In public debates biology is often used to assert essential sex differences. Yet, evolutionary biologists themselves disagree over sex differences and their causes. This project aims at understanding how and why this controversy over sex differences emerged. The last decades of biological research have revealed an extensive variability in sex and “sex roles” among animals. In the wake of these findings, perceptions of females and males have shifted. However, interpretations of this variability diverge between those biologists that consider the sexes as fundamentally different, emphasizing different sizes of egg and sperm as the basis for male and female patterns, and other biologists that highlight the variability of the sexes due to social and environmental influences, hence the dynamics of sex differences. How and why do the researchers’ ontological understandings of sex differences vary? In this three-year project, I will combine narrative analysis and methods from controversy studies to analyse scientific publications, scientific debates and interviews. This analysis will explore explicit and implicit presuppositions about sex differences and examine the controversy in a wider social and scientific context. This project will illuminate the interaction between society and the scientific process of evolutionary research at a hitherto unexamined central controversy, thereby contributing significantly to the international forefront of feminist science studies.