In a famous passage of Mark Twain's novel, Tom Sawyer, Tom is faced with the unenviable job of whitewashing his aunt's fence in full view of his friends who will pass by shortly and whose snickering promises to add insult to injury. When his friends do show up, Tom applies himself to the paintbrush with gusto, presenting the tedious chore as a rare opportunity. Tom's friends wind up not only paying for the privilege of taking their turn at the fence, but deriving real pleasure from the task – a win–win outcome if there ever was one. In Twain's words, Tom “had discovered a great law of human action, without knowing it – namely, that in order to make a man or a boy covet a thing, it is only necessary to make the thing difficult to attain.”

There are no mysteries in what painting a fence entails. Hence, Tom's “law” challenges the intuition that whether a familiar activity or experience is pleasant or unpleasant is a self-evident matter – at least to the person participating in that activity. If true, Tom's law would pose a fundamental challenge to economics. In a world where people don't reliably know what they like, it cannot be assumed that voluntary trades will improve well-being or that markets will increase welfare.

Recent research by psychologists and behavioral economists suggests that Twain's notions about human nature may be on the mark, at least in some situations (Frederick & Fischhoff, 1998; Hsee, Loewenstein, Blount, & Bazerman, 1999; Kahneman, Ritov, & Schkade, 1999; Slovic, 1995; Sunstein, Kahneman, Schkade, & Ritov, 2002).