In 1959, the American Association on Mental Retardation (then called the American Association on Mental Deficiency) adopted this definition of mental retardation:
Mental retardation refers to sub-average general intellectual functioning which originates during the developmental period and is associated with impairment in adaptive behavior." (Heber, 1959, cited in J.D. Smith 381)
The AAMD Manual on Terminology and Classification in 1959 "suggested a three-level division of those with an IQ below 50 into moderately, severely, and profoundly retarded. This classification replaced a similar hierarchy from 1910 based on the "mental age" of the individual – idiots, imbeciles, and morons.