The preventive approach in mental health remains marked by the impossibility of a fully objective appreciation of certain parameters and essential datThe theoretical reference, pointing out preferentially or more specifically one or the other of these data, can lead to conclusions, not always pertinent. The study of the rank of first born among the siblings, an element of exceptional objectivity apparently, shows, in the transitional world of the Algeria of today, the weight and limits of the cultural factors in the psychogenetical organization. It also underlines how much the individual is first of all a unique being, not reducible to statistics, the historical reconstruction of his development remaining almost undecipherable in his relations with the Unconscious.