When a couple is formed, the kin groups of each spouse are brought together. The couple’s relations with these two sets of kin are generally based on a tacit principle of equality, with neither family line taking preference. British and American urban surveys in the mid 1950s began to cast doubt upon this principle of equality. In everyday exchanges and in the practical routines of sociability, couples tend to show preference for one or other kin group, most often that of the woman. No recent research, in France at least, has examined this tendency, known as matrilaterality, to favour the woman’s family. Using the results of the Kinship networks and support (RPE) section of the INSEE permanent survey on living conditions of households (PCV) conducted in October 1997, this study assesses the scale of this matrilateral tendency from the viewpoint of exchanges within kin groups and suggests hypotheses to explain its origins and mechanisms.