Seguin: "The capacity of the mind for development was equal in all newborn, but... the nerves which transmit sensory messages to the brain were deficient or inefficient in some individuals, thus preventing the experiences from being effectively transmitted" (1907; as cited by Kolstoe, 1970, p. 13)

Tredgold: "Mental deficiency is 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 as to maintain existence independently of supervision, control or external support" (1937, p. 4).

Crane, writing on mental retardation, stated: "The fundamental condition here is failure of normal development of the neural cerebral cortex. This failure is best indicated by a standardized series of tests which measure the capacity for doing certain tasks of varying degrees of difficulty which in turn have been devised to represent the cortical capacity to function in basal conditions" (1939, p. 181).

Doll: "Mental deficiency is a state of social incompetence obtained at maturity, resulting from developmental arrest of intelligence because of constitutional (hereditary or acquired) origin: the condition is essentially incurable through treatment and unremediable through training and training instill habits which superficially compensate for the limitations of the person so afflicted while under favorable circumstances and for more or less limited periods of time" (1941, pp. 163-164).

Jervis: "Mental deficiency may be defined, from a medical point of view, as a condition of arrested or incomplete mental development induced by disease or injury before adolescence or arising from genetic causes" (1952, p. 175).

Porteus and Corbett: "Feebleminded persons are those who by reason of permanently retarded or arrested mental development existing from an early age are incapable of independent self-management and self-support" (1953, pp. 103-104).

Royal Medico-Psychological Association: "This 'condition of arrested or incomplete development of mind' may, however, be manifested in very varied ways. A usual manifestation is failure to develop what is commonly known as intelligence-functions which can be measured by psychometric methods and assessed under such terms as 'mental age' or 'intelligence quotient'; but this is by no means invariable, and in other cases the undeveloped mind may be manifested chiefly by failure to attain normal control of the emotions or to achieve the qualities needed for normal social behavior" (in Clarke & Clarke, 1975, p. 20).

Sarason: "[Mental retardation] refers to individuals who, for temporary or long standing reasons, function intellectually below the average of their peer groups but whose social adequacy is not in question or, if it is in question, there is the likelihood that the individual can learn to function independently and adequately in the community" (in Cruickshank, 1955, p. 440).

"[Mental deficiency] refers to individuals who are socially inadequate as a result of an intellectual defect which is a reflection of an impairment of the nervous system which is essentially incurable" (1955, p. 442).

Benoit: "Mental retardation may be viewed as a deficit of intellectual function resulting from varied intrapersonal and/or extra personal determinants, but having as a common proximate cause a diminished efficiency of the nervous system, thus entailing a lessened general capacity for growth in perceptual and conceptual integration and consequently in environmental adjustment" (1956, p. 56).

Great Britain Mental Health Act: Severe sub-normality indicates "a state of arrested or incomplete development of mind which includes subnormality of intelligence and is of such a nature and degree that the patient is incapable of leading an independent life or of guarding himself against serious exploitation, or will be so incapable when of an age to do so" (1959, sec. 4, par. 2). Subnormality refers to a condition "which includes sub-normality of intelligence and is of a nature or degree which requires or is susceptible to medical treatment or other special care or training of the patient" (1959, sec. 4, par. 3)

Clinical mental retardation: Mental retardation or deficiency is a condition in which we find the individual's mental processes are below the accepted normal as a result of arrested or incomplete mental development arising from genetic. When this condition develops after adolescence, it is more commonly known as "dementia." (From Carter, 1965, p. ix).

Bijou: "A retarded individual is one who has a limited repertory shaped by events that constitute his history" (1966, p. 2, italics in original).

Kolstoe: Kolstoe considers retardation to be "a condition of intellectual arrest at some level below Piaget's level of formal thought" (1972, p. 23). (Basing his ideas on Piaget's notions of intelligence and human development, Kolstoe lists several advantages of this approach, most notably an emphasis on the thinking processes possessed by the individual.)

Ehlers, Krishef, and Prothero: "Mental retardation is a permanent condition of impaired or incomplete mental functioning. It usually becomes evident at birth or at an early age, but by accepted definition, it must originate at some time during the developmental period. This means that in order to be classified as retarded, an individual must have acquired the condition at some time prior to his seventeenth birthday" (1973, p. 1).

Mercer: A clinical perspective regards mental retardation as an attribute of the individual, assuming "that the condition exists as an entity regardless of whether the person is aware of its presence of whether others recognize his pathology... A case of mental retardation can exist undiagnosed in much the same sense that a case of rheumatic fever can exist undiagnosed and unrecognized" (1973, p. 7).

Source: Neisworth, J.T. & Smith, R. M. (1978.) Figure 4-7, Chronological Listing of Definitions. In Retardation: Issues, Assessments, and Intervention (pp. 66-69). New York: