Stelvio, Martello and Curon, three villages of the Venosta Valley, South Tyrol (Italy), were recently included in a large genetic survey because of their isolation. This study focuses on the long-term reproductive behaviour of these villages. Family size, age at marriage and marital fertility were estimated based on a genealogy going back in the 17th century. Marriage behaviour was characterized by an elevated age at marriage and a large proportion of adults never getting married. Marital fertility was among the highest worldwide, because couples tried to use the short time at their disposal to have the largest possible number of children. Together with the already known null population expansion and high geographic endogamy rates, the reduced number of siblings who had the opportunity to get married could have favoured an increased genetic homogeneity.