In Chapter 2, Duhigg introduces Claude C. Hopkins, an American businessman who helped to shape the power of corporate marketing during the 20th century. Hopkins is most famous for convincing Americans to begin buying and using Pepsodent toothpaste. Before Hopkins took over Pepsodent, few in America brushed their teeth regularly, and even more rarely did they use a special paste.

Hopkins provided a guide to corporations on how to convince consumers to develop new habits to sell products—what Duhigg refers to as “Hopkins’s rules.” First, companies needed to find the consumer’s trigger, the reason why the consumer would want to use that product on a daily or weekly basis. In the Pepsodent example, Hopkins convinced consumers that their cue for needing to brush with toothpaste was a film coating on their teeth. Next, their routine would be to brush their teeth using Pepsodent toothpaste. The final step in the habit loop, the reward, would be a tingly feeling in the mouth.

In the 1980s, Wolfram Schultz’s lab at the University of Cambridge performed experiments on monkeys to understand how their brains learned to do new tasks. The scientists added another component, craving, to the tried-and-true habit loop (cue > routine > reward).