A fundamental distinction has to be made between intellectual impairment starting in early childhood (learning disability or mental retardation) and intellectual impairment developing later in life (dementia).
In 1845, Esquirol made this distinction when he wrote:
Idiocy is not a disease, but a condition in which the intellectual faculties are never manifested; or have never been developed sufficiently to enable the idiot to acquire such an amount of knowledge as persons of his own age and placed in similar circumstances with himself are capable of receiving.
Early in the twentieth century, Binet's tests of intelligence provided quantitative criteria for ascertaining the condition. These tests also made it possible to identify lesser degrees of the condition that might not be obvious otherwise (Binet and Simon 1905). Unfortunately, it was widely assumed that people with such lesser degrees of intellectual impairment were socially incompetent and required institutional care (Corbett 1978).
Similar views were reflected in the legislation of the time. For example, in England and Wales the Idiots Act of 1886 made a simple distinction between idiocy (more severe) and imbecility (less severe). In 1913, the Mental Deficiency Act added a third category for people who 'from an early age display some permanent mental defect coupled with strong vicious or criminal propensities in which punishment has had little or no effect'. As a result of this legislation, people of normal or near normal intelligence were admitted to hospital for long periods simply because their behavior offended against the values of society. Although some of these people had committed crimes, others had not, for example, girls whose repeated illegitimate pregnancies were interpreted as a sign of the 'criminal' propensities mentioned in the Act.
Although, in the past, the use of social criteria clearly led to abuse, it is unsatisfactory to define mental retardation in terms of intelligence alone. Social criteria must be included, since a distinction must be made between people who can lead a normal or near-normal life and those who cannot. A useful modern definition is the one of the American Association for Mental Deficiency (AAMD), which defines mental retardation (the term preferred in the USA ) as 'sub-average general intellectual functioning which originated during the development period and is associated with impairment in adaptive behavior' (Heber 1981).
DSM-IV defines mental retardation as a 'significantly sub-average general intellectual functioning, that is accompanied by significant limitations in adaptive functioning in at least two of the following skill areas: communication, self-care, home living, social/interpersonal skills used for community resources, self direction, functional academic skills, work, leisure, health and safety' and having an onset before the age of 18.