Intellectual Disabilities

Learning Objectives

  • Describe intelligence and how it is measured
  • Explain diagnostic criteria for intellectual disabilities

Understanding Intelligence

child reading on the floor

Figure 1. Children with high scores on intelligence tests tend to learn more of what is taught in school than their lower-scoring peers

Intellectual functioning, generally called intelligence, includes a wide range of mental activities such as the ability of logical reasoning and practical intelligence (problem-solving), ability in learning, verbal skills, and so on. It manifests and expresses itself through a numerous set of capabilities, behaviors, thoughts, and emotions. In other words, intellectual functioning is definable as the global ability that allows the individual to understand reality and interact with it.

Intellectual functioning is commonly measured by the intelligence quotient (IQ), which represents a total score obtained from standardized tests (IQ tests) developed for evaluating human intelligence. IQ test score has a median of 100 and a standard deviation of 15.

Intellectual Development Disorder (Intellectual Disability)

Intellectual development disorder, also called intellectual disability (ID) or a general learning disability, is a generalized neurodevelopmental disorder characterized by significantly impaired intellectual and adaptive functioning. Intellectual development disorder (intellectual disability) is defined by an IQ under 70 (two standard deviations below the median), in addition to deficits in two or more adaptive behaviors that affect everyday, general living.

Once focused almost entirely on cognition, the definition now includes both a component relating to mental functioning and one relating to an individual’s functional skills in their daily environment. As a result of this focus on the person’s abilities in practice, a person with an unusually low IQ may still not be considered to have intellectual development disorder.

Intellectual development disorder is subdivided into syndromic intellectual development disorder, in which intellectual deficits associated with other medical and behavioral signs and symptoms are present, and non-syndromic intellectual development disorder, in which intellectual deficits appear without other abnormalities. Down syndrome and fragile X syndrome are examples of syndromic intellectual development disorders.

Intellectual development disorder (intellectual disability) affects about 2%-3% of the general population, of which 75–90% of the affected people have mild intellectual disability.

Intellectually Disabled Criminals and Capital Punishment

The case of Atkins v. Virginia was a landmark case in the United States Supreme Court. On August 16, 1996, two men, Daryl Atkins and William Jones, robbed, kidnapped, and then shot and killed Eric Nesbitt, a local airman from the U.S. Air Force. A clinical psychologist evaluated Atkins and testified at the trial that Atkins had an IQ of 59. The mean IQ score is 100. The psychologist concluded that Atkins was mildly intellectually disabled. The jury found Atkins guilty, and he was sentenced to death. Atkins and his attorneys appealed to the Supreme Court. In June 2002, the Supreme Court reversed a previous decision and ruled that executions of mentally retarded criminals are “cruel and unusual punishments” prohibited by the Eighth Amendment. The court wrote in their decision:

Clinical definitions of mental retardation require not only subaverage intellectual functioning, but also significant limitations in adaptive skills. Mentally retarded persons frequently know the difference between right and wrong and are competent to stand trial. Because of their impairments, however, by definition they have diminished capacities to understand and process information, to communicate, to abstract from mistakes and learn from experience, to engage in logical reasoning, to control impulses, and to understand others’ reactions. Their deficiencies do not warrant an exemption from criminal sanctions, but diminish their personal culpability (Atkins v. Virginia, 2002, par. 5).

The court also decided that there was a state legislature consensus against the execution of the mentally retarded and that this consensus should stand for all the states. The Supreme Court ruling left it up to the states to determine their own definitions of mental retardation and intellectual disability. The definitions vary among states as to who can be executed. In the Atkins case, a jury decided that because he had many contacts with his lawyers and thus was provided with intellectual stimulation, his IQ had reportedly increased, and he was now smart enough to be executed. He was given an execution date and then received a stay of execution after it was revealed that lawyers for co-defendant, William Jones, coached Jones to “produce a testimony against Mr. Atkins that did match the evidence” (Liptak, 2008). After the revelation of this misconduct, Atkins was re-sentenced to life imprisonment. Atkins v. Virginia (2002) highlights several issues regarding society’s beliefs around intelligence. In the Atkins case, the Supreme Court decided that intellectual development disorder does affect decision-making and therefore should affect the nature of the punishment such criminals receive. Where, however, should the lines of intellectual disability be drawn? In May 2014, the Supreme Court ruled in a related case (Hall v. Florida) that IQ scores cannot be used as a final determination of a prisoner’s eligibility for the death penalty (Roberts, 2014).

