This research analyses the extent to which the father in differentiating his role from that of the mother opens new aspects of the world to the child and influences the development of inter-subjectivity (Malrieu, 1986; Pinol-Douriez, 1984). A survey questionnaire was used in combination with semi-directive interviews concerned with paternal differentiation. The research population consisted of ten 9-month-old infants (5 boys and 5 girls) of differentiated fathers and ten 9-month-old infants (5 boys and 5 girls) of non-differentiated fathers. The research focused on inter-subjective processes in the young child by studying an interactive play situation with the father (5 min). The results show that children of differentiated fathers construct the outside world (human and non-human) through the opening provided by their father. Paternal differentiation exercises a favourable influence on the socio-affective development of the young child through his/her engagement in interaction with father.