Repo shows that gender is not originally a feminist term, but emerged from the study of intersex and transsexual persons in the fields of sexology and psychology in the1950s and 1960s. Prior to the 1950s gender was used to refer to various types of any number of phenomena. In the mid-twentieth century, gender shifted from being a nominator of types to designating the sexual order of things. Over the last sixty years, the notion of gender has become an entire field of knowledge. Feminists took up the term in the 1970s to challenge biological determinism. Gender has also become a key variable in social scientific surveys of different socio-political phenomena like voting, representation, employment, salaries, and parental leave decisions. Repo analyzes the strategies and tactics of power involved in the use of 'gender' in sexology and psychology, and subsequently its reversal and counter-deployment by feminists in the 1970s and 1980s. It critiques the emergence of gender in demographic science and the implications of this genealogy for feminist theory and politics today.