It is widely assumed that material circumstances strongly affect human happiness. However, as the example of the “poor little rich girl” suggests, objective outcomes and happiness are not perfectly correlated. Indeed, many studies suggest that they are hardly correlated at all. For example, winners of lotteries do not report themselves as being much happier than other people, and those who were paralyzed in an accident do not report themselves as being much less happy. Similarly, as nations get wealthier, the reported wellbeing of its citizens does not increase.

The lack of evidence for a relation between objective circumstances and reported well-being has given rise to the concept of a hedonic treadmill, on which humans' happiness remains stationary, despite efforts or interventions to advance it. The metaphor ...