Video: Special Olympics. (2013, June 27). Nelson Mandela on Eunice Kennedy Shriver and justice for people with intellectual disabilities. YouTube. https://www.youtube.com/watch?v=ULBpl8sN9NQ

Introduction

Individuals with intellectual disabilities have always been part of our communities, yet throughout history they have often been excluded, segregated, or denied meaningful educational opportunities. Over time, however, our understanding of learning differences has grown and attitudes toward disability have become more inclusive, leading to changes in terminology, definitions, educational opportunities, and expectations. Today, educators recognize that individuals with intellectual disabilities are capable of learning, developing meaningful relationships, contributing to their communities, and leading fulfilling lives when provided with appropriate opportunities and supports.

This chapter examines the characteristics of intellectual disability, including how it is defined and how assessments of intellectual functioning and adaptive behavior are used to identify students' strengths and support needs. It explores the prevalence, causes, learner characteristics, and instructional implications associated with intellectual disability while emphasizing the importance of high expectations, individualized supports, and meaningful inclusion. Throughout the chapter, the discussion reflects the growing emphasis on dignity, respect, and person-centered supports that has shaped contemporary understandings of 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 laws and policy documents. Shortly thereafter, the Diagnostic and Statistical Manual of Mental Disorders: DSM-5, used by healthcare professionals, and the International Classification of Diseases (ICD-11), published by the World Health Organization, also adopted the term intellectual disability in place of mental retardation (Harris, 2013). This change in terminology reflects a broader shift toward language that emphasizes dignity and respect while recognizing advances in our understanding of intellectual disability.

The American Association of Intellectual and Developmental Disabilities (AAIDD) defines intellectual disability as follows: "Intellectual disability is a disability characterized by significant limitations in both intellectual functioning and adaptive behavior, which covers many everyday social and practical skills. This disability originates before the age of 22." According to this definition, an intellectual disability is characterized by three essential criteria: significant limitations in intellectual functioning, significant limitations in adaptive behavior, and onset during the developmental period. Adaptive behavior refers to the conceptual, social, and practical skills that individuals use to function independently and participate successfully in everyday life.

Determining what constitutes a significant limitation in intellectual functioning has evolved over time. Early definitions relied primarily on Intelligence Quotient (IQ) scores. Originally, intellectual disability was defined as an IQ score at least two standard deviations below the mean. In 1959, this guideline was changed to one standard deviation below the mean, and adaptive behavior was introduced as an additional diagnostic criterion. In 1973, the IQ criterion was returned to two standard deviations below the mean, while the requirement for significant limitations in adaptive behavior was retained and given greater emphasis. In 1983, additional flexibility was introduced by allowing professional judgment when interpreting IQ scores between approximately 70 and 75. Revisions published in 1992, 2002, and 2010 continued to shift the emphasis away from IQ scores alone and toward an individual's present level of functioning within the context of their everyday environment and the supports needed to participate successfully in home, school, work, and community life (NASET, 2024).

The inclusion of adaptive behavior represents an important conceptual shift in the understanding of intellectual disability. Rather than defining intellectual disability solely by an IQ score, current definitions recognize that assessment should also consider how individuals function in everyday environments. Consequently, comprehensive evaluations include assessments of both intellectual functioning and adaptive behavior, allowing educators and other professionals to develop a more complete understanding of an individual's strengths and needs so they can identify the instructional and environmental supports that promote learning, independence, and meaningful participation.

Assessing Intellectual Functioning

Cognitive ability is typically measured using standardized tests of general intelligence. These assessments evaluate a person's ability to think, reason, solve problems, and learn by measuring skills such as verbal reasoning, working memory, processing speed, visual-spatial reasoning, and fluid reasoning (OPM, n.d.). Although some intelligence tests include tasks involving mathematics or language, they are designed to assess underlying cognitive processes rather than academic achievement. Many intelligence tests report scores with a mean of 100 and a standard deviation of 15. In a population with a normal distribution of scores, approximately 50% of individuals score above the mean and 50% score below it. Approximately 68% of scores fall within one standard deviation of the mean (85–115), while about 95% fall within two standard deviations (70–130).

Historically, an IQ score approximately two standard deviations below the mean (below 70) has been one criterion used in identifying an intellectual disability, provided that significant limitations in adaptive behavior are also present. Understanding the normal distribution helps illustrate the importance of the IQ cutoff used in the definition of intellectual disability. When the IQ guideline was temporarily changed from two standard deviations below the mean to one standard deviation below the mean in 1959, the proportion of individuals who could potentially qualify on the basis of IQ alone increased from approximately 2.5% of the population to about 16%. While the guideline soon reverted back to two standard deviations, this change demonstrated the importance of considering adaptive behavior in addition to IQ when determining whether an individual meets the criteria for an intellectual disability.

