An Overview of Intellectual Disability: Levels of Severity, Causes and Diagnosis

By Elizabeth Stannard Gromisch

Previously called mental retardation, an intellectual disability is a type of disability in which the patient has limitations involving her adaptive behavior, learning and problem solving.

In the United States, up to three out of every 100 people are identified as having types of intellectual disabilities, according to the National Dissemination Center for Children with Disabilities. Intellectual disability was previously called mental retardation, though some organizations and publications, such as the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR), still use the term “mental retardation." The American Association on Intellectual and Developmental Disabilities (AAIDD) explains that the name was changed to “intellectual disability" because it is a less offensive term.

If a child has one of the different types of intellectual disabilities, she may have problems with comprehending social rules, solving problems, thinking logically and understanding consequences. Other signs of an intellectual disability include reaching motor milestones later than other children, memory problems and difficulty learning to talk.

Several conditions may cause an intellectual disability, though the National Women's Health Information Center notes that between 40 and 50 percent of intellectual disability case causes are not known. Some children may have an intellectual disability due to a genetic condition, such as Down syndrome. If the mother drinks during pregnancy and the child develops one of the fetal alcohol spectrum disorders, an intellectual disability may occur. Conditions such as meningitis, whooping cough and the measles may also cause an intellectual disability. Other possible causes include poor nutrition, toxin exposure, and problems during the pregnancy or while giving birth.

Levels of Severity

The types of intellectual disabilities are actually different levels of severity, which differ by IQ score. These levels include mild, moderate, severe and profound intellectual disability. The DSM-IV-TR also includes “mental retardation, severity unspecified," in which the clinician has a strong feeling that the patient has an intellectual disability, but for some reason, such as the patient being too impaired or uncooperative, the clinician cannot test her successfully.

With a mild intellectual disability, the patient has an IQ between 52 and 69, and she can learn up to around the sixth grade level, notes the Merck Manual Home Edition. The next type of intellectual disability is moderate intellectual disability, in which the patient has an IQ of between 36 and 51. Children with a moderate intellectual disability can learn communication, but have poor social awareness. With severe intellectual disability, the patient's IQ is between 20 and 35, she she can speak a couple of words, but has poor motor coordination. The last type of intellectual disability, profound intellectual disability, involves an IQ of 19 or below. Children with this type have extremely limited cognition, according to the Merck Manual Home Edition.


Diagnosis for types of intellectual disabilities involves testing the patient's intelligence and her adaptive behavior. If the patient is a child, the Wechsler Intelligence Scale for Children (WISC) may be used to test intelligence. The AAIDD notes that adaptive behavior testing includes gauging the patient's social skills, conceptual skills and practical skills.


National Women's Health Information Center: Intellectual Dsaibility

National Dissemination Center for Children with Disabilities: Intellectual Disability (formerly Mental Retardation)

Merck Manual Home Edition: Mental Retardation/Intellectual Disability

AAIDD: FAQ on Intellectual Disability

American Psychiatric Association (2000). Diagnostic and Statistical Manual of Mental Disorders. American Psychiatric Publishing