Chapter 4: Intellectual Disability

Definition

With the signing of Rosa’s Law on October 5, 2010, the term mental retardation was replaced with intellectual disability in all federal policy documents. A few years later, The Diagnostic and Statistical Manual of Mental Disorders: DSM-5, used by health care professionals, and the International Classification of Diseases (ICD-11), used by the World Health Organization, also adopted the term intellectual disability in place of mental retardation (Harris, 2013). An intellectual disability is often present at birth and is characterized by two main components, a significant limitation in cognitive ability and an impairment in adaptive skills, which are the skills needed for practical everyday functioning.

The American Association of Intellectual and Developmental Disabilities (AAIDD) provides this definition, “Intellectual disability is a disability characterized by significant limitations in both intellectual functioning and in adaptive behavior, which covers many everyday social and practical skills. This disability originates before the age of 22.” There has been some debate over the years as to what constitutes a “significant limitation” in intellectual functioning. Originally, this was classified as an Intelligence Quotient (IQ) score that is at least 2 standard deviations below the mean, but in 1959 the IQ guideline for intellectual disability was raised to 1 standard deviation below the mean. It was at this time that the additional criterion of a measurable impairment in adaptive behavior was added. In 1973, the IQ cutoff was again changed back to 2 standard deviations below the mean; however, the benchmark of a measurable impairment in adaptive behavior was retained and given greater attention. In 1983, some flexibility around IQ was built into the definition, with professional judgement of the evaluator being added as an acceptable consideration for persons with an IQ between 70-75. In 1992, 2002, and 2010, the definition was again updated to focus more on present functioning in the community and to further refine adaptive behavior (NASET, 2024).

Assessing Intellectual Functioning

Cognitive ability is often calculated using tests of general intelligence. These tests assess thinking and reasoning abilities using measures of skills such as perception, memory, and problem solving. Specific subtests may include assessments of math computation, verbal reasoning, reading comprehension, working memory, and spatial relationships (OPM, n.d.). Many assessments of general intelligence have a mean score of 100 with a standard deviation of 15. This means that, given a typical population with a normal distribution of scores, it would be expected that 50% of the population would have an IQ above 100 and 50% would have an IQ below 100. Further, based on the standard deviation of 15, it would be expected that 68% of the population would have an IQ that falls between 85-115, with 95% of all the IQ scores falling between 70-130. Thus, given the IQ guideline for intellectual disability of 2 standard deviations below the mean, a person with an IQ below 70 would be considered intellectually disabled if an impairment in adaptive skills was also present. Raising the IQ guideline to 1 standard deviation below the mean, as was done in 1959, could potential increase the number students qualifying as intellectually disabled from 2.5% to 16%, depending on the concomitant impairment in adaptive skills.

Image: Kemp, J. (2005, December 19). Normal distribution and scales. Wikimedia Commons. Public domain.

While intellectual functioning is generally measured using an IQ test, this is not without controversy. Intelligence is an abstract and complex concept. It cannot be measured directly and must be derived from performance on various cognitive tasks. There have also been concerns about the potential for bias on IQ tests. In the 1970 case of Diana v. State Board of Education, it was determined that students could not be placed in special education classes solely on the basis of an IQ test, which the courts recognized as being potentially culturally biased. The 1972 Larry P. v. Riles case similarly upheld that students could not be classified as having mental retardation based on an IQ test that could be culturally biased. One outcome of this case was that the court tasked the school officials with developing a nondiscriminatory assessment to be used for identifying students with mental retardation. Unfortunately, they failed to create such a measure. Interestingly, this failure led to the 1979 ruling in the state of California which prohibits the use of IQ testing for identifying African American children as intellectually disabled. It should be noted, though, that this ruling only applies to students in the state of California.

