Levitt and Dubner discuss the power of incentives, highlighting that they are important components of thinking like a Freak. The key is finding the right incentives—that is, those that actually work to shape people’s behavior as intended. They begin with an anecdote of one of them trying to potty train his young daughter. He devised a scheme to reward her with M&Ms if she did it correctly, and she soon learned the routine perfectly to get her reward. Before long, however, she also learned to game the system. She would use the toilet to pee only a little but hold the rest, then ask for her M&Ms; then she returned to pee a little more and demand more candy. Thus, the author learned a lesson about unintended consequences.

An obvious and common incentive is money. Levitt and Dubner even tie it to the rise in the rate of obesity in the United States in the last several decades: Highly processed fattening food has gotten cheaper, so people now eat more of it. To demonstrate how money can deter even behavior with a high degree of social conformity, they describe an incident in China in which the driver of a van struck and killed a young child in the street.