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Committee to Evaluate the Supplemental Security Income Disability Program for Children with Mental Disorders; Board on the Health of Select Populations; Board on Children, Youth, and Families; Institute of Medicine; Division of Behavioral and Social Sciences and Education; The National Academies of Sciences, Engineering, and Medicine; Boat TF, Wu JT, editors. Mental Disorders and Disabilities Among Low-Income Children. Washington (DC): National Academies Press (US); 2015 Oct 28.

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Mental Disorders and Disabilities Among Low-Income Children.

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9Clinical Characteristics of Intellectual Disabilities


Intelligence is the general mental capacity that involves reasoning, planning, solving problems, thinking abstractly, comprehending complex ideas, learning efficiently, and learning from experience (AAIDD, 2010). Historically, intellectual disability (previously termed “mental retardation”) has been defined by significant cognitive deficits—which has been established through a standardized measure of intelligence, in particular, with an IQ score of below 70 (two standard deviations below the mean of 100 in the population)—and also by significant deficits in functional and adaptive skills. Adaptive skills involve the ability to carry out age-appropriate daily life activities. Two different systems for classifying intellectual disability (ID) used in the United States are that of the American Association on Intellectual and Developmental Disabilities (AAIDD) and the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5), which is published by the American Psychiatric Association. Both of these systems classify severity of ID according to the levels of support needed to achieve an individual's optimal personal functioning (see Table 9-1).

TABLE 9-1. Classifications of Intellectual Disability Severity.


Classifications of Intellectual Disability Severity.

DSM-5 defines intellectual disabilities as neurodevelopmental disorders that begin in childhood and are characterized by intellectual difficulties as well as difficulties in conceptual, social, and practical areas of living. The DSM-5 diagnosis of ID requires the satisfaction of three criteria:


Deficits in intellectual functioning—“reasoning, problem solving, planning, abstract thinking, judgment, academic learning, and learning from experience”—confirmed by clinical evaluation and individualized standard IQ testing (APA, 2013, p. 33);


Deficits in adaptive functioning that significantly hamper conforming to developmental and sociocultural standards for the individual's independence and ability to meet their social responsibility; and


The onset of these deficits during childhood.

The DSM-5 definition of ID encourages a more comprehensive view of the individual than was true under the fourth edition, DSM-IV. The DSM-IV definition included impairments of general mental abilities that affect how a person functions in conceptual, social, and daily life areas. DSM-5 abandoned specific IQ scores as a diagnostic criterion, although it retained the general notion of functioning two or more standard deviations below the general population. DSM-5 has placed more emphasis on adaptive functioning and the performance of usual life skills. In contrast to DSM-IV, which stipulated impairments in two or more skill areas, the DSM-5 criteria point to impairment in one or more superordinate skill domains (e.g., conceptual, social, practical) (Papazoglou et al., 2014).

Classifications of Severity

The terms “mild,” “moderate,” “severe,” and “profound” have been used to describe the severity of the condition (see Table 9-1). This approach has been helpful in that aspects of mild to moderate ID differ from severe to profound ID. The DSM-5 retains this grouping with more focus on daily skills than on specific IQ range.

Mild to Moderate Intellectual Disability

The majority of people with ID are classified as having mild intellectual disabilities. Individuals with mild ID are slower in all areas of conceptual development and social and daily living skills. These individuals can learn practical life skills, which allows them to function in ordinary life with minimal levels of support. Individuals with moderate ID can take care of themselves, travel to familiar places in their community, and learn basic skills related to safety and health. Their self-care requires moderate support.

Severe Intellectual Disability

Severe ID manifests as major delays in development, and individuals often have the ability to understand speech but otherwise have limited communication skills (Sattler, 2002). Despite being able to learn simple daily routines and to engage in simple self-care, individuals with severe ID need supervision in social settings and often need family care to live in a supervised setting such as a group home.

Profound Intellectual Disability

Persons with profound intellectual disability often have congenital syndromes (Sattler, 2002). These individuals cannot live independently, and they require close supervision and help with self-care activities. They have very limited ability to communicate and often have physical limitations. Individuals with mild to moderate disability are less likely to have associated medical conditions than those with severe or profound ID.

Evaluation of Severity

Currently AAIDD publishes a framework for evaluating the severity of ID, the Supports Intensity Scale (SIS), which focuses on the types and intensities of supports needed to enable an individual to lead a normal and independent life, rather than defining severity in terms of deficits. The SIS evaluates the support needs of an individual across 49 life activities, divided into six categories: home living, community living, life-long learning, employment, health and safety, and social activities.

