It is helpful to think of people as having two fundamental motivations: the desire to see ourselves as honest, good people, and the desire to gain the benefits that come from cheating - on our taxes or on the football field.
Because cheating is easier when we can justify our behavior, people often cheat in small amounts: We can come up with an excuse for stealing Post-It notes, but it is much more difficult to come up with an excuse for taking $10,000 from petty cash.
Dishonesty is all about the small acts we can take and then think, 'No, this not real cheating.' So if you think that the main mechanism is rationalization, then what you come up with, and that's what we find, is that we're basically trying to balance feeling good about ourselves.