14.1 Intellectual Disability (Intellectual Developmental Disorder)

When either Lela or Carlos calls Richie’s name, he often seems to ignore it. They can tell that he’s not deaf—he clearly notices street noises and other sounds, and he startles in response to loud noises. His cognitive abilities are noticeably less developed than Javier’s and Pia’s were when they were his age; Richie’s intellectual functioning doesn’t seem normal. Could he have intellectual developmental disorder?

What Is Intellectual Disability?

Intellectual disability A neurodevelopmental disorder characterized by cognitive abilities that are significantly below normal, along with impaired adaptive functioning in daily life; also called intellectual developmental disorder and previously referred to as mental retardation.

Intellectual disability (also called intellectual developmental disorder [IDD], and related to what was previously called mental retardation, a term that is generally no longer used) is characterized by deficits in cognitive abilities (significantly below normal), as determined both by standardized intelligence testing and by clinical assessment, along with impaired adaptive functioning in daily life. Specific deficits are in academic learning, in social understanding, and in “practical understanding” (such as knowing how to manage money) and involve a variety of cognitive abilities: reasoning and problem solving, planning, abstract thinking, judgment, and understanding complex ideas.

Although the DSM-5 criteria themselves do not specify a specific IQ score cutoff, the part of the manual that discusses the criteria in detail suggests that impaired intelligence would entail having an IQ score that is at least two standard deviations below average. On a standard IQ test where the mean is set at 100, an IQ approximately equal to or less than 70 (plus or minus 5 points) would be two standard deviations below average.

But DSM-5 is clear that a low IQ by itself is not enough to diagnose IDD; the child must also have impaired adaptive functioning in daily life as a result of the cognitive deficits. Adaptive functioning involves three domains, assessed relative to the person’s age and background:

The deficits in intellectual ability and adaptive functioning (see TABLE 14.1) must have emerged during childhood and thus cannot be the result of brain trauma in adulthood.

Table : TABLE 14.1 • DSM-5 Diagnostic Criteria for Intellectual Disability (Intellectual Developmental Disorder)
  1. Deficits in intellectual functions, such as reasoning, problem solving, planning, abstract thinking, judgment, academic learning, and learning from experience, confirmed by both clinical assessment and individualized, standardized intelligence testing.
  2. Deficits in adaptive functioning that result in failure to meet developmental and socio-cultural standards for personal independence and social responsibility. Without ongoing support, the adaptive deficits limit functioning in one or more activities of daily life, such as communication, social participation, and independent living, across multiple environments, such as home, school, work, and community.
  3. Onset of intellectual and adaptive deficits during the developmental period.
Reprinted with permission from the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, (Copyright ©2013). American Psychiatric Association. All Rights Reserved.
Like Sean Penn’s character in the movie I Am Sam, people with mild intellectual disability can, with training, learn to function independently. In this scene Penn is shown with Michelle Pfeiffer, who plays his lawyer in his fight to retain custody of his young daughter.
New Line Cinema/Courtesy Everett Collection

Although a person’s IQ score may serve as a rough guide to evaluating mental abilities, the most important criterion for determining the level of intellectual disability is the level of impaired adaptive functioning. DSM-5 specifies four levels of severity of these impairments—mild, moderate, severe, and profound—which reflect functioning in conceptual, social, and practical domains. In general, the greater the severity, the more impaired the person is likely to be:

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Information from parents and teachers may help the clinician to determine a person’s ability to function. Depending on IQ and level of adaptive functioning, someone with IDD may require supervision, ranging from minimal to constant care. Like Larry in Case 14.1, many of these people have needs and abilities that fall somewhere in the middle.

CASE 14.1 • FROM THE OUTSIDE: Intellectual Disability

Larry, a 34-year-old man with moderate [intellectual disabilities]…had been referred for complaints of seeing “monsters,” “scary faces,” and “the bogeyman.” He initially appeared paranoid and delusional, describing the feared bogeyman in detail. An assessment of possible neurochemical or physical factors that might help explain the recent onset of these symptoms yielded no significant diagnostic information….

Larry’s perception of monsters was specific to certain situations, such as being alone in a dark room….

Specifically, when asked to go alone to any dark place [including the dark stairwell in his group home, where Larry must go to carry out his assigned chore of taking down the trash], he became agitated, resisted, and made loud statements about monsters and scary faces….

