Why do people cheat? 

You might guess that it is a consideration of the potential gain and the potential risk (of being caught). 

In this video, Dan Ariely, Alfred P. Sloan Professor of Behavioral Economics, speaks about his surprising findings on why people cheat- who does it and when. 

His findings would suggest that rather than personal gain, and rather than level of risk, a large factor on whether people cheat are the norms of the group.  In part of the video above, he relates an experiment where students were paid for correct answers.  An actor was planted in the group and displayed behavior that made it obvious that he was cheating.  There were no repercussions to his actions.  They found that when the actor wore a sweatshirt with the name of the university where most of these students attended, the incidence of cheating increased.  When the actor wore a sweatshirt with the name of a different university, the incidence of cheating decreased. 

All the more reason why it is so important to understand your values -- and reflect on how your actions are supporting or foresaking them.  Group norms are powerful influencers and can lead you to compromises - if you don't know yourself and what you truly believe.