While intellectual functioning is commonly assessed using standardized intelligence tests, their use has not been without controversy. Intelligence is a complex and abstract construct that cannot be measured directly. Instead, it must be inferred from performance on a variety of cognitive tasks. In addition, concerns have been raised about the potential for cultural and linguistic bias in some intelligence tests and the ways in which assessment results have been used to make educational decisions. These concerns led to several landmark court cases. In the 1970 case of Diana v. State Board of Education, the court determined that students could not be placed in special education programs solely on the basis of an IQ test, recognizing that assessments administered in English to students from culturally and linguistically diverse backgrounds could produce misleading results (Disability Rights California, 2025a).

Similarly, in the 1972 case of Larry P. v. Riles, the court ruled that African American students could not be identified as having intellectual disabilities on the basis of intelligence tests that had not been shown to be free from cultural bias (Disability Rights California, 2025b). The court directed California education officials to develop a nondiscriminatory method for identifying students with intellectual disabilities. When no satisfactory alternative assessment was developed, a subsequent 1979 ruling prohibited the use of IQ tests for identifying African American students as intellectually disabled in California. It is important to note that this restriction applies only within the state of California. These cases significantly influenced contemporary assessment practices by reinforcing the importance of using multiple sources of information, considering cultural and linguistic diversity, and avoiding reliance on any single assessment when making eligibility decisions (Evaluation Procedures, 2026).

In addition to concerns about potential bias, concerns have also been raised about the interpretation and stability of IQ scores. An intelligence test provides an estimate of a person's cognitive functioning based on performance at a particular point in time and under specific testing conditions. Although intelligence tests generally demonstrate good reliability, an obtained IQ score represents an estimate of performance and should always be interpreted within the broader context of multiple assessment findings. The inclusion of measures of adaptive behavior helps address this concern by providing information about how an individual functions in everyday settings. Nevertheless, some professionals believe that too much emphasis is still placed on IQ scores and not enough on other sources of information. As a result, some parents choose not to have their children participate in intelligence testing. While intelligence tests are useful predictors of many aspects of academic learning, they provide only one piece of the assessment process. Comprehensive evaluations should also include information from multiple sources, such as work samples, classroom observations, interviews with parents and teachers, and other formal and informal assessments. Collecting information from a variety of sources provides a more complete understanding of a student's strengths and needs, allowing educators to identify the instructional and environmental supports that best promote learning and participation (Evaluation Procedures, 2026).

Assessing Adaptive Skills

In addition to significant limitations in intellectual functioning, an intellectual disability is also characterized by significant limitations in adaptive behavior. Adaptive behavior refers to the conceptual, social, and practical skills that individuals use to function independently and meet the demands of everyday life. Conceptual skills involve learning and applying knowledge, social skills involve interacting effectively with others, and practical skills include the everyday tasks needed for independent living. These skills provide important information about a person's level of independence and ability to meet the everyday demands of home, school, work, and community life. Measures of adaptive behavior should be interpreted in relation to the individual's age and the cultural and community expectations that influence daily functioning. Depending on the individual's age, assessments of adaptive behavior may examine areas such as communication, self-care, social skills, independent living, vocational skills, and leisure skills. Information about adaptive behavior is typically gathered through a combination of observations, interviews with parents, caregivers, and teachers, and standardized rating scales or questionnaires. Finally, by definition, an intellectual disability must originate before age 22, meaning it develops during the developmental period. Although individuals with intellectual disabilities generally learn at a slower pace than their peers, they continue to learn, develop new skills, and increase their independence throughout their lives (AAIDD, 2024; APA, 2024).

Prevalence

According to the American Psychiatric Association (2024), approximately one percent of the population meets the criteria for an intellectual disability. Of those individuals, approximately 85% are identified as having a mild intellectual disability. Although intelligence testing remains an important component of the evaluation process, current diagnostic practice classifies the severity of an intellectual disability primarily on the basis of adaptive functioning rather than IQ scores alone. Historically, more males than females have been diagnosed with an intellectual disability. The table below summarizes the prevalence and levels of severity associated with intellectual disability.

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. It is important to recognize that support needs are not fixed. They may increase or decrease over time as individuals develop new skills, experience changes in health or living circumstances, or receive effective instruction and appropriate supports. The four levels of support typically used are intermittent, limited, extensive, and pervasive.

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.

