Intellectual Disability vs Autism: The Difference Between

As a former special education teacher, I've sat in many, many, many IEP meetings where eligibility testing scores and reports were shared with students and families.  To the untrained ear, it can feel like lots of numbers, confusing terms, and psychology jargon.    Hearing everything for the first time can easily overwhelm parents or family members.  

If you're a parent or family member who has heard the terms intellectual disability and autism in one of those types of IEP meetings and is curious if they are one and the same, they are not. Intellectual disability and autism spectrum disorder, or ASD, are separate conditions. Below, I'll share the different definitions, the tests that determine these disabilities, what these disabilities are and are not, and how someone could be both autistic and have an intellectual disability. 

For parents and loved ones, that diagnosis likely doesn't matter. However, the diagnosis will matter when it comes to the supports, resources, and benefits that the individual may want or need to pursue either now or in the future. A diagnosis or disability does not define someone. However, it may be the evidence needed to qualify for appropriate support.  

For example, if the individual were school-age, then this diagnosis would be considered during the evaluation portion of the IEP consideration process.  An IEP opens up the opportunity for school-provided occupational therapy, speech therapy, and additional in-school support.  

For individuals older than school age, the diagnosis of intellectual disability or autism spectrum disorder would shared during the application process for benefits like Social Security Administration's Supplemental Security Income (Disability) or establishing an estate plan where a special needs trust.  

Defining Intellectual Disability 

An intellectual disability is a neurodevelopmental disorder, meaning it has been present since early childhood, where an individual's IQ score is 70 or lower.  An intellectual disability is a lifelong condition.  

An average IQ is 100, and an intellectual disability is defined as two standard deviations below average.  The standard deviation is 15, so two standard deviations is 30, therefore 100 (average intelligence score) - 30 = 70.  What is a standard deviation?  A standard deviation is a statistics term used to measure an amount away from a fixed number.    

If the individual test scores show a Full Scale Intelligence Quotient, or FSIQ, of 70 or lower, they may have a diagnosis of intellectual disability. 

There are levels of intellectual functioning:  

• Mild intellectual disability is defined as an IQ score between 50-69  
• Moderate intellectual disability is defined as an IQ score between 35-49
• Severe intellectual disability is defined as an IQ score between 20-34  
• Profound intellectual disability is defined as an IQ score between 0-19  

How are IQ scores determined?  The individual will be given an IQ test, which consists of various subtests.  Each subtest assesses a set of specific skills.  Examples of these specific areas include verbal comprehension, processing speed, and working memory, among others.    Each subtest is scored and those various scores are then combined to determine an individual's overall intellectual functioning.  According to the American Psychiatric Association Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) diagnostic criteria, for an accurate diagnosis of intellectual disability, the scores should indicate deficits in both intellectual functioning and adaptive skills, such as conceptual skills, social skills, and practice skills.  

For individuals with significant limitations, a clinician may attempt to give them an IQ test. If they are unable to respond, they may be given a score of 'Untestable.' 

Standardized Intelligence Tests for Determine IQ Test Scores:

• Wechsler Adult Intelligence Scale (WAIS)
• Wechsler Intelligence Scale for Children (WISC)

An intellectual disability was previously referred to as mental retardation.  It is not uncommon for disability terms to be updated and clarified over time (see Defining Autism below for a very recent example), so you may still see this term used in testing titles, organization names, and medical field reports.  

An intellectual disability may also be referred to as a Cognitive Disability because it impacts cognitive functioning.  Cognitive functioning means how the brain thinks, acquires knowledge, and understands what it experiences.   

Defining Autism

Autism spectrum disorder, or ASD, is a neurodevelopmental disorder under the developmental disabilities category, meaning the disability was present in early childhood when the brain was developing, 

The American Psychiatric Association Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5-TR), which is used by medical professionals, psychologists, social workers, and others, defines autism as experiencing "persistent challenges with social communication and social interactions, specifically deficits in social-emotional reciprocity, nonverbal communication skills, and developing, maintaining, and understanding relationships, restricted and repetitive patterns of behaviors, interests, or activities such as stereotype or repetitive motor movements, use of objects, or speech, insistence on sameness, inflexible adherence to routines, or ritualized patterns of verbal or nonverbal behavior, highly restricted, fixated interests that are abnormal in intensity or focus, hyper or hyporeactivity to sensory input or unusual interest in sensory aspect of the environment, and be present since early childhood and significantly impairs social, occupational, or other important areas of current functioning."  (American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders. 5th ed., Text Revision, American Psychiatric Publishing, 2022, p.56-57)

The American Psychiatric Association Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5-TR) also recently updated the terms used to identify autism, specifically removing Asperger's syndrome and Pervasive Developmental Disorder—Not Otherwise Specified (PDD-NOS). Therefore, the blanket term is now Autism Spectrum Disorders, or ASD.  

An individual may receive a diagnosis of ASD after a standardized test.  

Standardized Tests Used to Diagnosis Autism:

• Adaptive Behavior Assessment Scales (ABAS)
• Vineland Adaptive Behavior Scales 
• Autism Diagnostic Observation Schedule
• Gilliam Autism Rating Scale (GARS)
• Childhood Autism Rating Scale (CARS) 

For the American Psychiatric Association: The Diagnostic and Statistical Manual of Mental Disorders definition of autism, IDEA definition of autism, and Social Security Administration definition of autism, check out my Disability Benefits for Adults with Autism blog post.  

Autism and Intellectual Disabilities Might:

Autism and an intellectual disability can be 'comorbid' with other disabilities and disorders.  What does comorbid mean?  Comorbid means that a diagnosis may occur simultaneously as another disability or disorder.  Therefore, an individual may be autistic and have attention deficit hyperactivity disorder or they may have Down Syndrome and an intellectual disability.  And yes, an individual may have both an intellectual disability and have autism spectrum disorder.  

There are disabilities that have a higher rate of comorbidity, such as Down syndrome and intellectual disability or fetal alcohol syndrome and intellectual disability.  

Individuals diagnosed with an autistic disorder or intellectual disability might need support with independent living skill development, therapies to strengthen their language skills, behavioral interventions to address repetitive behaviors and may benefit from appropriate interventions for learning and completing everyday tasks.  

Teaching students with autism comes with its own unique challenges since autism presents differently in each person, because the old adage holds true, 'If you've met one person with autism, then you've met one person with autism'.   For research-based approaches, check out my blog about 11 Methods for Teaching Nonverbal Students with Autism.

Autism and Intellectual Disabilities are Not:

An intellectual disability is different from a learning disability.  While both impact a person's ability to acquire knowledge and understanding, they are different.  A learning disability indicates a challenge in acquiring skills and knowledge.  An intellectual disability indicates significant limitations in acquiring skills and knowledge.  A person may have one learning disability or more than one.  However, a person with an intellectual disability may not also have a learning disability, rather their intellectual disability may be greatly impacting a particular area of learning, like math, more than others.  

An individual's autism or intellectual disability diagnosis does not define their level of happiness, joy, or quality of life!   I'll let the many other voices of individuals with intellectual disabilities and autism speak for themselves, as they are growing in size and volume and deserve that spotlight, but a diagnosis is just that, a part of what makes someone who they are.