This article explores how the Portuguese legal system’s efforts to determine paternity of children born outside legal marriage, automatically initiated by the Registry Office when a birth registration does not indicate the father, reveal cultural models which reinforce the naturalisation of the differences between mothers and fathers, with significant effects on the social construction of parental roles and on expectations of family organisation and female sexual behaviour. The article relies on ethnographic data drawn from direct observation of court proceedings for (...) the determination of paternity, as well as interviews with judges and prosecuting counsels all over the country. It is argued that judicial practices in the specific context of courtroom investigations of paternity reinforce gender inequalities in two interrelated ways. On the one hand, they are strengthened in the discursive practices performed during the course of the interactions between judges, prosecuting counsels and the mother of the child, as well as the alleged father. On the other hand, the normative model of family life and the dominant ideology of women’s and men’s relationships, which emphasise women’s socially subordinate position, are revealed by the selective use of DNA testing in paternity cases, based on the judge’s evaluation of the mother’s sexual behaviour. The article argues that legal attempts to establish the paternity of children born outside marriage—though based on novel technical and supposedly objective procedures—tend, nevertheless, to reproduce the prevailing patriarchal structures. (shrink)