1. An idiot is one who hath no understanding from his nativity. Prerogative of the King of England, 1255 to 1290 A. D. (Payne and Patton, 1981)

 The word idiot is derived from the Latin word idiota, meaning an ignorant person and from the Greek word idiotos , meaning unfit for public life. It was used to refer to individuals with mental retardation (of all levels) well into the 20th Century.

2.  An idiot is such a person who cannot account or number, nor can tell who was his father or mother, nor how old he is, so as it may appear he has not understanding or reason of what shall be his profit or his loss. Sir Anthony Fitzherbert, about 1510 to 1520 (Payne and Patton, 1981)

Sir Anthony Fitzherbert was an English judge who was appointed the king's "serjeant-at-law "on 18 November, 1510. His work, La Graunde Abridgement, was the first systematic attempt to provide a summary of English law.

3.. Idiocy is a specific infirmity of the cranio-spinal axis, produced by deficiency of nutrition in utero and neo-nati. It incapacitates mostly the functions which give rise to the reflex, instinctive, and conscious phenomena of life; consequently, the idiot moves, feels, understands, wills, but imperfectly; does nothing, thinks of nothing, cares for nothing (extreme cases), he is legally irresponsible; isolated, without associations; a soul shut up in imperfect organs, an innocent. Edouard Seguin, France/United States, 1866 (Simpson, 1997)

Edouard Seguin was an associate of Jean Itard, whose work with Victor, the famous "Wild Boy of Aveyron," was the first documented (but unsuccessful) attempt to educate a child thought to have idiocy (mental retardation). Seguin became head teacher of a class of "idiot" children at Salpetriere, France and later started a private school for "idiots." He later moved to the United States, where in 1866 he published his famous work, Idiocy: and Its Treatment by the Physiological Method, in which the above definition appeared. In 1876, Seguin became the first president of The Association of Medical Officers of American Institutions for Idiotic and Feeble-Minded Persons , which evolved into the American Association on Mental Retardation (Smith, 2000).

4. Idiocy (involves) deficiency in the ordinary mental powers, due to disease or failure of development of the central nervous system. More especially, idiocy often relates to the severer forms of such mental deficiency , while children of subnormal capacity or backward development are spoken of as feeble-minded, or children. There is some tendency to use the term feeble-minded to include all degrees of defect from the slightest to the most severe. James Mark Baldwin, Canada, 1901 (Green, 2000).

James M. Baldwin founded the first psychological laboratory in the British Empire in 1889 at the University of Toronto (Pantalony, 2000). His most famous work, Mental Development in the Child and the Race, first published in 1895, is recognized as an important text in the history of social psychology.

5. Mental Deficiency: A state of incomplete mental development of such a kind and degree that the individual is incapable of adapting himself to the normal environment of his fellows in such a way to maintain existence independently of supervision, control or external support. Tredgold, Great Britain, 1937 (Tredgold, 1937, p.4)

A. F. Tredgold was a member of the British Eugenics Society, which like its counterpart in the United States favored reducing or eliminating reproduction by those deemed "unfit" through segregation and sterilization (See The Roots of the I.Q. Debate: Eugenics and Social Control ). In his article, "The Feebleminded - A Social Danger," which appeared in Eugenics Review (1909), Tredgold wrote, "... in the animal world or in a primitive state of human society these degenerate members would very soon be extinguished ... our modern legislation ... is actually tending to breed degenerates." (Eugenics Watch U.K).

6. Six criteria . . . have been generally considered essential to an adequate definition and concept (of mental deficiency): These are 1) social incompetence; 2) mental subnormality; 3) which has been developmentally arrested; 4) which obtains at maturity 5) is of constitutional origin; and 6) is essentially incurable. E. A. Doll, United States, 1941 (Doll, 1941, p. 215).

Edgar Doll is best known for his work at the Vineland Training School where he developed the Vineland Social Maturity Scale (now the Vineland Adaptive Behavior Scales) . First published in 1935, this instrument was designed to assess the daily living skills of individuals with mental retardation.

7. Mental retardation refers to significantly subaverage general intellectual functioning which originates during the developmental period and is associated with impairment in adaptive behavior.

Subaverage Intellectual Functioning: one or more standard deviations (SD's) below the mean. (An IQ score of 85 or below)

Adaptive Behavior: adaptation to the demands of the environment.

Developmental Period: birth to age 16
American Association on Mental Deficiency, United States, 1961 (Heber, 1961)

This definition, with its three components (subaverage intelligence, impaired adaptive behavior, and origination during the developmental period), became widely accepted. Subsequent AAMD/AAMR definitions are essentially refinements of Heber's 1961 definition.

Subaverage Intellectual Functioning: at least two standard deviations below the mean (current functioning - not permanent status). (An IQ of 70 score or below)

Adaptive Behavior: ability to meet the standards of personal independence and social responsibility expected of his/her age level and cultural group.