Larry’s monsters could be attributed to his limited means of communicating his fear of being alone in the dark. [It appears, then, that Larry has a phobic response to the dark.]…

(Nezu et al., 1992, pp. 78, 164)

Larry had two disorders: intellectual disability and a specific phobia. because of his intellectual disability, he had difficulty explaining his fears. He received CBT for his phobia, which was successful, and he was then able to go down to the basement without fear.

Like people of normal intelligence, people with intellectual disability exhibit a wide variety of personality characteristics: some are passive or easygoing, and others are impulsive or aggressive; some people with intellectual disability, like Larry, may have difficulty communicating verbally—which can heighten aggressive or impulsive tendencies. People with intellectual disability are more likely than average to be exploited or abused by others. TABLE 14.2 lists additional facts about IDD.

Table : TABLE 14.2 • Intellectual Disability Facts at a Glance
Prevalence
  • Approximately 1% of the general population has intellectual disability; however, prevalence estimates vary depending on the age, the survey method used, and the particular population studied.
Onset
  • Severe and profound intellectual disability are generally identified at birth, although in some cases intellectual disability is caused by a medical condition later in childhood, such as head trauma.
  • Mild intellectual disability is sometimes not diagnosed until relatively late in childhood, although the onset may have been earlier.
Comorbidity
  • Compared to the general population, people with intellectual disability are three to four times more likely to have an additional psychological disorder, cerebral palsy, or epilepsy.
  • Among the most common comorbid disorders are major depressive disorder, attention-deficit/hyperactivity disorder, and autism spectrum disorder.
Course
  • The diagnosis is typically lifelong for moderate to severe intellectual disability, but beneficial environmental factors can improve adaptive functioning for those with mild intellectual disability to the point where they no longer meet all the criteria for the disorder.
  • Educational opportunities, support, and stimulation can improve the level of functioning.
Gender Differences
  • Intellectual disability occurs more frequently in males, with a male-to-female ratio of about 1.5 to 1.
Cultural Differences
  • Although the criteria for intellectual disability used in many other countries are similar to those used in the United States, they are not always the same; such differences may account for the higher prevalence rates in some other countries, such as 4.5% in France (Oakland et al., 2003).
Source: Unless otherwise noted, the source for information is American Psychiatric Association, 2013.

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CURRENT CONTROVERSY

Changing Mental Retardation to Intellectual Disability: Will Such a Switch Be Beneficial?

People with intelligence levels low enough to affect their daily life have been labeled in various ways over the decades. The newest change in the DSM-5 is to replace the diagnosis of mental retardation with that of intellectual disability. However, this is not simply a change in label; the criteria have changed: The diagnosis and its severity are no longer a function of the assessed intelligence level. (Previously the diagnosis and its severity were anchored in IQ scores of various ranges.) Rather, the diagnosis and its severity are now based on levels of adaptive functioning—how well the person can adapt and function—in three domains: conceptual, social, and practical.

On the one hand, the new name more accurately describes the disorder in that it is not simply that mental abilities are delayed (that is “retarded”). Moreover, changing the criteria to deemphasize IQ scores and emphasize the three domains of functioning is ultimately more relevant to people’s ability to function and their need for services.

On the other hand, if the name change has been made in order to reduce stigma, it could be just a matter of time before this new diagnosis becomes a stigmatizing label. The diagnosis may be more descriptive, but is the description accurate? If the emphasis is more on adaptive functioning than intellectual level, then using the diagnostic term “intellectual disability” could be confusing.

CRITICAL THINKING Do you think that the term intellectual disability will come to be as stigmatizing as mental retardation? Why or why not? Is there a better word than intellectual, given that the diagnosis focuses more on the ability to adapt and function than on intellect?

(James Foley, College of Wooster) Teratogens

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Understanding Intellectual Disability

Teratogens Substances or other stimuli that are harmful to a fetus.

Many neurological events can lead to intellectual disability, some of which reflect the fact that the fetus was exposed to certain types of substances (such as drugs or a virus) or to other stimuli (such as radiation); such harmful substances and stimuli are referred to as teratogens. Intellectual disability may also result from particular complications during labor (such as occurs when a newborn receives insufficient oxygen during birth) or from exposure to high levels of lead prenatally or during childhood.

