Copyright © 1999 Massachusetts Institute of Technology
Definitions of mental retardation characteristically include three criteria: (1) significantly subaverage INTELLIGENCE accompanied by (2) significant limitations in adaptive skills with (3) an onset during the developmental period. Thus, according to the DSM-IV (American Psychiatric Association 1994), individuals are considered to have mental retardation if (1) they have a current IQ, based on an individually administered test, of approximately two or more standard deviations below the mean (e.g., ~70); (2) they have significant limitations (relative to those expected for chronological age and sociocultural background) in two or more of the following domains: communication, social/interpersonal skills, self-care, home living, self-direction, leisure, functional academic skills, use of community resources, work, health, and safety; and (3) these difficulties were first evidenced prior to age 18 years. Each of the other major organizations involved in the treatment of individuals with mental retardation -- the American Association on Mental Retardation (AAMR), the American Psychological Association (APA), and the World Health Organization (WHO) -- accepts the same three criteria, but implements them in slightly different ways (see Luckasson et al. 1992; American Psychological Association 1996; World Health Organization 1996). The DSM-IV, APA, and WHO ICD-10 definitions further divide mental retardation into four levels: mild (IQ between 50-55 and approximately 70); moderate (IQ between 35 - 40 and 50 - 55); severe (IQ between 20 - 25 and 35 - 40); and profound (IQ below 20). The AAMR definition also recognizes four levels of mental retardation, based on the intensity of support needed to enhance independence, productivity, and community integration: intermittent, limited, extensive, and pervasive.
Epidemiological research indicates a prevalence of mental retardation at between 0.8 and 1.2 percent. Per 1,000 individuals, approximately 3-6 have mild mental retardation, 2 have moderate mental retardation, 1.3 have severe mental retardation, and 0.4 have profound mental retardation. The cause of mental retardation is known for approximately 33 - 52 percent of individuals with IQs between 50 and 69: chromosomal: 4 - 8 percent; prenatal (multifactorial or environmental): 11 - 23 percent; perinatal or postnatal: 21 percent. For individuals with IQs below 50, the cause of mental retardation is known for 60 - 75 percent: chromosomal: 20 - 40 percent; prenatal: 20 - 30 percent; perinatal or postnatal: less than 20 percent (Pulsifer 1996). The most common known causes of mental retardation are fetal alcohol syndrome, Down's syndrome, and fragile X syndrome. Individuals with mental retardation often have additional disabilities (Batshaw and Shapiro 1997). Seizure disorders and cerebral palsy are present in about 10 percent of individuals with mild mental retardation and more than 20 percent of individuals with severe mental retardation. Individuals with mental retardation are three or four times more likely than the general population to have a psychiatric disorder (Kymissis and Leven 1994), with the likelihood of comorbid psychiatric disorder increasing as a function of severity of mental retardation (mild mental retardation: 25 percent, severe: 50 percent). Sensory impairments are also frequent (mild mental retardation: 24 percent, severe: 55 percent).
Historically, psychologists and educators have focused on level of mental retardation rather than cause (etiology), both for research purposes and for educational intervention (Goodman 1990; Hodapp 1997). Extensive characterizations are provided in the Manual of Diagnosis and Professional Practice in Mental Retardation (American Psychological Association 1996). Individuals with mild mental retardation are expected to attain a mental age of between 8 and 12 years. Many individuals acquire fluent language by adolescence; READING and arithmetic skills are usually between the first and sixth grade levels. Independence in both employment and daily living is typically attained. Individuals with moderate mental retardation are expected to attain a mental age of between 6 and 8 years, to acquire functional language abilities, but not functional reading or arithmetic skills, and to require supervision during adulthood. Individuals with severe mental retardation are expected to attain a mental age of between 4 and 6 years, although language abilities will be at a lower level. During adulthood, assistance is required for self-care skills. Individuals with profound mental retardation attain a mental age of between birth and 4 years. Many of these individuals are medically fragile, with a very high early mortality rate. During adulthood, some individuals will be able to walk and to produce single words. Pervasive supervision is required throughout the life span.
More recently, some researchers have begun to emphasize the importance of etiology. There are more than 500 known genetic causes of mental retardation (Flint and Wilkie 1996), in addition to a wide range of teratogenic causes (e.g., prenatal alcohol exposure). Each of these may be expected to affect brain structure and function. The particular areas and functions impacted will vary due to differences in which genes are affected and the roles the particular genes play in development, or which aspects of the brain were developing most rapidly at the time of exposure to a particular teratogen. It is likely that some aspects of cognition will be more severely impacted than others, and that areas of severe impact will vary from syndrome to syndrome. If so, the overall mental age attributed to a given individual (which is used to indicate level of mental retardation) may not accurately reflect his or her abilities in specific domains. For example, consider Williams syndrome, which is caused by a hemizygous microdeletion of chromosome 7q11.23, encompassing at least fifteen genes. Full-scale IQs range from less than 40 to about 90, with a mean of 55-60. Despite the fact that this mean IQ is in the range of mild mental retardation, individuals with Williams syndrome evidence a wide range of ability levels, as a function of domain. Their auditory rote memory ability is typically within the normal range (greater than second percentile), and about half have vocabulary or grammatical abilities, or both, within the normal range. In contrast, levels of visual-spatial constructive ability typically fall within the moderate to severe range of mental retardation (Mervis et al. forthcoming; see also Bellugi, Wang, and Jernigan 1994). In addition, unlike the typical characterization of individuals with mild mental retardation, most individuals with Williams syndrome evidence additional psychopathology: attention deficit hyperactivity disorder, anxiety disorder, or both (e.g., Dykens and Hodapp 1997). Perhaps because of these problems, individuals with Williams syndrome also differ from the typical characterization of individuals with mild mental retardation, by seldom being able to live independently.
As evidenced by the example of Williams syndrome, summary test scores often do not represent well the level of ability within individual domains. Thus it is crucial to take into account etiology when planning either basic research or intervention. At the same time, it is important to remember that there is within-syndrome variability, both in overall IQ and fit to the behavioral phenotype associated with the syndrome. Explication of both within- and between-syndrome variability depends on coordination of research efforts among researchers studying cognition, personality, brain structure and development, and genetics. Such interdisciplinary efforts should lead to a deeper understanding of basic processes and their relation to intelligence and adaptive functioning, whether the goal is to explain more fully a specific etiology or to elucidate processes relevant to mental retardation as a whole.
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Copyright © 1999 Massachusetts Institute of Technology