The book began with a New York Times Magazine article. Stephen Dubner, who's one of the co-authors of Freakonomics, was working on a story about the psychology of money and that's how he met Stephen Levitt. So the book is co-written by both of them, but it's not kind of clear who wrote what. There's lots in the book that is based on Levitt studies, but there's also lots in thebook that isn't based onLevitt studies.