31.3 Intellectual Disability
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functioning (reasoning, learning, and problem solving) and in
adaptive behavior (conceptual, social, and practical skills) that
emerges before the age of 18 years. Wide acceptance of this
definition has led to the international consensus that an assess-
ment of both social adaptation and intelligence quotient (IQ) are
necessary to determine the level of intellectual disability. Mea-
sures of adaptive function assess competency in social func-
tioning, understanding of societal norms, and performance of
everyday tasks, whereas measures of intellectual function focus
on cognitive abilities. Although individuals with a given intel-
lectual level do not all have identical levels of adaptive func-
tion, epidemiologic data suggest that prevalence of intellectual
disability is largely determined by intellectual level and a level
of adaptive function, which typically corresponds closely with
cognitive ability.
In the
Diagnostic and Statistical Manual of Mental Dis-
orders,
Fifth Edition (DSM-5), various levels of severity of
intellectual disability are determined on the basis of adaptive
functioning, not on IQ scores. This change in emphasis from
prior diagnostic manuals has been adopted by DSM-5 because
adaptive functioning determines the level of support that is
required. Furthermore, IQ scores are less valid in the lower por-
tions of the IQ range. Making a determination of severity level
of intellectual disability, according to DSM-5, includes assess-
ment of functioning in a conceptual domain (e.g., academic
skills), a social domain (e.g., relationships), and a practical
domain (e.g., personal hygiene).
Societal approaches to children with intellectual disability
have shifted significantly over time. Historically, in the mid-
1800s many children with intellectual disability were placed
in residential educational facilities based on the belief that
with sufficient intensive training, these children would be able
to return to their families and function in society at a higher
level. However, the expectation of educating these children in
order to overcome their disabilities was not realized. Gradually,
many residential programs increased in size, and eventually the
focus began to shift from intensive education to more custodial
care. Residential settings for children with intellectual disabil-
ity received their maximal use in the mid-1900s, until public
awareness of the crowded, unsanitary, and, in some cases, abu-
sive conditions sparked the movement toward “deinstitution-
alization.” An important force in the deinstitutionalization of
children with intellectual disability was the philosophy of “nor-
malization” in living situations, and “inclusion” in educational
settings. Since the late 1960s, few children with intellectual
disability have been placed in residences, and the concepts of
normalization and inclusion remain prominent among advocacy
groups and parents.
The passage of Public Law 94–142 (the Education for all
Handicapped Children Act) in 1975 mandates that the public
school system provide appropriate educational service to all
children with disabilities. The Individuals with Disabilities Act
in 1990 extended and modified the above legislation. Currently,
provision of public education for all children, including those
with disabilities, “within the least restrictive environment” is
mandated by law.
In addition to the educational system, advocacy groups,
including the Council for Exceptional Children (CEC) and
the National Association for Retarded Citizens (NARC) are
well known parental lobbying organizations for children with
intellectual disability and were instrumental in advocating for
Public Law 94–142. The American Association on Intellectual
and Developmental Disabilities, formerly known as The Ameri-
can Association on Intellectual disability (AAMR), is the most
prominent advocacy organization in this field. It has been very
influential in educating the public about, and in supporting
research and legislation relating to, intellectual disability.
The AAIDD promotes a view of intellectual disability as a
functional interaction between an individual and the environ-
ment, rather than a static designation of a person’s limitations.
Within this conceptual framework, a child or adolescent with
intellectual disability is determined to need intermittent, limited,
extensive, or pervasive “environmental support” with respect
to a specific set of adaptive function domains. These include
communication, self-care, home living, social or interpersonal
skills, use of community resources, self-direction, functional
academic skills, work, leisure, health, and safety.
The United Nations Convention on the Rights of Persons
with Disabilities (2006) has created a forum to promote the full
social inclusion of people with intellectual disability. Through
its recognition and focus on social barriers, this international
forum aims to provide protections for individuals with intellec-
tual disability, and to seek more inclusion of those with intellec-
tual disability in social, civic, and educational activities.
Nomenclature
The accurate definition of intellectual disability has been a chal-
lenge for clinicians over the centuries. All current classification
systems underscore that intellectual disability is based on more
than cognitive deficits, that is, it also includes impaired social
adaptive function. According to DSM-5, a diagnosis of intel-
lectual disability should be made only when there are deficits
in intellectual functioning and deficits in adaptive functioning
(Table 31.3-1). Once intellectual disability is recognized, the
level of severity is determined by the level of adaptive func-
tional impairment.
Classification
DSM-5 criteria for intellectual disability include significantly
subaverage general intellectual functioning associated with
concurrent impairment in adaptive behavior, manifested before
the age of 18. The diagnosis is made independent of coexisting
physical or mental disorders. Table 31.3-2 presents an overview
of developmental levels in communication, academic func-
tioning, and vocational skills expected of persons with various
degrees of intellectual disability.
If the clinician chooses to use a standardized test of intelli-
gence—which is still common practice—the term
significantly
subaverage
is defined as an IQ of approximately 70 or below
or two standard deviations below the mean for the particular
test. Adaptive functioning can be measured by using a standard-
ized scale, such as the
Vineland Adaptive Behavior Scale.
This
scale scores communications, daily living skills, socialization,
and motor skills (up to 4 years, 11 months) and generates an
adaptive behavior composite that is correlated with the expected
skills at a given age.
Approximately 85 percent of individuals who have intel-
lectual disability fall within the DSM-5 mild intellectual
