There is a high incidence of psychiatric disorders in mentally retarded subjects: one third to two thirds of mentally retarded subjects exhibit psychiatric disorders, a proportion which is much higher than that found in subjects with normal intelligence. The issue is to clarify the nature of the relationship between cognitive and psychiatric disorders (generally analyzed in a dichotomous approach). A way to analyze the phenomenon is to consider a psychopathological approach, which can define the underlying mechanisms responsible for this incidence. The aim of this paper is to analyze the explicatory value of deficient cognitive development, as the main factor determining a specific personality organization. Direct and indirect effects of cognitive impairment on the development of personality disorders are described: the first, in terms of how cognitive deficit (i.e. severity, homogeneity in several cognitive domains, pattern of development) disorganizes personality; the second, in terms of impact that cognitive deficit could have on the child's relationship with the external world, especially with the mother. In order to illustrate these viewpoint, the paper discusses the role of cognitive functions in the development of personality. Specifically, the way the normal child processes his perceptual and motor experiences is analyzed, that is pursuit of new causal links in his knowledge seeking activity of mastering the world. The child's primitive relationship with the world is then aimed at learning, exploring and searching for new causal links. In the light of these considerations, what the child with Mental Retardation experiences is discussed. A series of psychopathological mechanisms in Mental Retardation are postulated. The organization of the Mentally Retarded child's internal world is described, as reflected in Rorschach protocols, which outline a chaotic and primitive internal world, but with a specificity of its own. Finally, the paper discusses the hampering effect that cognitive impairment has on the quality of the relationship with the caregiver. This effect can be seen in terms of the child's interactive capacity and, at the same time, in terms of the emotional impact on the caregiver that derives from interacting with a mentally retarded child. From the above considerations a global approach to the psychopathology of cognitive and affective aspects of Mental Retardation seems warranted. Both aspects acquire a specific significance when seen in light of a specific personality organization. Defining the characteristic of this specific organization seems to be the key to a more comprehensive approach to psychiatric disorders of Mental Retardation.