Neurological Factors: Teratogens and Genes

One type of teratogen is environmental toxins, to which a fetus typically is exposed through the placenta after the toxin has entered the mother’s bloodstream. Examples of environmental toxins include synthetic chemicals such as methyl mercury, polychlorinated biphenyls (PCBs), and pesticides. Exposure to these toxins in the first trimester of pregnancy can disrupt early developmental processes of the central nervous system (Lanphear et al., 2005). Intellectual disability may also arise because of a variety of genetic abnormalities, listed in TABLE 14.3.

Table : TABLE 14.3 • Genetic Causes of Intellectual Disability
Cause of intellectual disability Genetic abnormality
Down syndrome Abnormality in chromosome 21
Rett’s disorder (females only) Abnormality in X chromosome (which is lethal for male fetuses)
Fragile X (the most common cause of inherited intellectual disability) Repetition of a piece of genetic code on the X chromosome that becomes progressively more severe in each generation
Prader-Willi and Angelman syndromes Deletion on chromosome 15 that has different consequences depending on which parent’s genes contribute the deletion
Phenylketonuria (PKU) A genetically based defect in an enzyme, phenylalanine hydroxylase, that leads to a failure to convert phenylalanine to tyrosine. Unconverted phenylalanine is toxic to brain cells, leading to intellectual disability, which can be prevented if PKU is identified (through a blood test at birth) and the person adheres to a diet that restricts phenylalanine.
Congenital hypothyroidism Inadequate production of thyroid hormone caused by a genetic mutation. The fetus gets thyroid hormone from the mother, but after birth, the deficiency leads to defects in the developing brain. If hypothyroidism is not detected within the first 3 months of life, the damage cannot be reversed, even with thyroid hormone replacement.
Source: For more information see the Permissions section.
The characteristic facial features associated with fetal alcohol syndrome include small eyes; a smooth philtrum—the space between the upper lip and the nose; and a thin upper lip.
Gwen Shockey/Science Source

The cognitive and behavioral deficits observed in people with intellectual disability occur because the brain does not process information appropriately, often because of abnormal brain structure. For example, fetal alcohol syndrome is a set of birth defects caused by the mother’s alcohol use during pregnancy. (Alcohol is a teratogen.) People who have this syndrome have an unusually small head size, which occurs in part because the frontal lobes are smaller than normal—specifically the portions involved in planning, carrying out tasks, and controlling impulsive behavior (Riley & McGee, 2005). All of these activities are difficult for many children with fetal alcohol syndrome.

Although severe or profound intellectual disability often involves global abnormalities, each case of mild or moderate IDD may have a unique profile of specific impaired abilities, related to the particular cause of the disability.

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Psychological Factors: Problem Behaviors

When this child gets excited, she engages in the stereotyped behavior of hand flapping. Other stereotyped behaviors exhibited by people with intellectual disability include rocking back and forth and repeatedly moving a finger.
Maria Platt-Evans/Science Source

People with intellectual disability often engage in two types of problematic behaviors that are not specifically mentioned in the DSM-5 criteria: (1) stereotyped behaviors (also referred to as stereotypies), which are repetitive behaviors that don’t serve a function, such as hand flapping, slight but fast finger and hand motions, and body rocking; and (2) self-injurious behaviors, such as hitting the head against something and hitting or biting oneself. People with intellectual disability who exhibit both stereotypic behaviors and self-injurious behaviors have greater deficits in nonverbal social skills than those with only one type of problematic behavior (Matson et al., 2006).

Stereotyped behaviors Repetitive behaviors—such as body rocking—that do not serve a function; also referred to as stereotypies.

Other problematic behaviors that often go along with intellectual disability include consistently choosing to interact with objects rather than people, inappropriately touching others, and resisting physical contact or affection.

Social Factors: Understimulation

If an infant is severely understimulated (e.g., the child is not played with enough) or is undernourished, he or she may subsequently develop intellectual disability (Dennis, 1973; Dong & Greenough, 2004; Skeels & Dye, 1939). For example, children raised in orphanages where they are essentially warehoused, ignored and neglected, may develop this disorder.

In sum, most cases of intellectual disability arise primarily from neurological factors—teratogens or genes—that produce abnormal brain structure and function, which then cause cognitive deficits. Children with intellectual disability may exhibit stereotyped or self-injurious behaviors.

