Center for Neurodiversity and Behavioral Health

Intellectual Disability

Intellectual disability is a below-average cognitive ability with three (3) characteristics:

  • Intelligent quotient (or I.Q.) is between 70-75 or below
  • Significant limitations in adaptive behaviors (the ability to adapt and carry on everyday life activities such as self-care, socializing, communicating, etc.)
  • The onset of the disability occurs before age 18. 

Intelligence refers to general mental capability and involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly, and learn from experience. 

Studies show that somewhere between one (1) percent and three (3) percent of Americans have intellectual disability.  There are many causes of intellectual disability, factors include physical, genetic and/or social. 

The most common syndromes associated with intellectual disability are autism, Down syndrome, Fragile X syndrome and Fetal Alcohol Spectrum Disorder (FASD).

Common causes occur from genetic conditions (Down syndrome and Fragile X syndrome are examples), problems during pregnancy (a pregnancy of the mother who drinks alcohol while pregnant can result in FASD), problems at the time of birth, health problems such as whooping cough, measles or meningitis and exposure to environmental toxins like lead or mercury.

The impact of having an intellectual disability varies considerably, just as the range of abilities varies considerably among all people.  Children may take longer to learn to speak, walk and take care of their personal needs, such as dressing or eating.  It may take longer to learn in school. 

As adults, some people are able to lead independent lives in the community without paid supports.  A small percentage will have serious, lifelong limitations in functioning.  However, with early intervention, an appropriate education and supports as an adult, all can lead satisfying lives in the community. 

Sometimes intellectual disability is also referred to as developmental disability which is a broader term that includes ASD (autism spectrum disorders), epilepsy, cerebral palsy, developmental delay, fetal alcohol syndrome (or FASD) and other disorders that occur during the developmental period (birth to age 18). 

The major differences are in the age of onset, the severity of limitations, and the fact that a person with a developmental disability definition may or may not have a low I.Q. While some people with intellectual disability will also meet the definition of developmental disability, it is estimated that at least half do not meet the requirements for the developmental disability definition. 

(SOURCE: The Arc)


Autism is defined by the presence of difficulties in three (3) areas: social deficits, communication problems and repetitive or restricted behaviors, with an onset in at least one area by age three (3).  It may or may not be associated with language delays or intellectual disability.  

It is estimated that one (1) in every 68 children is diagnosed with autism (and one (1) in every 42 boys), making it more common than childhood cancer, juvenile diabetes and pediatric AIDS combined.  An estimated 1.5 million individuals in the U.S. and tens of millions worldwide are affected by autism.

Autism Spectrum Disorder

Autism Spectrum Disorder (or ASD) is a general term used to describe a group of complex developmental brain disorders known as Pervasive Developmental Disorders (PDD). 

The other pervasive developmental disorders are PDD-NOS (Pervasive Developmental Disorder – Not Otherwise Specified), Asperger's Syndrome, Rett Syndrome and Childhood Disintegrative Disorder.  Many parents and professionals refer to this group as Autism Spectrum Disorders.

ASD is a "spectrum disorder" because it affects individuals differently and to varying degrees.  There is no known single cause for ASD, but increased awareness and funding can help families today.

(SOURCE: The Arc)