...an exceedingly interesting question. This topic is covered by the well-renown professor of psychology and behavioral economics at Duke University, Dan Ariely in an article in WSJ - Why we lie
I'm currently reading his fascinating, highly original and thought-provoking book Predicably Irrational. His WSJ article on lying covers some of the themes of his new book, which I'm also looking forward to reading, entitled The Honest Truth about Dishonesty: How we lie to everyone - especially ourselves