The incidence of mental retardation is approximately 3% of the population. Hence, the problem of mental retardation is of considerable magnitude. Ninety out of one hundred defectives fall within the category of the morons or feebleminded with an intelligence quotient (IQ) between 75 and 50, eight to nine are imbecile (IQ between 50 to 25), and one to two are idiots (IQ below 25). A number of all the cases of mental retardation are hereditary in origin. Environmental factors are likewise important in causing mental retardation. Some of them are listed in Table 1. In some instances, both hereditary and environmental factors act conjointly. Interaction between nature and nurture may play a role in the pathogenesis of some cases of mental retardation. Mental retardation occurring after a post- or parainfectious or postvaccinal encephalopathy is classified usually as environmental in origin, though a hereditary component may be at least partly responsible
ZELLWEGER H. Genetic Aspects of Mental Retardation. Arch Intern Med. 1963;111(2):165–177. doi:10.1001/archinte.1963.03620260025006
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