DSM-5 notes that intellectual functioning reflects several different components: verbal comprehension, working memory, perceptual reasoning, quantitative reasoning, abstract thought, and cognitive efficacy (APA, 2013). Accurate measurement requires an instrument that is psychometrically valid, culturally appropriate, and individually administered. In the absence of appropriate measurement instruments, screening instruments are still able to assist in the identification individuals who need further testing. IQ test results fall along the normal (bell-shaped) curve, with an average IQ of 100, and individuals who are intellectually disabled are usually two standard deviations below the average (IQ below 70). Various issues (e.g., co-occurring communication problems, sensory or motor difficulties) can affect assessment, and psychologists must address these in considering which tests to use. IQ scores are usually reported with an associated confidence interval which indicates a range within which the “true” score is likely to fall.

A frequently used IQ measure for children in the United States is the Wechsler Intelligence Scale for Children (WISC-V). It historically measured verbal IQ, performance IQ, and full performance IQ (Wechsler et al., 2004). In its most recent edition, the WISC-V provides an overall IQ score as well as five other scores for verbal comprehension, visual spatial skills, fluid reasoning, working memory, and processing speed (Pearson Education, 2015). Because IQ scores have been constantly rising since the 1930s, standardized IQ tests have been periodically renormed based on the current population (Flynn, 1987).


Environmental factors such as exposure to toxic substances (e.g., prenatal alcohol exposure, prenatal or postnatal lead exposure), nutritional deficiencies (e.g., prenatal iodine deficiency), brain radiation, childhood brain infections, traumatic brain injury, and maternal infections (e.g., rubella, cytomegalovirus) can lead to ID. Additionally, prenatal and postnatal complications—e.g., complications of prematurity such as hypoxemia and periventricular hemorrhage—may cause brain injury resulting in ID (Gustafsson, 2003).

Genetic factors play a major role in ID. Different genetic causes may lead to ID. Down syndrome (trisomy 21) is the most common genetic cause of ID in the United States, occurring approximately once every 700 live births (Parker et al., 2010). Fragile X syndrome is the most common known inherited cause of ID, and it affects approximately 1 per 5,000 males (Coffee et al., 2009). Many cases of ID in the population are of unknown etiology.

Because of the varied causes and consequences of ID, an initial evaluation should address intellectual and life skills, the identification of genetic and nongenetic etiologies, and the diagnosis of conditions that need treatment (e.g., epilepsy and phenylketonuria). Prenatal and perinatal medical histories, a physical examination, genetic evaluations, and metabolic screening and neuroimaging assessment may aid in the determination of characteristics that may influence the course of the disorder.


ID begins in the first two decades of life. The age and characteristics of onset depend on the cause of the disability and the severity of the neuropsychiatric dysfunction. The identification of children with more severe ID (what previously would have been termed severe and profound mental retardation) typically occurs early in life. These children often have dysmorphic features and associated medical conditions and higher rates of behavioral and psychiatric disturbances. Individuals with severe intellectual disability may show delayed motor, language, and social accomplishments within the first 2 years of life. Individuals with mild intellectual disability may not be recognized until early school age because that is when their difficulties with academic learning become apparent.

Depending on its cause, ID may be stable and nonprogressive or it may worsen with time. After early childhood, the disorder is chronic and usually lasts an individual's lifetime; however, the severity of the disorder may change with age. For example, visual or hearing difficulties, epilepsy, childhood psychological or head trauma, substance abuse, and other medical conditions may affect the course of the disorder. Conversely, an early intervention may improve adaptive skills.


Males are more likely than females to be diagnosed with ID. According to the National Health Interview Survey, from 1997 to 2008 the prevalence of ID was 0.78 percent in boys and 0.63 percent in girls (Boyle et al., 2011). Overall, studies of prevalence show a male excess in the prevalence of ID, which is partially explained by x-linked causes of the disability, such as fragile X syndrome (Durkin et al., 2007).


In the United States, the prevalence of ID varies by race/ethnicity, probably due to confounding by socioeconomic status (SES). Black non-Hispanic children are approximately twice as likely, and Hispanic children approximately one and a half times as likely, to be diagnosed with ID as white non-Hispanic children (Bhasin et al., 2006; Boyle and Lary, 1996; Boyle et al., 2011; Camp et al., 1998; Van Naarden Braun et al., 2015). Language differences and poverty likely contribute to the racial and ethnic differences in performance on cognitive tests and to the corresponding disparities in prevalence. Even after taking the effects of SES into account, there is evidence that test bias and diagnostic bias affects the rates of the diagnosis of ID (Jencks and Phillips, 1998).

