People don’t make purely rational decisions based on careful analysis of cost and expected utility, despite what classical economics taught us. Research findings confirm that our decisions are driven more by our emotions than logical and conscious thinking.
However, our irrationality is predictable. Good designers, therefore, can learn about human decision making and go beyond usability to create products that effectively influence our behavior.
via UX Myths – Myth #29: People are rational. The post is an awesome read and ends with a nice list of books to read to explore the subject further.