Cognition: Thinking, Intelligence, and Language 289DIAGNOSIS Previous editions of the Diagnostic and Statistical Manual of Mental Disorders
(DSM) relied heavily on IQ tests for determining the diagnosis of mental retardation and
level of severity. Recognizing tests of IQ are less valid as one approaches the lower end
of the IQ range, and the importance of adaptive living skills in multiple life areas, lev-
els of severity are now based on the level of adaptive functioning and level of support
the individual requires (American Psychiatric Association, 2013). Thus, a Diagnostic and
Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) (American Psychiatric Asso-
ciation, 2013) diagnosis of intellectual disability is based on deficits in intellectual func-
tioning, determined by standardized tests of intelligence and clinical assessment, which
impact adaptive functioning across three domains. The domains include: conceptual
(memory, reasoning, language, reading, writing, math, and other academic skills), social
(empathy, social judgment, interpersonal communication, and other skills that impact
the ability to make and maintain friendships), and practical (self-management skills that
affect personal care, job responsibilities, school, money management, and other areas;
American Psychiatric Association, 2013). Symptoms must begin during the developmen-
tal period.
Intellectual disability can vary from mild to profound. According to the DSM-5
(American Psychiatric Association, 2013), individuals with mild intellectual disability may
not be recognized as having deficits in the conceptual domain until they reach school age,
where learning difficulties become apparent; as adults, they are likely to be fairly concrete
thinkers. In the social domain, they are at risk of being manipulated, as social judgment and
interactions are immature as compared to same-age peers. In the practical domain, they are
capable of living independently with proper supports in place but will likely require assis-
tance with more complex life skills such as health care decisions, legal issues, or raising a
family (American Psychiatric Association, 2013). This category makes up the vast majority
of those with intellectual disabilities. Other classifications in order of severity are moderate,
severe, and profound. Conceptually, individuals with profound intellectual disability have
a very limited ability to learn beyond simple matching and sorting tasks and, socially, have
very poor communication skills, although they may recognize and interact nonverbally
with well-known family members and other caretakers. In the practical domain, they may
be able to participate by watching or assisting but are likely totally dependent on others for
all areas of their care (American Psychiatric Association, 2013). All of these skill deficits are
likely compounded by multiple physical or sensory impairments.
CAUSES What causes intellectual disability? Unhealthy living conditions can affect brain
development. Examples of such conditions are lead poisoning from eating paint chips
(Lanphear et al., 2000), exposure to PCBs (Darvill et al., 2000), prenatal exposure to mercury
(Grandjean et al., 1997), as well as other toxicants (Eriksson et al., 2001; Eskenazi et al., 1999;
Schroeder, 2000). Deficits may also be attributed to factors resulting in inadequate brain
development or other health risks associated with poverty. Examples include malnutrition,
health consequences as the result of not having adequate access to health care, or lack of
mental stimulation through typical cultural and educational experiences.
Some of the biological causes of intellectual disability include Down syndrome
( to Learning Objective 8.3), fetal alcohol syndrome, and fragile X syndrome.
Fetal alcohol syndrome is a condition that results from exposing a developing embryo to
alcohol, and intelligence levels can range from below average to levels associated with
intellectual disability (Olson & Burgess, 1997). In fragile X syndrome, an individual (more
frequently a male) has a defect in a gene on the X chromosome of the 23rd pair, leading to
a deficiency in a protein needed for brain development. Depending on the severity of the
damage to this gene, symptoms of fragile X syndrome can range from mild to severe or
profound intellectual disability (Dykens et al., 1994; Valverde et al., 2007).
There are many other causes of intellectual disability (Murphy et al., 1998). Lack of
oxygen at birth, damage to the fetus in the womb from diseases, infections, or drug use
This middle-aged man, named Jack, lives
in a small town in Arkansas and serves as
a deacon in the local church. He is loved
and respected and leads what, for him, is
a full and happy life. Jack also has Down
syndrome, but he has managed to find his
place in the world.