There are three primary definitions of intellectual disabilities. These include definitions from the Individuals with Disabilities Education Act (IDEA), the American Association on Intellectual and Developmental Disabilities (AAIDD), and the Diagnostic and Statistical Manual of Mental Disorders (DSM) of the American Psychiatric Association. The IDEA defines intellectual disabilities as the following.
Intellectual disability means significantly subaverage general intellectual functioning, existing concurrently with deficits in adaptive behavior and manifested during the developmental period, that adversely affects a child’s educational performance (IDEA, Part B, Subpart A § 300.8(c)(6)(2004)).
Subaverage general intellectual functioning is defined as a score on a standardized intelligence test below 68 (significantly below an average score of 100). In addition, the developmental period refers to the time between birth and 18 years of age (Smiley et al., 2022).
The AAIDD Definition
The AAIDD is an advocacy group that promotes the rights of people with intellectual and developmental disabilities. The AAIDD publishes a manual titled Intellectual Disability: Definition, Diagnosis, Classification, and Systems of Supports. This manual defines intellectual disability as the following.
Intellectual disability is a disability characterized by significant limitations in intellectual functioning and adaptive behavior as expressed in conceptual, social, and practical skills. This disability originates during the developmental period, defined operationally as before the individual attains age 22 (Schalock et al., 2021).
The IDEA and AAIDD definitions refer to adaptive behavior. Adaptive behaviors are “learned behaviors that reflect an individual’s social and practical competence to meet the demands of everyday living” (AAIDD, 2022). AAIDD refers to adaptive behavior as a collection of conceptual, social, and practical skills. Conceptual skills include memory, language, reading, writing, math reasoning, acquisition of practical knowledge, problem solving, and judgment in novel situations. Social skills include empathy, interpersonal communication skills, friendship abilities, social judgment, and awareness of others’ thoughts, feelings, and experiences. Practical skills involve learning and self-management across life settings, including personal care, job responsibilities, money management, recreation, self-management of behavior, and school and work task orientation (DSM-5-TR, 2022, p. 42).
Although the IDEA and AAIDD definitions provide criteria to support the identification of students with an intellectual disability, it is important to recognize that adaptive behaviors are malleable. With appropriate interventions, students with intellectual disabilities can improve their adaptive behaviors.
The APA Definition
The definition included in the DSM refers to intellectual disability as “a disorder with onset during the developmental period that includes both intellectual and adaptive functioning deficits in conceptual, social, and practical domains” (DSM-5-TR, 2022, p. 37). The following three criteria must be met for a child to be diagnosed with an intellectual disability.
- Deficits in intellectual functions, such as reasoning, problem solving, planning, abstract thinking, judgment, academic learning, and learning from experience, confirmed by both clinical assessment and individualized, standardized intelligence testing.
- Deficits in adaptive functioning that result in failure to meet developmental and sociocultural standards for personal independence and social responsibility. Without ongoing support, the adaptive deficits limit functioning in one or more daily life activities, such as communication, social participation, and independent living, across multiple environments, such as home, school, work, and community.
- Onset of intellectual and adaptive deficits during the developmental period (DSM-5-TR, 2022, p. 37).
The diagnosis of intellectual disability must also be based on “both clinical assessment and standardized testing of intellectual functions, standardized neuropsychological tests, and standardized tests of adaptive functioning” (DSM-5-TR, 2022, p. 38). In addition, the DSM specifies four severity levels of intellectual disability: mild, moderate, severe, and profound, which are diagnosed on the basis of adaptive functioning. Schalock et al. (2021) indicate that a classification is considered an optional post-diagnosis organizing scheme that helps educators and clinicians provide appropriate interventions for students.
The Illinois Definition of Intellectual Disability
“Intellectual Disability means significantly subaverage general intellectual functioning, existing concurrently with deficits in adaptive behavior and manifested during the developmental period, that adversely affects a child’s educational performance” (Illinois State Board of Education, 2022).
Activity \(\PageIndex{1}\): Compare and Contrast
Directions: Compare and contrast the IDEA, AAIDD, APA, and Illinois definitions of intellectual disabilities. Consider the differences and similarities between diagnostic criteria.
The AAIDD has also published guidance on how to classify the severity of an intellectual disability, as well as the Supports Intensity Scale (SIS), which allows professionals to measure the support needs of students with intellectual disabilities across categories of adaptive behavior. The SIS changes the focus from the individual’s deficits to the support they need to succeed in different settings.
Individuals With Disabilities Education Act, Part B, Subpart A § 300.8(c)(6)(2004).
Smiley, L. R., Richards, S.B., & Taylor, R. (2022). Exceptional students: Preparing teachers for the 21st century (4th ed.). McGraw Hill.
Schalock, R. L., Luckasson, R.., & Tassé, M. J. (2021). Intellectual disability: Definition, diagnosis, classification, and systems of supports (12th ed.). American Association on Intellectual and Developmental Disabilities.
American Psychiatric Association. (2022). Diagnostic and statistical manual of mental disorders (5th ed. Text Revision). https://doi.org/10.1176/appi.books.9780890425596
Illinois State Board of Education. (2022, September 27). Special education disability areas. https://www.isbe.net/Pages/Special-Education-Disability-Areas.aspx
