MIT Encyclopedia of the Cognitive Sciences - Cryptome

530 Mental Retardation
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