Why do we want - and do - so many things that are bad for us? We vow to lose those extra five pounds, put money in the bank, and mend neglected relationships, but our attempts often end in failure. Our toughest battles, it seems, are with ourselves. To understand this fundamental aspect of human nature, Terry Burnham and Jay Phelan argue, we need to stop looking to Sigmund Freud - and start looking to Charles Darwin.