Tufts OpenCourseware
Authors: John Morgan, Bonnie F. Zimble
Intellectual Disability:

Diagnosis, Classification and Expectations

Special Care in Dentistry
John Morgan, DDS
Tufts University School of Dental Medicine, 2008

Study Questions:

  1. Describe the two types of functioning that are assessed when diagnosing intellectual disability.

  2. Name the four categories of intellectual disability, giving the IQ range and percentage estimates of each.

  3. How common is intellectual disability in the general population?

  4. What is the proportion of children with intellectual disability who receive special education services?

  5. Describe the most common signs of intellectual disability.

  6. Summarize the levels of functioning expected of individuals with intellectual disability in the mild, moderate and severe/profound categories.

How is Intellectual Disability diagnosed?

The following sections, with the exception of Classification and Functional Expectations, have been adapted from the National Dissemination Center for Children with Disabilities

Intellectual disability is diagnosed by looking at two main things.  These are:

  • The ability of a person to learn, think, plan, reason, solve problems, and make sense of an often complex world - called intellectual functioning, and

  • The extent to which the person has the skills he or she needs to live independently - called adaptive behavior, or adaptive functioning.

Intellectual functioning. Intellectual functioning, or IQ, is usually measured by a standardized test called an IQ test (there are several).  The average score for the general population is 100.  People scoring below 70 to 75 are thought to have intellectual disability.   
Adaptive functioning
. To measure adaptive behavior, professionals look at what a person can do in comparison to others of his or her age. Standardized tests (again, there are several) are used to assess conceptual, social and practical skills.  

To diagnose intellectual disability, professionals look at the combination of a person’s intellectual functioning (IQ) and adaptive skills. 


The categories mild, moderate, severe, and profound are often used to describe the extent of impairment.  The Stanford-Binet IQ test, though it is no longer legal for use as the sole determinant of intelligence, provides rough equivalents for the categories of intellectual disability.



IQ Range

Approximate % of Individuals













Individuals in the severe and profound categories of intellectual disability are apt to be institutionalized. The patients with intellectual disability encountered in private practice for the most part will live independently or semi-independently.  

How common is Intellectual Disability?

As many as 3 out of every 100 people in the country have intellectual disability (The Arc, 2001).  Over 611,000 children ages 6-21 have some level of intellectual disability and need special education in school (Twenty-second Annual Report to Congress, U.S. Department of Education, 2000).  In fact, 1 out of every 10 children who need special education has some form of intellectual disability.

What are the signs of Intellectual Disability?

There are many signs of intellectual disability.  For example, children with 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,

  • not understand how to pay for things,

  • have trouble understanding social rules,

  • have trouble seeing the consequences of their actions,

  • have trouble solving problems, and/or

  • have trouble thinking logically.

About 87% of people with intellectual disability will only be a little slower than average in learning new information and skills.  The remaining 13% of people with intellectual disability score below 50 on IQ tests.  A person with more severe retardation will need more intensive support his or her entire life.

What are the functional expectations of persons with intellectual disability? 

Functional Expectations


Description of Functioning


Less than normal physical coordination; can learn simple skills (in great detail and understanding); shorter than normal attention span, memory; less than average ability to deal in abstraction.


May have poor hand and finger coordination; short memory and attention span; limited imagination and ability to deal with abstract concepts.


Poor coordination; may not be ambulatory; very short attention span and memory.