Dev. Period: birth to age 18.
American Association on Mental Deficiency, United States, 1973 (Grossman, 1973)

The 1973 definition, though retaining the three basic AAMR components, makes the following significant changes: 1) defining "subaverage intelligence" as meaning an IQ score of two or more standard deviations below the mean (70) rather than one standard deviation below the mean (85). This reduced the number of individuals who met this criterion from 16 % of the population to a little more than 2 % of the population; 2) clarifying the meaning of deficient adaptive behavior; and 3) extending the developmental period to age 18.

Subaverage Intellectual Functioning: two or more standard deviations below average--but a small number of persons with scores up to 10 points above the guideline ceiling may be classified as mildly mentally retarded if their adaptive behavior is significantly impaired (current functioning - not permanent status) (An IQ score of 70 - 80 or below).

Adaptive Behavior: same as 8

Dev. Period: same as 8.
American Association on Mental Deficiency, United States, 1977 Grossman, 1977)

This 1977 revision allows for individuals with borderline intelligence (70 to 80) to be classified as having mental retardation and, thus, makes them eligible for services provided to those with MR, including special education.

Mental retardation manifests before age 18.

Significantly Subaverage Intellectual Functioning: same as 9 (An IQ score of 70 - 80 or below)
American Association on Mental Retardation, United States, 1992 (AAMR, 1992)

This AAMR definition, though still retaining Heber's three components, provides even more specificity regarding the meaning of the phrase "deficits in adaptive behavior." Other changes, particularly in regard to levels of mental retardation, though not part of the definition, are included in the AAMR publication, Mental retardation: Definition, classification, and systems of supports, in which the new definition appears. Click here for an ERIC Clearinghouse article which summarizes differences between the current AAMR definition and previous ones.

11.  Mental Retardation is a disability characterized by significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social, and practical adaptive skills. This disability originates before the age of 18.

Significant Limitations in Intellectual Functioning : Same as 9.

Adaptive Behavior: “The collection of conceptual, social, and practical skills people have learned so they can function in their lives. Significant limitations . . .  impact a person’s daily life and affect the ability to respond to . . . the environment.” In order to meet this criterion, the person must perform at a level two or more standard deviations below the mean (average) in one of the three areas (conceptual, social, or practical) or have an overall score that is two or more standard deviations below average on a standardized measure of adaptive behavior.
American Association on Mental Retardation, United States, 2002 (AAMR, 2002).

This definition again tinkers with the adaptive behavior criteria and now makes limitations in adaptive behavior more consistent with those in intellectual functioning by requiring that they also be two or more standard deviations below average. See Mental Retardation: Update 2002 for further information on this definition.

12.  Intellectual Disability is a disability characterized by significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social, and practical adaptive skills. This disability originates before the age of 18. 
American Association on Intellectual and Developmental Disabilities , United States, 2002 (AAIDD, 2008).

This is the 2002 AAMR (now AAIDD) definition, reflecting the recent change in terminology. No other aspects of the definition have been changed. The term Intellectual Disability is now the preferred term for "the same population of individuals who were diagnosed previously with mental retardation" (AAIDD, 2008 )    

References:
American Association on Mental Retardation (2002). Mental retardation: Definition, classification, and systems of supports (10th ed.).
Washington, D. C.: American Association on Mental Retardation.

American Association on Mental Retardation. (1992). Mental retardation: Definition, classification, and systems of supports (9th ed.).
Washington, D. C.: American Association on Mental Retardation.

Baldwin, J. M. Dictionary of Philosophy and Psychology in Green, C. D. (2000). Classics in the history of psychology. Toronto: York University. (http://psychclassics.yorku.ca/Baldwin/Dictionary/I1defs.htm)

Doll, E. A. (1941). The essentials of an inclusive concept of mental deficiency. .American Journal of Mental Deficiency. 46, 214 - 219.

Eugenics Watch U. K. (undated). Eugenic Society Members by Surname. (http://www.africa2000.com/endx/british.htm)

Grossman, H. J. (Ed.). (1973). Manual on terminology in mental retardation (1973 rev.). Washington, D. C.: American
Association on Mental Deficiency.

Grossman, H. J. (Ed.). (1977). Manual on terminology in mental retardation (1977 rev.). Washington, D. C.: American
Association on Mental Deficiency.

Heber, R. A. (1961). A manual on terminology and classification in mental retardation (2nd ed.). Monograph Supplement to the American Journal of Mental Deficiency.

Pantalony, D. (2000), Brass Instrument Psychology. Toronto: University of Toronto (http://www.psych.utoronto.ca/museum/Welcome.html)

Payne, J. and Patton, J. (1981). Mental retardation. Columbus, Ohio: Charles E. Merrill.

Seguin, E. (1866). "Idiocy: and its treatment by the physiological method, " in Simpson, M. K. (1997). Resources on the History of Idiocy. Dundee, Scotland: University of Dundee. (http://www.personal.dundee.ac.uk/~mksimpso/SegIdio.htm).

Simpson, M. K. Resources on the History of Idiocy. Dundee, Scotland: University of Dundee. (http://www.personal.dundee.ac.uk/~mksimpso/).

Smith, B. J. (2000). An American History of Mental Retardation. (http://member.aol.com/MRandDD/introhx.htm)

Tredgold, A. F. (1937). A textbook of mental deficiency. Baltimore: Wood.