Americans today “know” that a majority of the population supports the death penalty, that half of all marriages end in divorce, and that four out of five prefer a particular brand of toothpaste. Through statistics like these, we feel that we understand our fellow citizens. But remarkably, such data—now woven into our social fabric—became common currency only in the last century. Sarah Igo tells the story, for the first time, of how opinion polls, man-in-the-street interviews, sex surveys, community studies, and consumer research transformed the United States public. Igo argues that modern surveys, from the Middletown studies to the Gallup Poll and the Kinsey Reports, projected new visions of the nation: authoritative accounts of majorities and minorities, the mainstream and the marginal. They also infiltrated the lives of those who opened their doors to pollsters, or measured their habits and beliefs against statistics culled from strangers. Survey data underwrote categories as abstract as “the average American” and as intimate as the sexual self. With a bold and sophisticated analysis, Igo demonstrates the power of scientific surveys to shape Americans’ sense of themselves as individuals, members of communities, and citizens of a nation. Tracing how ordinary people argued about and adapted to a public awash in aggregate data, she reveals how survey techniques and findings became the vocabulary of mass society—and essential to understanding who we, as modern Americans, think we are.