Dan Ariely in an interview with Evan Davis on BBC Today 21/05/09 throws light on the Psychology of cheating.
He has done a number of experiments which demonstrate that it is “normal” for most people to cheat a little, and he has examined the circumstances in which this may increase.

With regard to the MPs expenses his take on it is that we gave them a test to discover if they were human and we discovered that they are human.

The full interview can be heard here. http://news.bbc.co.uk/today/hi/today/newsid_8060000/8060758.stm