Duhigg opens the book with the story of 34-year-old Lisa Allen, an American woman who lived for years with a smoking addiction, an unhealthy body, financial problems, and relationship struggles. A range of medical researchers from neurologists to geneticists were researching how the brain forms habits, and Lisa’s story provided a perfect example of how people can change their habits to improve their lives. Lisa turned her life around by addressing one key habit first: She quit smoking. By changing just this one habit, what Duhigg identifies as a keystone habit, Lisa retrained her brain with an entirely new set of habits that overhauled her life.

The problem with habits, as Duhigg explains, is that humans often feel they are making active decisions in their lives, but our habits unfold as deeply subconscious routines. Thanks to a robust field of scientists who have researched habit formation in the brain, we do not necessarily need to be victimized by our habits. Duhigg then introduces both the book’s premise and its structure: “Each chapter revolves around a central argument: Habits can be changed, if we understand how they work” (xvii).

The book’s nine chapters are organized into three broad themes: 1) how habits form within the brains of individual humans; 2) how corporations and organizations have used habits to market their products; and 3) how habit formation works within societies.