Background: The paper plots some shifts in educational policy between 1988 and 2009 in England that launched the rhetoric of a "gender gap" as a key political and social concern. The rhetoric was fuelled by a rise in the importance of quantification in technologies of accountability and global comparisons of achievement. A focus on boys and attainment emerged, along with new requirements for measuring educational achievement in the context of debates about standards and the growing marketisation of education following the 1988 Educational Reform Act (ERA) in England and Wales. Purpose: Theoretically, the paper explores the effect of "gender gap" rhetoric on pedagogy. The arguments about pedagogy presented here are based on the premise that "sex-group" is different from "gender." Sex-group is a form of labelling and categorising persons as either male or female with reference to a biological classification that focuses on genitalia and reproductive organs. The emergence of "gender gap" rhetoric is investigated within a temporal perspective, through an overview of guidance to teachers about pedagogy published between 1932 and 2007. This temporal lens becomes a heuristic for presenting the main point of the paper, which is that technologies of measurement construct reified representations of the learner. This is used to demonstrate how gender, as a sociocultural and political phenomenon, morphed into sex-group, a biological categorisation, and how this has had unintended effects of pedagogy. Sources of evidence: Analysis of three landmark educational documents focuses on changes in representations of society, the learner and pedagogy. The documents are the Hadow Report (1931), the Plowden Report (1967) and a guidance document for teachers called "Confident, Capable and Creative: supporting boys' achievements" (Department for Children, Schools and Families 2007, http://www.foundationyears.org.uk/wp-content/uploads/2011/10/Confident_Capable_Boys.pdf). Main argument: Analysis demonstrates the way that technologies of measurement construct reified or "ideal" representations of the learner and how technologies used for measuring sex-group difference have changed across time. Shifts in representations of the learner, from the "bone child" to the "gene child" and eventually to the "masculine child" were detected. These shifts represent a gradual decline in the emphasis on pedagogy as nurture, towards a heightened focus on the supposedly innate characteristics of individuals, in line with neoliberalism. Conclusions: The discussion points to some of the unintended effects on pedagogy and practice that take place when gender becomes sex. If teachers are constantly presented with the message that boys and girls learn differently due to innate genetic make-up, they may assume that whatever pedagogic strategies they employ, these will be ineffective in the face of what some educational consultants tell them are boys' and girls' innate genetic features. In effect, teachers are being told that biology controls learning and that social and cultural contexts, and thus their own classroom environments, cannot counter the forces of nature. Some methodological implications of studying gender as opposed to sex-group are discussed. The conclusion advocates a shift back to the study of gender as a historical, sociocultural phenomenon.