In the Afterword, Duhigg recounts the story of the Harvard Study of Adult Development, one of the longest and most comprehensive studies of human life ever conducted (See: Background). The study, which began in 1937 with funding from a chain store magnate named Billy Grant, initially aimed to help businesses select the best employees by examining the lives of Harvard undergraduates and a group of teens from South Boston’s tenements. The study followed these participants for decades, collecting data on their physical and mental health, personalities, and life experiences.

Duhigg focuses on two participants, Godfrey Camille and John Marsden, who had strikingly different backgrounds and life trajectories. Camille, who had a troubled childhood and struggled with mental health issues, was predicted to have a bleak future. In contrast, Marsden, an exceptional student from a wealthy family, was expected to achieve great success. However, when the study resumed in the 1970s after a period of dormancy, researchers discovered that their predictions were wrong. Camille had transformed his life, becoming a nationally-recognized expert, exemplary father, and beloved community member. Marsden, on the other hand, had become lonely, angry, and dissatisfied with his life.

As researchers analyzed the data, they found that the most important factor in determining happiness and health was the quality of the participants’ relationships.