tendency to justify our actions is more powerful and deceptive than an explicit lie. They write:

[Self-justification] allows people to convince themselves that what they did was the best thing they could have done. In fact, come to think of it, it was the right thing. "There was nothing else I could have done." "Actually, it was a brilliant solution to the problem." "I was doing the best for the nation." "Those [jerks] deserved what they got." "I'm entitled."

[For example], when researchers ask husbands and wives what percentage of the housework they do, the wives say, "Are you kidding? I do almost everything, at least 90 percent." And the husbands say, "I do a lot, about 40 percent." Although the specific numbers differ from couple to couple, the total always exceeds 100 percent by a significant margin. It's tempting to conclude that one spouse is lying, but it is more likely that each is remembering in a way that enhances his or her contribution.

Over time, as the self-serving distortions of memory kick in … we come to believe our own lies, little by little. We know we did something wrong, but we gradually begin to think that it wasn't our fault, and after all, the situation was complex. We start underestimating our own responsibility, whittling away at it until it is a mere shadow of its former hulking self.