The geneticist and the so-called "socio cultural" familial mental retardation.

Five hundred children and adolescents affected by mild mental retardation were studied together with their families with regard to a number of psycho-social, medical and genetic factors. Statistical studies were performed with these data and showed that this apparently homogeneous population could in fact be divided into three subgroups. The first and by far the largest group (66%) was constituted by a clinically normal population but in whom adverse social and cultural factors were dominant. In the second subgroup (20%) the mental retardation was much more pronounced (IQ of 50----65) and the socio-cultural and socio-economic levels were particularly low. Finally, organic causes (prematurity, dystocic, labour, etc.) seemed to constitute the principal factor explaining the retardation observed in the third subgroup (14%). The second subgroup comprising of 50 children and their families, were observed more closely; in this group 20 cases of known genetic diseases were found.