Sur le postulat de la comparabilité dans la methode des jumeaux

Abstract

SUMMARYAmong the postulates of the twin method the Author describes especially the fundamental one of comparability. Whenever one assumes that the differences between monozigotic twins are due only to heredity (this is the basis of the twin method), one postulates in fact that environmental influences are of equal importance and act in the woy to differentiate one from the other both monozygotic and dizygotic co-twins.The comparison between the two types of twins implies the identity of environmental influences and a conception of heredity and environment as indépendant variants.As far as the psychological environment is concerned, the Author faithfully reports works on twin-condition: psychologically environment is defined according to the structure of the pair, of the duty of each co-twin the pair, and thus: a) two identical twins never hare an exactly identical environment; b) the environment of identical twins is never the same as that of non-identical twins.Concerning pre-birth and peribirth conditions, the Author reaches the same conclusions of non-comparability between monozigotic twins, dizygotic twins and control groups of non-twins: difference in lenght of pregnancy, in weight at birth, in the rate of stillbirth and new-born deaths, in the mother's age.The Author concludes not for abandoning the twin-method but for a revision of the criteria, and for a change of perspective.The failure of comparison and of mathematical analysis applied to the difference between heredity and environment is not. completely negative. It reveals the inadequacy of such criteria at least in the field of psychology, and leads to overcoming them. The psychological environment does not appear as a still picture, but rather as a situation defined by the inter-social and differential duties. Furthermore, heredity and environment are variants closely dependent on each other.