In addition to the potential for bias, there are also concerns about the stability of IQ scores. An IQ test only provides a picture of a person’s performance at a particular moment in time under particular conditions. A person’s IQ is not static. It can change and, in some cases, can change quite substantially. The inclusion of measures of adaptive behavior helps with this concern, although some feel that educators still put too much emphasis on the IQ score and not enough on other factors. This has led some parents to refuse IQ testing for their children. While IQ tests are fairly accurate at predicting academic success, there are other sources of information that could also be considered such as work samples, classroom observations, and interviews with parents and teachers. The goal in collecting information from a variety of sources is to provide the most accurate picture possible of the student’s strengths and challenges so that the appropriate supports and services can be identified.

Assessing Adaptive Skills

In addition to a significant limitation in intellectual functioning, an intellectual disability is also characterized by an impairment in adaptive behavior. Adaptive skills are those skills that are necessary to meet the requirements of everyday living. Adaptive skills are an indication of social ability and level of independence. Measures of adaptive behavior should take into account the age of the child and the community norms that may inform their behavior. Depending on the age of the child, measures of adaptive behavior can include assessments of communication, self-care, social skills, independent living, vocational skills, and leisure skills. Assessment of adaptive skills is usually completed through a combination of observations, interviews with teachers and caregivers, and questionnaires. Finally, by definition, an intellectual disability must manifest itself before age 22, meaning it is evident during the developmental period of a child’s life. It is important to note that while persons with an intellectual disability will learn more slowly than their peers, they will continue to learn throughout their lifetime.

Prevalence

According to the American Psychiatric Association (2024), approximately one percent of the population meets the requirements to be identified as having an intellectual disability. Of those, the vast majority, about 85%, have a mild intellectual disability, which is characterized in part by an IQ between 55-69. Historically, more males than females have been diagnosed with an intellectual disability. The table below shows classifications of the severity and prevalence of intellectual disabilities.

Image credit: National Academies of Sciences, Engineering, and Medicine. 2015. Mental Disorders and Disabilities Among Low-Income Children. https://doi.org/10.17226/21780. Reproduced with permission from the National Academy of Sciences, Courtesy of the National Academies Press, Washington, D.C.

Subcategories of Intellectual Disability

The category of Intellectual Disability is further divided into levels based on the type and intensity of the supports a person needs to participate in typical daily activities. These supports can be provided by parents, siblings, friends, and teachers, as well as by paid support staff. The four levels of support are intermittent, limited, extensive, and pervasive. The intermittent level of support is the least intensive level and applies to those who may need occasional assistance, but do not need help on a daily basis. An example is a person who might need support in finding a job or for making the move from living in the family home to living in an independent setting. The second level of support is limited support. Persons needing limited support require assistance over a longer time period, but the time frame is still a finite one. An example would be the provision of a job coach for a limited time when starting a new job. The third level, extensive support, applies to those who have a need for assistance on a daily basis, although not necessarily in all environments. These supports may be required in a combination of home, work, and community settings. These supports will also be needed long term rather than temporarily. Pervasive support is the most intensive level of support. Persons who need pervasive support require assistance across all environments throughout their day.

Alternatively, intellectual disability can be divided into subcategories based on IQ rather than by levels of support. This method of categorization is not as widely used as the levels of support, but it is still used widely enough to warrant a quick overview. As mentioned earlier, those with mild intellectual disability make up about 85% of those with intellectual disability and are characterized by an IQ between 55-69. This is the first subcategory in this system. Persons in this group will require additional assistance, including special education services, but most are capable of holding jobs and being relatively independent as adults. The next subcategory, persons with an IQ between 35-54, are described under this system as having a moderate intellectual disability. Approximately 10% of those with intellectual disability are in this group. These students are often taught in self-contained special education classrooms, although more of these students are now spending at least part of their day in inclusive settings. As adults, persons in this group may hold jobs but will likely require additional supervision in both the home and work setting. The third subcategory of intellectual disability includes persons with an IQ of 20-34. This is described as a severe intellectual disability. This group makes up about 3% of the population with intellectual disability. Educational programs for these students focus on daily living skills. Finally, those with an IQ below 20 are described as profoundly intellectually disabled. Those in this category will require round-the-clock supervision, especially as many who are affected by this level of intellectual disability will have significant medical needs as well.

