Approximately 2,000,000 individuals in the United States are mentally retarded. The diagnosis as described by the DSM-IV requires an IQ of 70 or below as measured by standardized IQ testing, impairment in general functioning (adaptive functioning) in two areas, and onset before the age of 18. If onset is after age 18, this is considered Dementia.
About 80-85% of these individuals are mildly retarded, (IQ of 50-70- considered educable); about 10% are moderately retarded (IQ of 35-50- considered trainable); 3-4% are severely retarded (IQ scores of 20-35), and 1% of individuals are profoundly retarded (IQ below 20). Individuals with severe and profound retardation will undoubtedly need supportive or institutional care for life. Mentally Deficient and Intellectually Deficient are terms sometimes used in reports and documents as a substitute for Mental Retardation. These terms are interchangeable but are often chosen due to personal preference or bias.
There is an identifiable cause for retardation in about 50% of the cases. Factors are often biological, which are more likely to create moderate to profound retardation. Environmental factors, including perinatal problems, infant illness, neglect, and malnutrition all are factors that can lead to retardation in intellectual development. Moderate to profound Mental Retardation is distributed evenly across all social classes. Mild Mental Retardation appears more commonly in lower classes. One interesting point- infant developmental measures are not a good predictor of future retardation. However, they are a good predictor of exceptional intellectual development in the future. The WISC-III is not a good measure of more severe retardation because it has an inadequate floor. Tests such as the Stanford-Binet or Woodcock Johnson-Cognitive Battery are better measures on the lower end of the scale.
The following are examples of cases:
Bobby is 6 years old. His parents report that he started school this year and cannot keep up with the other kids. He is unable to identify letters, cannot yet write his own name. Interviewing him, you find that he is very difficult to understand and often uses words that make no sense. His parents are of average education level and speak well. Apparently, Bobby also has trouble tying his shoes and has trouble bathing on his own. His IQ is 68 as measured on the WISC -III in both Verbal and Performance areas (Full Scale of 67).
This child may very likely be diagnosed with Mild Mental Retardation. He meets criteria with the IQ score below 70. Further, he has communication problems and has problems in self-care skills. Of course, it is important to get a good history of childhood illnesses and injuries. This can help to clarify the diagnosis.
Tommy is 12 years old. His mother says he can’t read and even has trouble writing his own name. He is difficult to understand when you speak with him. His Full Scale IQ in the WISC-III is 69, with a verbal of 62 and a Performance IQ score of 75. His mother reports that he has some trouble with self-care skills. His mother is of a low SES; she has a 5th grade education.
This is a difficult case to make a differential diagnosis. However, several factors lend themselves to not diagnosing Mental Retardation. First, The IQ scores show a significant split (12 points on the WISC-III is considered significant). His performance IQ score is in the Borderline Range. Furthermore, his mother is of a low education level (carefully look into early stimulation, e.g. reading, talking, etc.). This child probably has some language problems or learning disabilities that affect his intellectual development and school performance.
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