According to the latest edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV),[8] three criteria must be met for a diagnosis of mental retardation: an IQ below 70, significant limitations in two or more areas of adaptive behavior (as measured by an adaptive behavior rating scale, i.e. communication, self-help skills, interpersonal skills, and more), and evidence that the limitations became apparent before the age of 18.
It is formally diagnosed by professional assessment of intelligence and adaptive behavior.
IQ below 70
The first English-language IQ test, the Terman-Binet, was adapted from an instrument used to measure potential to achieve developed by Binet in France. Terman translated the test and employed it as a means to measure intellectual capacity based on oral language, vocabulary, numerical reasoning, memory, motor speed and analysis skills. The mean score on the currently available IQ tests is 100, with a standard deviation of 15 (WAIS/WISC-IV) or 16 (Stanford-Binet). Sub-average intelligence is generally considered to be present when an individual scores two standard deviatons below the test mean. Factors other than cognitive ability (depression, anxiety, etc.) can contribute to low IQ scores; it is important for the evaluator to rule them out prior to concluding that measured IQ is "significantly below average".
The following ranges, based on Standard Scores of intelligence tests, reflect the categories of the American Association of Mental Retardation, the Diagnostic and Statistical Manual of Mental Disorders-IV-TR, and the International Classification of Diseases-10:
| Class | IQ |
|---|---|
| Profound mental retardation | Below 20 |
| Severe mental retardation | 20–34 |
| Moderate mental retardation | 35–49 |
| Mild mental retardation | 50–69 |
| Borderline intellectual functioning | 70–84 |
Since the diagnosis is not based only on IQ scores, but must also take into consideration a person's adaptive functioning, the diagnosis is not made rigidly. It encompasses intellectual scores, adaptive functioning scores from an adaptive behavior rating scale based on descriptions of known abilities provided by someone familiar with the person, and also the observations of the assessment examiner who is able to find out directly from the person what he or she can understand, communicate, and the like.
Significant limitations in two or more areas of adaptive behavior
Adaptive behavior, or adaptive functioning, refers to the skills needed to live independently (or at the minimally acceptable level for age). To assess adaptive behavior, professionals compare the functional abilities of a child to those of other children of similar age. To measure adaptive behavior, professionals use structured interviews, with which they systematically elicit information about persons' functioning in the community from people who know them well. There are many adaptive behavior scales, and accurate assessment of the quality of someone's adaptive behavior requires clinical judgment as well. Certain skills are important to adaptive behavior, such as:
- daily living skills, such as getting dressed, using the bathroom, and feeding oneself;
- communication skills, such as understanding what is said and being able to answer;
- social skills with peers, family members, spouses, adults, and others.
Evidence that the limitations became apparent in childhood
This third condition is used to distinguish it from dementing conditions such as Alzheimer's disease or due to traumatic injuries with attendant brain damage.
http://en.wikipedia.org/wiki/Mental_retardation
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