Socioeconomic Status

Poverty is one of the most consistent risk factors for ID (Cooper and Lackus, 1983; Durkin et al., 1998; Stein and Susser, 1963). Boyle and colleagues reported that in the United States between 1997 and 2008, the prevalence of ID among children below 200 percent of the federal poverty level (FPL) was 1.03 percent, while for those above 200 percent FPL the rate was 0.5 percent (Boyle et al., 2011). Similarly, Camp and colleagues found the prevalence of ID among children of low SES to be more than twice as high as that among middle- or high-SES children (Camp et al., 1998). The association between low SES and poverty is considerably stronger for mild than for more severe levels of ID (Drews et al., 1995; Durkin et al., 1998).


Many neurodevelopmental, psychiatric, and medical disorders co-occur with ID, especially communication disorders, learning disabilities, cerebral palsy, epilepsy, and various genetically transmitted conditions (APA, 2013). Estimates of the rates of psychiatric coexisting conditions vary. For many years there was an underestimation of the increased risk for development of comorbid conditions (“diagnostic overshadowing”). As research was conducted, it became clear that the risk for comorbid conditions is greater than previously believed. For example, Rutter and colleagues reported rates of 30 to 42 percent of psychopathology in children with “mental retardation” compared with 6 to 7 percent in children without the disability (Rutter et al., 1970). Gillberg and colleagues reported that 57 percent of subjects with mild and severe “mental retardation” met diagnostic criteria for affective, anxiety, conduct, schizophrenia, and somatoform disorders and attention deficit hyperactivity disorder (ADHD) (Gillberg et al., 1986). Most studies indicate a four- to fivefold increase in mental health problems among individuals with ID. In general, at least 25 percent of persons with ID may have significant psychiatric problems, with the population experiencing, in particular, significantly increased rates of schizophrenia, depression, and ADHD (Bouras and Holt, 2007; Fletcher et al., 2007).


The diagnosis of ID requires evidence of impairments in real life (adaptive) skills; thus all people with ID demonstrate functional impairment. These adaptive abilities relate to such things as understanding rules, the ability to navigate the tasks of daily living, and participation in family, school, and community activities. Various assessments of such skills are available, such as the Vineland Adaptive Behavior Scales which is a widely used instrument (Sparrow et al., 2005). Assessment of these skills helps to plan remediation, i.e., teaching specific skills and working on generalization of skills.


Treatments for ID generally fall into three main categories: (1) treatments that address or mitigate any underlying cause of ID, such as restricting phenylalanine in the diet of patients who have phenylketonuria; (2) treatments of comorbid physical and mental disorders with the aim of improving the patient's functioning and life skills, such as targeted pharmacologic treatments of behavioral disorders among children with fragile X syndrome (Hagerman and Polussa, 2015); and (3) early behavioral and cognitive interventions, special education, habilitation, and psychosocial supports (Szymanski and Kaplan, 2006).

Guidelines for the assessment and management of ID generally focus on psychosocial interventions. Challenges vary with the age of the individual and the level of impairment as well as with the presence of other associated conditions (Curry et al., 1997; Moeschler et al., 2006; Shevell et al., 2003; Szymanski and King, 1999). Because individuals with ID can manifest the full range of psychiatric disorders, medications can sometimes help manage these disorders (Reiss et al., 1982). However, the cognitive and verbal limitations of patients with ID make the psychiatric diagnostic process difficult. These disorders frequently respond to standard psychiatric treatment, i.e., medication and psychosocial support, although in the main ID makes treatment more complex.


  • Historically, intellectual disability has been defined by significant cognitive deficits, typically established by the testing of IQ and adaptive behaviors. There are no laboratory tests for ID; however, many specific causes and genetic factors for ID can be identified through laboratory tests.
  • Males are more likely than females to be diagnosed with ID. Poverty is a risk factor for ID, especially for mild ID.
  • The functional impairments associated with ID are generally lifelong. However, there are functional supports that may enable an individual with ID to function well and participate in society.
  • As a diagnostic category, IDs include individuals with a wide range of intellectual functional impairments and difficulties with daily life skills. The levels of severity of intellectual impairment and the need for support can vary from profound to mild.
  • Comorbidities, including behavioral disorders, are common.
  • Treatment usually consists of appropriate education and skills training, supportive environments to optimize functioning, and the targeted treatment of co-occurring psychiatric disorders.


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Copyright 2015 by the National Academy of Sciences. All rights reserved.
Bookshelf ID: NBK332877


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