Measuring Intelligence

The value of IQ testing is most evident in educational or clinical settings. Children who seem to be experiencing learning difficulties or severe behavioral problems can be tested to ascertain whether the child’s difficulties can be partly attributed to an IQ score that is significantly different from the mean for her age group. Without IQ testing—or another measure of intelligence—children and adults needing extra support might not be identified effectively. In addition, IQ testing is used in courts to determine whether a defendant has special or extenuating circumstances that preclude him from participating in some way in a trial. People also use IQ testing results to seek disability benefits from the Social Security Administration.

Bell Curves

The results of intelligence tests follow the bell curve, a graph in the general shape of a bell. When the bell curve is used in psychological testing, the graph demonstrates a normal distribution of a trait, in this case, intelligence, in the human population. Many human traits naturally follow the bell curve. For example, if you lined up all your female schoolmates according to height, it is likely that a large cluster of them would be the average height for an American woman: 5’4”–5’6”. This cluster would fall in the center of the bell curve, representing the average height for American women. There would be fewer women who stand closer to 4’11”. The same would be true for women of above-average height: those who stand closer to 5’11”.

The trick to finding a bell curve in nature is to use a large sample size. Without a large sample size, it is less likely that the bell curve will represent the wider population. A representative sample is a subset of the population that accurately represents the general population. If, for example, you measured the height of the women in your classroom only, you might not actually have a representative sample. Perhaps the women’s basketball team wanted to take this course together, and they are all in your class. Because basketball players tend to be taller than average, the women in your class may not be a good representative sample of the population of American women. But if your sample included all the women at your school, it is likely that their heights would form a natural bell curve.

A graph of a bell curve is labeled “Height of U.S. Women.” The x axis is labeled “Height” and the y axis is labeled “Frequency.” Between the heights of five feet tall and five feet and five inches tall, the frequency rises to a curved peak, then begins dropping off at the same rate until it hits five feet ten inches tall.

Figure 2. Are you of below-average, average, or above-average height?

The same principles apply to intelligence tests scores. Individuals earn a score called an intelligence quotient (IQ). Over the years, different types of IQ tests have evolved, but the way scores are interpreted remains the same. The average IQ score on an IQ test is 100. Standard deviations describe how data are dispersed in a population and give context to large data sets. The bell curve uses the standard deviation to show how all scores are dispersed from the average score (Figure 3). In modern IQ testing, one standard deviation is 15 points. So a score of 85 would be described as “one standard deviation below the mean.” How would you describe a score of 115 and a score of 70? Any IQ score that falls within one standard deviation above and below the mean (between 85 and 115) is considered average, and 82% of the population has IQ scores in this range. An IQ score of 130 or above is considered a superior level.

A graph of a bell curve is labeled “Intelligence Quotient Score.” The x axis is labeled “IQ,” and the y axis is labeled “Population.” Beginning at an IQ of 60, the population rises to a curved peak at an IQ of 100 and then drops off at the same rate ending near zero at an IQ of 140.

Figure 3. The majority of people have an IQ score between 85 and 115.

Only 2.2% of the population has an IQ score below 70 (American Psychological Association [APA], 2013). A score of 70 or below indicates significant cognitive delays, major deficits in adaptive functioning, and difficulty meeting “community standards of personal independence and social responsibility” when compared to same-aged peers (APA, 2013, p. 37). An individual in this IQ range would be considered to have an intellectual development disorder and exhibit deficits in intellectual functioning and adaptive behavior (American Association on Intellectual and Developmental Disabilities, 2013). Formerly known as mental retardation, the accepted term now is intellectual development disorder (intellectual disability), and it has four subtypes: mild, moderate, severe, and profound. The DSM lists criteria for each subgroup (APA, 2013).

