Dan Ariely, author of Predictably Irrational, is an insightful guy who recently performed some studies regarding cheating. (You’ll need to scroll down to the 3rd article.)
What he found suggests that some predisposition toward cheating may come from events that occur just prior the incident. He discusses, for example, one case where people were found more likely to cheat on a math test if they were a step removed from the cash payoff, say getting tokens to be redeemed later rather than cash. In one interesting variation of that test, there was no virtually no cheating after each of the participants was tested on his/her recall of the Ten Commandments.
In another case, subjects who wore “knock-offs” were twice as likely to cheat than those who were wearing the original product, suggesting some self-image aspects that are quite fascinating.