the first studies on early parent-child links focused mainly on the child-parent relationship, the mother being considered to be the main source of influence for the baby. However, in order to understand the social context in which the child operates, it is necessary to go beyond the study of dyadic relationships and to consider the importance of triadic relationships. Our longitudinal study based on a three-stage research programme (pregnancy, 12 months postpartum and 18 months postpartum) aimed to determine the process of building the family alliance (i.e. family engagement and cooperation) as assessed by the Lausanne Trilogique Jeu. A model with individual parental characteristics (i.e. personality traits and attachment patterns) as distal variables, co-parenthood as mediator, temperament of the child as moderator and family alliance as output variable was tested using structural equations (N = 62 families). The results showed that the conscientious nature of the parents was positively and indirectly (through co-parenthood) associated with the family alliance, while the parent’s escaping attachment and the father’s anxiety attachment were negatively and indirectly (via co-parenthood) associated with the family alliance. The discussion stressed the importance of the characteristics of the father and mother in early family interactions and of co-parenthood as a central mechanism in the development of the family alliance.