Causes of Intellectual Disability

There are many potential causes of intellectual disability. Any condition that affects brain development before birth, during the birth process, or throughout childhood can result in an intellectual disability. Fortunately, researchers have identified many of the risk factors that can affect cognitive development, allowing healthcare providers and educators to help families reduce or prevent some of these risks. Advances in genetic research, including the mapping of the human genome, have also greatly increased our understanding of the genetic causes of intellectual disability. However, many causes of intellectual disability cannot be prevented, and in many cases no specific cause can be identified. Currently, the underlying cause remains unknown in approximately one-third to one-half of all cases.

A genetic disorder may be inherited or may result from genetic changes that occur during development. Although genetic disorders cannot be cured, many associated symptoms can be managed or treated. For example, Phenylketonuria (PKU), which can result in intellectual disability when left untreated, can be successfully managed through a specialized diet. Other genetic conditions commonly associated with intellectual disability include Down syndrome and Fragile X syndrome. Maternal factors during pregnancy can also contribute to intellectual disability. Examples include malnutrition, exposure to infections such as rubella, cytomegalovirus, or syphilis, and prenatal exposure to drugs or alcohol. Exposure to radiation and certain environmental toxins may also interfere with normal brain development. Complications during labor and delivery can likewise increase the risk of intellectual disability. For example, prolonged oxygen deprivation during birth may result in brain injury that affects cognitive development. Prematurity and low birth weight are also associated with an increased risk of intellectual disability.

Once the baby is born, the child may still experience conditions that can affect brain development and result in an intellectual disability. Traumatic brain injuries caused by events such as motor vehicle accidents or near-drowning incidents can damage the developing brain. Serious childhood illnesses, including measles, meningitis, and whooping cough, may also result in neurological damage. In addition, severe malnutrition, exposure to environmental toxins such as lead or mercury, and prolonged environmental deprivation or a lack of appropriate cognitive and social stimulation can negatively affect brain development. According to the U.S. Centers for Disease Control and Prevention, an estimated 3–15% of intellectual disabilities are acquired after birth, many of which are preventable (CDC, 1996). One of the most serious preventable causes is abusive head trauma (AHT), formerly known as shaken baby syndrome. It is estimated that between 1,000 and 3,000 children in the United States experience abusive head trauma each year. Approximately one-quarter of these children die as a result of their injuries. Among those who survive, many experience permanent brain damage that may result in cerebral palsy, blindness, hearing loss, epilepsy, paralysis, intellectual disability, and long-term learning and behavioral challenges (Department of Health, 2010).

Learner Characteristics

Children with intellectual disabilities generally demonstrate global delays across multiple areas of development. Cognitive development, including skills such as reasoning, problem solving, and working memory, is typically delayed compared to that of their peers. Similarly, academic achievement in reading, writing, and mathematics is often significantly below age-level expectations. These students may also have shorter attention spans and be more easily distracted than their classmates. In addition, many experience difficulties with metacognition, which refers to understanding how one learns, selecting appropriate learning strategies, monitoring one's own understanding, and evaluating or adjusting those strategies as needed.

Children with intellectual disabilities often experience challenges with learning and remembering new information. Difficulties with attention, working memory, and metacognition can make it more difficult to identify important information, organize it effectively, and retrieve it later. Many also have difficulty generalizing what they have learned, meaning they may struggle to apply a skill or concept learned in one setting to a new situation or slightly different context. As a result, they often require explicit instruction, repeated practice across multiple settings, and ongoing opportunities to apply newly learned skills. In addition, repeated experiences of academic difficulty may contribute to frustration, anxiety, reduced confidence, and decreased motivation to engage in learning. Together, these factors can further influence academic progress.

Understanding these learning characteristics helps educators design instruction that addresses students' individual strengths and needs. Effective instructional strategies include actively engaging students in learning, using concrete materials whenever possible, and providing numerous examples and non-examples to promote concept development. Whenever appropriate, instruction should occur in authentic, real-life settings where students can immediately apply new skills. When real-life experiences are not feasible, carefully designed simulations can provide meaningful opportunities for practice. Complex skills should be broken down into smaller, manageable steps and taught systematically with frequent opportunities for guided practice, reinforcement, and corrective feedback. Finally, regular cumulative review helps students retain previously learned skills and promotes the transfer of learning to new situations. Ultimately, the goal of these instructional strategies is not simply to improve academic performance, but to help students develop the knowledge, skills, and independence needed to participate as fully as possible in their homes, schools, workplaces, and communities.