Treating Intellectual Disability

Intellectual disability cannot be “cured,” but interventions can help people with IDD to function more independently in daily life. Such interventions are designed to improve specific skills and abilities, such as the person’s ability to communicate. But more than that, clinicians try to prevent intellectual disability from arising in the first place. Prevention efforts seek to avert or reduce the factors that cause intellectual disability.

Targeting Neurological Factors: Prevention

Because the key causes of intellectual disability are neurological, this type of factor is the target of prevention efforts. For example, one successful prevention effort focuses on phenylketonuria (PKU). Since the 1950s, virtually all newborns in the United States have received a test to detect whether they have PKU, which addresses a problem metabolizing the enzyme phenylalanine hydroxylase. For newborns who test positive, lifelong dietary modifications can prevent any brain damage, thereby preventing intellectual disability.

Another successful prevention effort addresses childhood exposure to lead, which can trigger brain abnormalities. Lead was banned as an ingredient in paint in 1978; laws were passed that required landlords and homeowners to inform any prospective renters or buyers of any known lead paint on the property. Beginning in the 1970s, lead was also phased out as an additive to gasoline. As a result of these measures, lead exposure—and lead-induced intellectual disability—has decreased.

There are no neurological treatments for intellectual disability, although symptoms of comorbid disorders and some self-injurious behaviors may respond to medication (Unwin & Deb, 2011).

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Targeting Psychological and Social Factors: Communication

Given the deficits and heterogeneous symptoms that accompany intellectual disability, no single symptom is the focus of all psychological and social treatments. Rather, psychological and social treatments depend on the person’s specific constellation of symptoms and comorbid disorders. In some cases, treatment targets significant communication deficits. Such treatment may teach nonvocal communication, for example, using a technique called the Picture Exchange Communication System (PECS) (Bondy & Frost, 1994). With this system, children learn to give a picture of the desired item to someone in exchange for that item.

Targeting Social Factors: Accommodation in the Classroom—It’s the Law

With the Picture Exchange Communication System, a child who has intellectual disability with poor verbal communication skills can make his or her desires known: The child presents a card with the picture of the desired object to another person, who then may give the actual object to the child.
Used with permission of Pyramid Educational Consultants, Inc./www.pecs.com

With the passage of the Americans with Disabilities Act in 1990 and the subsequent Individuals with Disabilities Education Act (IDEA) in 1997, eligible children with disabilities between the ages of 3 and 21 are guaranteed special education and related services that are individually tailored to the child’s needs, at no cost to the parents. An individualized education program (IEP) specifies educational goals as well as supplementary services or products that should be used to help the student benefit from the regular curriculum. Each child with disabilities receives a comprehensive evaluation, and the child is placed in the least restrictive environment that responds to his or her needs.

For many children, one goal of the IEP is to facilitate inclusion—placing students with disabilities in a regular classroom, with guidelines for any accommodations that the regular classroom teacher or special education teacher should make. Note that mainstreaming is not the same as inclusion; mainstreaming simply refers to placing a child with disabilities into a regular classroom, with no curricular adjustments to accommodate the disability.

Legal mandates have brought people with intellectual disability (and other disabilities) out from the shadows of institutional living into society: Depending on the severity of their retardation, they live in communities, hold jobs, and have families.

Like this girl with Down syndrome, youngsters with mild to moderate intellectual disability may be placed in regular classrooms, either as part of inclusion (in which the teacher makes specific accommodations based on the child’s special needs) or mainstreaming (in which the teacher doesn’t make specific accommodations).
Paul Conklin/Photo Edit

Is intellectual disability an appropriate diagnosis for Richie Enriquez? It may well be, but in the next section, we’ll examine a set of disorders that might better account for these problems.

Thinking Like A Clinician

Clare just graduated from college and started working in a center for adults with various intellectual disabilities. She is trying to get to know each client—his or her strengths and weaknesses. The clients with intellectual disability are classified in the moderate-to-severe range. Based on what you’ve learned, what can—and can’t—you assume about those clients? Why might people with mild or profound intellectual disability not be at the center?

Inclusion The placement of students with disabilities in a regular classroom, with guidelines for any accommodations that the regular classroom teacher or special education teacher should make.