The nosology of mental retardation: including the report of a survey of 1378 mentally retarded individuals at the Walter E. Fernald State School.

The mentally retarded here are divided into two groups. The first group consists of the mildly retarded (IQ 50-70) whose deficit, for the most part is a reflection of the expected variability of human intelligence and of adverse sociocultural factors. The second and smaller group includes the moderately, severely and profoundly retarded (IQ smaller than 50), most of whom have brain damage. A new clinical survey indicates that acquired brain disease is the most common cause (26 percent), followed by chromosome disorders (18.7 percent).