Table 1. Characteristics of Cognitive Disorders
Intellectual Development Disorder (Intellectual Disability) Subtype Percentage of Intellectually Disabled Population Description
Mild 85% third- to sixth-grade skill level in reading, writing, and math; may be employed and live independently
Moderate 10% Basic reading and writing skills; functional self-care skills; requires some oversight
Severe 5% Functional self-care skills; requires oversight of daily environment and activities
Profound <1% May be able to communicate verbally or nonverbally; requires intensive oversight

Diagnostic Criteria

According to both the American Association on Intellectual and Developmental Disabilities (Intellectual Disability: Definition, Classification, and Systems of Supports (11th Edition) and the DSM-5, three criteria must be met for a diagnosis of intellectual development disorder (intellectual disability):

  • significant limitation in general mental abilities (intellectual functioning),
  • significant limitations in one or more areas of adaptive behavior across multiple environments (as measured by an adaptive behavior rating scale, i.e., communication, self-help skills, interpersonal skills, and more)
  • evidence that the limitations became apparent in childhood or adolescence.

It is formally diagnosed by an assessment of IQ and adaptive behavior. A third condition requiring onset during the developmental period is used to distinguish intellectual development disorder from other conditions, such as traumatic brain injuries and dementias (including Alzheimer’s disease).

Levels of intellectual disability vary greatly in children. Children with intellectual development disorder might have a hard time letting others know their wants and needs, and taking care of themselves. Intellectual development disorder could cause a child to learn and develop more slowly than other children of the same age. It could take longer for a child with intellectual development disorder to learn to speak, walk, dress, or eat without help, and they could have trouble learning in school.

Intellectual development disorder (intellectual disability) can be caused by a problem that starts any time before a child turns 18 years old—even before birth. It can be caused by injury, disease, or a problem in the brain. For many children, the cause of their intellectual development disorder is not known. Some of the most common known causes of intellectual development disorder—like Down syndrome, fetal alcohol syndrome, fragile X syndrome, genetic conditions, birth defects, and infections—happen before birth. Others happen while a baby is being born or soon after birth. Still other causes of intellectual development disorder do not occur until a child is older; these might include serious head injury, stroke, or certain infections.

What are some of the signs of intellectual development disorder (intellectual disability)?

Usually, the more severe the degree of intellectual development disorder (intellectual disability), the earlier the signs can be noticed. However, it might still be hard to tell how young children will be affected later in life.

There are many signs of intellectual development disorder. For example, children with intellectual development disorder (intellectual disability) may

  • sit up, crawl, or walk later than other children;
  • learn to talk later or have trouble speaking;
  • find it hard to remember things;
  • have trouble understanding social rules;
  • have trouble seeing the results of their actions; or
  • have trouble solving problems.

Dig Deeper: What’s in a Name? Mental Retardation

In the past, individuals with IQ scores below 70 and significant adaptive and social functioning delays were diagnosed with mental retardation. When this diagnosis was first named, the title held no social stigma. In time, however, the degrading word retard sprang from this diagnostic term. Retard was frequently used as a taunt, especially among young people, until the words mentally retarded and retard became an insult. As such, the DSM-5 now labels this diagnosis as intellectual development disorder (intellectual disability). Many states once had a Department of Mental Retardation to serve those diagnosed with such cognitive delays, but most have changed their name to Department of Developmental Disabilities or something similar in language. The Social Security Administration still uses the term mental retardation but is considering eliminating it from its programming (Goad, 2013). Earlier in the module, we discussed how language affects how we think. Do you think changing the title of this department has any impact on how people regard those with developmental disorders? Does a different name give people more dignity, and if so, how? Does it change the expectations for those with developmental or cognitive disorders? Why or why not?

Try It


intelligence: one’s intellectual functioning, the overall ability to gain and apply knowledge, including a wide range of mental activities such as logical reasoning, problem-solving, learning, verbal skills, etc.

intellectual development disorder (intellectual disability): a generalized neurodevelopmental disorder characterized by significantly impaired intellectual and adaptive functioning. It is defined by an IQ under 70 (two standard deviations below the median), in addition to deficits in two or more adaptive behaviors that affect everyday, general living

intelligence quotient (IQ): score on a test designed to measure intelligence

non-syndromic intellectual development disorder: intellectual deficits that appear without other abnormalities

norming: administering a test to a large population so data can be collected to reference the normal scores for a population and its groups

representative sample: subset of the population that accurately represents the general population

standard deviation: measure of variability that describes the difference between a set of scores and their mean

standardization: method of testing in which administration, scoring, and interpretation of results are consistent

syndromic intellectual development disorder: intellectual deficits associated with other medical and behavioral signs and symptoms that are present, such as in the case of those with Down syndrome or fragile X syndrome