The brilliant rationalist had encountered a central, frustrating tenet of human nature: behavior change is hard. The cleverest engineer or economist or politician or parent may come up with a cheap, simple solution to a problem, but if it requires people to change their behavior, it may not work. Every day, billions of people around the world engage in behaviors they know are bad for them--smoking cigarettes, gambling excessively, riding a motorcycle without a helmet. Why? Because they want to! They derive pleasure from it, or a thrill, or just a break from the daily humdrum. And getting them to change their behavior, even with a fiercely rational argument, isn't easy.