There are three components to the definition of MR/DD:
1) Significantly sub-average intellectual functioning.
2)Existing concurrently with related limitations in 2 or more adaptive skill arears: communication, home living, community use, health & safety, functional academics, self-care, social skills, self-direction, leisure, work
3)Manifesting before age 18.
The condition of MR/DD results in difficulties coping with the demands of day to day living which peers of similar age and social or cultural background are expected to deal with successfully.

What is Intellectual Functioning?

In general terms, 'intelligence' refers to: "...the ability to gather and make sense of the information that surrounds a person in the world, to act on it, and then to use it in other situations to make sense of new experiences"(Kosma & Stock, 1993; p.5)
Intellectual functioning is meaured by the individual's performance on a standardized test of intelligence (often referred to as an IQ test). The individual's score is compared to the scores of others his age in his culture. The more the individual's score differs from the average range of scores for his peers, the more significant the difference in intellectual functioning.
Measures of intelligence (IQ tests) are constructed in such a way that the average score is 100. However, thsoe who score slightly above and slightly blow 100 are also considered of average intelligence. In fact, a 15-point spread on either side of the average score of 100 is still considered to be within the average range.
Each 15 point range is called a "standard deviation". Suffice it to say, that if your IQ score is between 85-115, you are considered to be within one standard deviation of the mean and to have normal or "average" intelligence.
Scores falling an additional 15 points on either side of the average (70-85 and 115-130) are considered to be below and above average, respectively. Sometimes those falling in the lower range are reffered to as "borderline mentally retarted".
However, it is not until IQ scores differ more than 30 points (more than 2 standard deviations) on either side of the average score that a true exceptionality is considered. For those whose scores exceed 130, a label of "gifted" is often assigned. For those whose scores fall below 70, a label of "mental retardation" is often assigned
In terms of the formal (i.e., clinical) definition of Mental Retardation, then... Significantly sub-average intellectual functioning is defined as (AAMR, 1992): a measure of intellectual functioning (e.g., a score on a test of intelligence) which falls two or more standard deviations below the mean/average score. [an IQ score of 7-75 or below]

What is Adaptive Behaviour?

In general terms, adaptive behaviour, or adative "functioning" refers to the sets of skills that are necessary for independence in daily living. The formal devinition of adaptive behaviour is: the degree & efficiency with which the individual meets the standards of personal independence & social responsibility expected of his age & cultural group.
How independently a particular individual should get along at home, school and in the community are very much age-dependent. By the age of two, for instance, a child should know to avoid touching items that have been identified as "hot"; but traffic safety wouldn't be expected of this same child until he is much older.
How we 'measure' adaptive skils is usually through the use of behaviour checklists, often reffered to as "adaptive behavour scales". Reviewing the list of adaptive skill areas identified in the second part of the definition of MR/DD, you can see how extensive such scales might be. In fact, may such scales have upwards of 2000 'items' (questions/measures of specific skills). Sometimes the parent or other primary care giver completes the scale themselves. However, most times the parent or primary care giver is interviewed by a practitioner (e.g., a psychologish, psychometrist or other trained individual) who completes the scale items based on the care giver's answers to questions asked about specific skills.
The most important thing to remember is that, despite impairments in cognitive functioning, individuals are capable of on-going development in (learning of) adaptive skills, given the right kinds of training, experiences and supports. Regardless of our inborn potential, we are all capable of learning, growing and developing throughout the life span.

PAST AND CHANING DEFINITIONS OF MR/DD

There have been many different labels for the condition of MR/DD over the years. There have even been changes in terms of who "qualifies" to be given the label. Labels are a social invention: they reflect society's perceptions and values about people who are different. These perceptions and values change, and so do the labels.
The following outlines a history of how MR/DD has been understood, and how its description or definition has changed over time.

Stage One: Pre-1990's to Early 1900's

Prior to 1900's physicians described persons who "lacked reason" as "idiotic".
Later on, the term "imbecile" was used to describe persons who were less severely affected, while "idiot" conitinued to be used to describe those who were more severely affected.
It should be noted that these were medical/clinical terms (as the term "mental retardation continues to be today), and they did not have the same social meaning we apply to them today.
Labels were given by physicians according to their observatios of the person's behaviours and absense of skills, not at any particular "test" or criterion of capability.
In the early 1990's, in recognition that capabilities were centered in the brain, the terms "feeble-mindedness" and "mental deficiency" began to be used with increasing frequency.

