ABSTRACT
In modern history, severe mental retardation has been attributed to central nervous system pathology, and mild mental retardation to environmental factors or to an interaction between genetic and environmental factors (Zigler, 1967, 1978). In the 1950s, epidemiological studies identified a high frequency of perinatal complications in the birth records of mentally retarded children; the more severe retardation was associated with the most complications, the less severe with fewer and less pathogenic complications. This research led to the hypothesis of a continuum of reproductive casualty (Lilienfeld & Pasamanick, 1955; Pasamanick & Lilienfeld, 1955).