Causes of Intellectual Disability

There are many potential causes of intellectual disability. Any condition that affects the brain and brain development before birth, during the birth process, or during childhood can result in an intellectual disability. Fortunately, we have identified many of the potential risks factors that can affect cognitive development and can advise parents so that many of these risk factors can be avoided. In addition, the mapping of the human genome has led to an increased understanding of the genetic causes of intellectual disabilities. Even so, the cause of the intellectual disability is unknown in one-third to one-half of all cases.

A genetic disorder may be inherited, or it may be the result of various errors that can occur in the genetic code. Genetic disorders cannot be cured, but the resulting symptoms can often be managed. For example, Phenylketonuria (PKU) which can result in intellectual disability when untreated, can be managed through diet. Some other examples of genetic disorders that are linked to intellectual disability are Down syndrome, Fragile X syndrome, and Turner syndrome. Maternal factors during pregnancy can also lead to intellectual disability. Some examples of these factors are malnutrition, exposure to certain illnesses such as rubella, cytomegalovirus or syphilis, and prenatal use of drugs or alcohol. Exposure to x-rays and other environmental contaminants can also affect the developing baby. In addition, there are risk factors that occur during labor and delivery. Any stress during the birth process, especially insufficient oxygen, can result in damage to the baby’s brain which can lead to an intellectual disability. Additionally, prematurity and low birth weight are also strongly correlated with intellectual disabilities.

Once the baby is born, the child can still encounter adverse conditions that can lead to an intellectual disability. Car accidents and near drowning can cause brain damage resulting in an intellectual disability. Childhood diseases such as measles, meningitis, and whooping cough can also damage the brain. Extreme malnutrition and exposure to environmental toxins, such as lead or mercury, have a lasting impact on the brain and nervous system. Even under-stimulation can affect the developing brain. According to the U.S. Center for Disease Control and Prevention, 3-15% of all intellectual disabilities are acquired postnatally, many of which were preventable. One of the most common preventable postnatal causes of intellectual disability is child battering, also known as shaken baby syndrome (CDC, 1996). It is estimated that 1,000-3,000 children in the United States are the victims of shaken baby syndrome each year. Approximately 25% of these children die as a result of their injuries. Of those that survive, 80% have permanent damage to the brain which can result in cerebral palsy, blindness, hearing loss, seizures, paralysis, and learning and behavior struggles (Department of Health, 2010).

Learner Characteristics

Children with intellectual disabilities generally show global delays in all areas of development. Cognitive development, which includes areas such as working memory and problem solving, is significantly behind that of their peers. Similarly, achievement across the academic areas of reading, writing, and math is generally delayed compared to their peers. These students often have a shorter attention span and are more easily distracted than their classmates. In addition, they usually struggle with metacognition, which encompasses understanding how one learns, being able to plan the best strategies to use for different types of learning tasks, and being able to evaluate and adjust these strategies as needed during the task.

Children with intellectual disability usually struggle with memory tasks. Factors that contribute to their memory challenges are difficulties with attention, inefficient techniques for learning and remembering information as mentioned above, and struggles with generalizing their learning to new problems and situations. This means that the difficulties they have in focusing and maintaining their attention and in determining which elements are most important will affect their ability to remember what they are being taught. Difficulty with generalization means they find it hard to transfer what they have learned in one setting to a new context or slightly different application. In addition, these students often have a history of failure which can lead to anxiety and motivation challenges. These factors can all negatively impact learning success.

