Most people want to stay healthy. Few enjoy being sick. Given that health is such a universally shared value, one might think that people's thinking about health is easily understood, and that changing behaviours with health consequences should be relatively easy. Of course, this not the case at all. For example, fewer than 50 per cent of sexually active college students use condoms to prevent the spread of sexually transmitted diseases such as HIV, and less than 60 per cent of women over the age of 50 seek annual screening mammography, despite its obvious value for the early detection of...