Scientific claims about essential, categorical differences between women and men play a key role in a range of contemporary social and political debates. The biology of sex is used to contest economic issues like wage gaps and occupational segregation; educational ones like whether classrooms are ill-suited to boys or whether girls are inferior at math; civil rights issues like which bathrooms and ID cards trans people may use; and cultural ones about which toys and activities parents should encourage for their children. In these debates, the right often appeals to the authority of biology research, which supposedly offers a long-established, incontestable truth of sexual difference at odds with feminist calls for equality. Sex difference researchers often claim to be the latest in a long, rich lineage of scientific research in their favour, writing their critics out of history in order to establish a sense of unchallenged authority. In response, feminist biologists find themselves constantly ‘going over... old ground’, having to make and re-make the same arguments, to tell and re-tell the history of criticism in sex research. This essay explores the ways in which history is deployed and reconstructed in these debates.