Strategies that can help address these challenges include keeping students actively engaged in learning, using concrete materials whenever possible, and providing many examples as well as non-examples. Providing real-life opportunities for learning and application of concepts can also be helpful. If real-life opportunities are not possible, simulations can be useful. Skills should be broken down into small discrete steps which are then taught in a systematic way with many opportunities for practice, reinforcement, and corrective feedback. Frequent cumulative review is also helpful for ensuring learning is maintained.

Placement

Students with mental retardation are now included in general education classrooms in greater numbers than ever before. Even so, in 2022, less than 21% were fully included, which is defined by IDEA as included in general education at least 80% of the time, and nearly half (45.5%) received less than 40% of their education in the general education classroom. However, only 4.6% were educated in separate schools and an even smaller number, only 0.2%, were educated in residential facilities (National Center for Educational Statistics, 2024; National Center for Education Statistics, 2023). Education for children with intellectual disabilities has made great strides since the passage of the Education for All Handicapped Children Act, the precursor of IDEA, in 1975.

Instructional methods for students with intellectual disabilities should include such research-based strategies as scaffolding, direct instruction, task analysis, and cooperative learning. These methods will allow teachers to provide appropriately leveled support, break complex tasks down into smaller more attainable steps, and keep students actively involved in their learning. We will explore these strategies at greater length in a later chapter. In addition, educators need to be aware that learning tasks can be challenging and require a lot of energy and concentration from students with intellectual disabilities, so it can be helpful to build in breaks over the course of the day. Children with intellectual disabilities can also develop learned helplessness, which makes it even more important that these learners experience success and gain confidence. These successes can occur with learning tasks as well as through opportunities to complete typical classroom jobs. Feedback and positive reinforcement must be provided to the learner frequently. Research has shown that providing positive reinforcement will help students learn at a faster rate than if they do not receive this affirmation and reinforcement.

Stephanie’s Story

Instruction for students with intellectual disabilities, like instruction for other students with disabilities, must be individualized and responsive to the specific strengths and needs of the student. It is important that instructional goals be both challenging and realistic. An effective curricular plan for a student with an intellectual disability will address both academic and adaptive skills. For most students, this plan will include instruction in functional academics. Functional academic skills are found at the crossroads where academics and adaptive skills meet. These are academic skills that are specifically targeted to everyday life situations. Some examples of functional academic skills include the reading skills used in reading a recipe or in reading instructions, the math skills used in making a budget and making change, and the writing skills used to complete a job application or to write a to-do list. Functional academic skills are just one component of adaptive skills. Other adaptive skills that may need to be addressed include independent living skills, social skills, work skills, self-care skills, communication skills, leisure skills, and instruction in how to access community resources.

For some students with intellectual disability, the impact on academic learning will be significant enough that a conversation about diploma requirements will be necessary. How this challenge is addressed varies from state to state. In some states, students with disabilities will receive a standard diploma if they meet the graduation requirements through special education coursework based on their IEP goals. In other states, students who meet graduation requirements in this way will be issued a modified or special diploma rather than a standard diploma. Teachers should become familiar with the options available in their state. It is important that a conversation be held with students and their families around the various options and the programming implications that accompany these options. This conversation should happen early in the process of creating a plan for secondary education, if not before. Access to a free and appropriate public education under IDEA for students with disabilities ends when the student earns a standard diploma or reaches the age of 21, whichever comes first.

Kennedy’s Story

Down syndrome is an extra copy of the 21st chromosome. It’s the most common chromosomal condition diagnosed, and many times is detected during pregnancy.

Babies and children with Down syndrome are eligible for Early Intervention where they can receive multiple therapies to help with development and growth, and that continues into their school years where they can often thrive and learn in a general education classroom with the right supports.

Adults with Down syndrome lead full and happy lives! They can have lots of friends, go on dates, get married, hold jobs and are contributing members to society. Always expect more than others think is possible!

Renee Perkins Garcia, Personal Communication, October 2021. Used with permission. Image © Renee Perkins Garcia. Used with permission.