As a social scientific term, gender came into common use only in the final quarter of the twentieth century. But its core idea, that biological sex and its cultural expression are separable, had been evolving for over a hundred years. As rapid urbanization fostered greater sexual freedom and spurred a vibrant women’s movement at the end of the nineteenth century, a disparate group of sex reformers, feminists, and university-trained researchers began to question a number of conventional beliefs. Does effeminacy in men signal biological abnormality? Is politics, by nature, a masculine enterprise? Are geniuses disproportionately male? Do females lack sexual drive? At the turn of the century, most social theorists answered yes to these questions. But by the 1970s, even as researchers were mapping the human brain with ever-greater precision, scholars had ceased treating the cultural expression of sex as a direct product of physiology. Symbolic of this dramatic shift, social scientists abandoned “sex” in favor of “gender” when discussing human behavior. Long used exclusively as a grammatical category, “gender” appealed to those who found the biological associations of “sex” too limiting. Here was a term that freed investigators to explore with new intensity the multiple ways in which cultures distinguish males from females, structure sexual experience, and deploy power.