Stage Two: Early to Mid-1900's-The Advent of Intelligence Testing

In 1904, at the request of the French government, physician Alfred Binet developed a test to identify school aged children who were above-average, and below-average in their academic abilities.
This new test led to the "discovery" (identification) of a whole new group of persons: The mildly mentally retarded. These were persons who fell below the "average" range of scores on the IQ measure, but who were above the idiot & imbecile levels. (those at the idiot and imbecile levels would not have attended school due to their vast difference from their peers) This "new" group came to be known as "morons".
And thus the first classification system for mental retardation was born, in order of least to most affected: moron, imbecile, idiot.
Morons were separated from other students and placed in different classrooms, so they would not "hold back" the educational progress of their "more intelligent" peers.
The identification of this group let to the concept of "retardation" as opposed to "deficiency".
Retardation suggests: slower (retarted) development.
Deficieny suggests: absence of debelopment and capacity.
From the mid-1900's the term "mental retardation" gained in popularity.

Stage Three: Mid 1900's to Early 1990's AAMD/AAMR Definitions

Definitions of MR/DD began to reflect a shifting emphasis from an exclusive reliance on intellectual measures (the IQ test) to the consideration of social and personal competence. This was b/c some professionals observed that some peole with lower than average intelligence still adjusted well to their environments and were able to manage quite well in day to day life. Why then, would they be considered to be "retarded"?
As a result of this recognition of the importance of 'adaptive behaviour', the American Association on Mental Deficiency (The AAMD) altered their definition of mental retardation. Now, along with their criteria of "sub-average intellectual functioning", the individual labeled was also required to show "impaired adaptive behaviour" before a label would be given.
It was around this time that there were changes in the terms used to describe this population and their different classification levels.
Mildly Mentally Retarted (IQ 50/55 - 70)
Moderately Mentally Retarted (IQ 30/35 - 50/55)
Severly Mentally Retarded (IQ 20 - 30/35)
Then, in 1959, the AAMD proposed a new classification system which added two new categories.
1) There was the identification of the profoundly mentally retarded (IQ below 20). This had been a long-neglected group of persons who populated the back wards of the early institutions.
As a result of their identification, they became the subject of much research. This research demonstrated that these individuals had the ability to learn- (this had previously not been thought possible!)
2)There was the identification of a new group based on a higher IQ cut-off point. This group was designated as borderline mentally retarded. (IQ 70-85)
This single change vastly increased the number of individuals in the world who could now be considered mentally retarded! (Especially as, during this time, despite the AAMD requirement of deficits in adaptive behaviour before labeling, there was a persistent over-reliance on IQ measures alone as the primary measure for diagnosing retardation).
It wasn't until 1973, fourteen years later, that the AAMD rescinded this last classification, lowering the cut-off point for mental retardation from one (IQ 85) to two (IQ 70) standard deviations below the average score on a test of intelligence. Consequently, millions of citizens were no longer considered to be retarded!!

Stage Four: 1992 - To Present- Classifying Needs Instead of People

In 1992, the AAMR (formerly the AAMD) made another revision to their system of diagnosing and classifying persons with MR/DD. The new system has shifted diagnosis (labeling) away from estimating the person's level of deficiency (mild, moderate, severe, profound) to describing instead the level of supports required by the person.
This new system recognized that the primary reason for diagnosis (i.e. labeling) should be the provision of intervention (i.e. assistance or support)
Today the key question is no longer "at what level does this person function?", but what does this person need, in the way of assistance, services, opportunities, etc., which can allow him or her to become more independent, more productive & part of his/her own community?
To this end, the current AAMR (1992) classification system defines the Intensities of Supports (rather than the level of 'functioning'), and describes them as follows:
Intermittent Support Needs -Only short-term supports are required, on an as-needed basis; the person doesn't always require supports, may only need them episodically.
Limited Support Needs -Intensity of supports consistent over time, but may be time limited (e.g., employment training); while not intermittent, are less costly & require fewer staff hours than more intensive supports.
Extensive Support Needs -Regular (i.e. daily) supports are provided in at least some environments. Supports are on-going.
Pervasive Support Needs -Constant, high intensity supports provided across all environments. May be of a life-sustaining nature. Involve more staff hours/cost than do extensive supports.

What are Supports?

Supports have two components:
a) Sources-where they come from.
b) Functions-what they do/provide.
1) Sources of Supports:
Within the Person -Skills, abilities, talents, achievements and personal resources (possessions, finances) which could be used to enhance their own development & quality of life