Placement

Students with intellectual disabilities are educated in general education classrooms more often today than at any other time in history. Even so, in 2022, fewer than 21% of students with intellectual disabilities spent at least 80% of the school day in the general education classroom, the threshold IDEA uses to define full inclusion. Nearly half (45.5%) received less than 40% of their instruction in general education classrooms. However, only 4.6% of students with intellectual disabilities were educated in separate schools, and just 0.2% were served in residential facilities (National Center for Educational Statistics, 2024; National Center for Education Statistics, 2023). These statistics illustrate both the progress that has been made since the passage of the Education for All Handicapped Children Act, the precursor of IDEA, in 1975 and the continuing challenge of providing meaningful inclusion for students with more significant support needs (IDEA, 2024).

Instructional Strategies

Instruction for students with intellectual disabilities should incorporate research-based strategies such as scaffolding, explicit instruction, task analysis, and cooperative learning. These approaches allow teachers to provide appropriately leveled support, break complex skills into smaller, manageable steps, and actively engage students in the learning process. Many of these evidence-based instructional practices are discussed in greater detail in a later chapter. Educators should also recognize that learning new skills often requires considerable effort, concentration, and persistence for students with intellectual disabilities. As a result, it may be helpful to build planned breaks into the school day to reduce fatigue and help students maintain attention. Because some students have experienced repeated academic failure, they may also develop learned helplessness, a belief that their efforts will not lead to success. Creating frequent opportunities for success, both during academic instruction and through meaningful classroom responsibilities, helps build confidence, independence, and motivation. Provision of frequent, specific feedback and positive reinforcement further encourages learning and has been shown to improve rate of learning. When educators combine high expectations with systematic instruction, meaningful opportunities for success, and positive reinforcement, persons with intellectual disabilities can continue to learn throughout their lives.

Stephanie’s Story

In this video, a school celebrates Stephanie, a child with Down syndrome, as an important member of their school community.

Video: God’s Treasures (Tesoros de Dios). (2021, May 18). Stephanie at school. YouTube. https://youtu.be/a9Y4F7LXCns?si=ZgEcyETjEm8xtYCi

Instruction for students with intellectual disabilities, like instruction for all students with disabilities, should be individualized and responsive to each student's unique strengths and needs. Instructional goals should be both appropriately challenging and realistically attainable. An effective educational program addresses both academic achievement and adaptive behavior. For many students, this includes instruction in functional academics. Functional academic skills are taught at the intersection of academic learning and everyday living. They are academic skills that are directly applied to real-life situations. Examples include reading a recipe, following written instructions, managing a budget, making change, completing a job application, or writing a to-do list. Functional academics represent just one component of adaptive behavior. Depending on the student's individual needs, instruction may also address independent living skills, communication, social relationships, self-care, employment skills, leisure skills, self-determination, and learning to access community resources.

For some students with intellectual disabilities, the impact on academic learning will be significant enough that decisions about graduation requirements and diploma options will need to be considered. These requirements vary from state to state. In some states, students who meet graduation requirements through an individualized program based on their IEP goals may earn a standard diploma. In other states, students who complete an alternative course of study may receive a modified, alternate, or other state-defined diploma or certificate of completion instead of a standard diploma. Teachers should become familiar with the graduation options and diploma requirements in their own state. Conversations about these options should occur early in the secondary planning process and should actively involve both the student and the family. Understanding the implications of each option is important because earning a standard high school diploma generally ends a student's eligibility for services under the Individuals with Disabilities Education Act (IDEA), whereas students who have not earned a standard diploma may continue to receive special education services until they reach the maximum age established by state law, typically age 21 or 22.

Renee Garcia’s Story

Conclusion

An intellectual disability may affect the way a person learns, but it does not define who that person is or diminish their inherent worth. Like all individuals, people with intellectual disabilities continue to learn, develop, and grow throughout their lives. When educators maintain high expectations, provide systematic and evidence-based instruction, teach both academic and functional skills, and create inclusive learning environments, students can develop the knowledge, skills, confidence, and independence needed to participate meaningfully and contribute to their families, schools, workplaces, churches, and communities.

Notice and Reflect Challenge

This week, pay attention to the expectations educators have for students with intellectual disabilities and other learners who require significant support. Notice how those expectations influence the opportunities students are given to learn, participate, develop independence, and contribute to their school or community.

Reflect:

Glossary

References

About the Author

Dr. Kathleen VanTol has over 35 years of experience working in the field of education. She holds a doctorate in special education from Western Michigan University and is a professor of special education at Dordt University. She is also a Board Certified Behavior Analyst and a Board Certified Advocate in Special Education.

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