Why do Germans synchronize their clocks more precisely than Brazilians? Why do Americans marry for love while arranged marriages persist in much of the world? Social psychologist Michele Gelfand argues that these differences reflect a fundamental cultural dimension: the degree to which societies enforce social norms. Tight cultures have strong norms and low tolerance for deviance; loose cultures have weaker norms and greater acceptance of unconventional behavior. This tight-loose distinction, Gelfand shows, predicts an enormous range of outcomes. Tight cultures have lower crime rates but less creativity. Loose cultures produce more innovation but more disorder. The dimension shapes everything from architecture to childrearing to the structure of languages. Gelfand, a professor at Stanford, traces the origins of cultural tightness to ecological and historical threats. Societies that faced frequent invasions, famines, or natural disasters developed stronger norms to coordinate collective responses. Safety and stability favored tightness; abundance and isolation permitted looseness. The book examines how tight-loose dynamics operate within nations as well as between them. Urban areas tend to be looser than rural ones. Organizations vary in their normative strictness. Even individuals differ in their need for structure. Gelfand connects these patterns to contemporary challenges, from political polarization to corporate culture to international negotiations. Readers seeking a framework for understanding cultural differences without resort to stereotypes will find her research-based approach valuable.