Intellectual Disabilities

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.

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.

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).

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.