What must underlie successful epidemics, in the end, is a bedrock belief that change is possible, that people can radically transform their behavior or beliefs in the face of the right kind of impetus. This, too, contradicts some of the most ingrained assumptions we hold about ourselves and each other. We like to think of ourselves as autonomous and inner-directed, that who we are and how we act is something permanently set by our genes and our temperament. But if you add up the examples of Salesmen and Connectors, of Paul Revere's ride and Blue's Clues, and the Rule of 150 and the New York subway cleanup and the Fundamental Attribution Error, they amount to a very different conclusion about what it means to be human. We are actually powerfully influenced by our surroundings, our immediate context, and the personalities of those around us. Taking the graffiti off the walls of New York's subways turned New Yorkers into better citizens. Telling seminarians to hurry turned them into bad citizens. The suicide of a charismatic young Micronesian set off an epidemic of suicides that lasted for a decade. Putting a little gold box in the corner of a Columbia Record Club advertisement suddenly made record buying by mail seem irresistible. To look closely at complex behaviors like smoking or suicide or crime is to appreciate how suggestible we are in the face of what we see and hear, and how acutely sensitive we are to even the smallest details of everyday life. That's why social change is so volatile and so often inexplicable, because it is the nature of all of us to be volatile and inexplicable.

How good people's decisions are under the fast-moving, high-stress conditions of rapid cognition is a function of training and rules and rehearsal.

"Preschoolers make a number of assumptions about words and their meaning as they acquire language, one of the most important of which is what the psychologist Ellen Markman calls the principle of mutual exclusivity. Simply put, this means that small children have difficulty believing that any one object can have two different names. The natural assumption of children, Markman argues, is that if an object or person is given a second label, then that label must refer to some secondary property or attribute of that object. You can see how useful this assumption is to a child faced with the extraordinary task of assigning a word to everything in the world. A child who learns the word elephant knows, with absolute certainty, that it is something different from a dog. Each new word makes the child's knowledge of the world more precise. Without mutual exclusivity, by contrast, if a child thought that elephant could simply be another label for dog, then each new word would make the world seem more complicated. Mutual exclusivity also helps the child think clearly. "Suppose," Markman writes, "a child who already knows 'apple' and 'red' hears someone refer to an apple as 'round.' By mutual exclusivity, the child can eliminate the object (apple) and its color (red) as the meaning of 'round' and can try to analyze the object for some other property to label.