If you want some real insights into why people cheat, both in RPGs and in real life, then watch this 15 minute presentation by Dan Ariely, a behavioral economist. Mr Ariely has run a series of experiments to discover why people cheat and what situations make us more or less likely cheat. The results are astounding, revealing how susceptible we are subtle difference in the situation and how they can make an enormous differences to when and how much we all cheat.
Note: If you are short of time, jump to 4:28, where he really starts to talk about cheating. On the other hand, if you some time to kill, have a look at this hour long talk by Mr Ariely as part of the Authors@Google series or check out his book: Predictably Irrational.