Autism vs Intellectual Disability: Are they Synonymous? (2024)

For parents who want to find out more about autism, it is important to know what autism is… and what it’s not. For others there is concern that promoting pride in autism sometimes ventures into adding to the stigma of other mental conditions or intellectual disabilities.

A LinkedIn post by Russell Lehmann caught my attention recently; in a video the motivational speaker talks about how the autism community insulting the mental health community “grinds his gears”. Defining autism, especially for those who found their identity in the condition, elicits strong emotions. Many on the spectrum, with high intellect, are vocal about autism not being like other intellectual conditions or disabilities—to them autism exists in a separate sphere to disabilities, disorders and the uber offensive term “mental retardation”.

Believe it or not, “mental retardation” was a medical term used for people with intellectual disabilities decades ago, but the terminology was officially changed when then President Obama signed Rosa’s Law. Unfortunately, individuals with intellectual disabilities are often more stigmatized and socially excluded than any other group.

In a misguided way of advocating for autism, the condition is sometimes favorably compared to other mental conditions, or intellectual disabilities. This not only adds to stigmatizing intellectual disabilities, it also ignores the fact that it is possible–and probable–to be on the spectrum and have a co-occuring intellectual disability.

While these conversations are important, many parents are not overly concerned with precise boundaries of inclusivity when it comes to diagnosis. Instead they want to know where autism and intellectual disability meet (or diverge) for purposes of treatment and intervention. In this article, autism and intellectual disability will be examined to find the link and differences between these conditions and help those confused by their apparent synonymous nature.

Difference between autism and intellectual disability

The Diagnostic and Statistical Manual of Mental Disorders (5th ed.; DSM–5; American Psychiatric Association, 2013) is a good place to start to discover the differences between autism spectrum disorder (ASD) and intellectual disability.

DSM-5 diagnostic criteria for intellectual disability

The DSM-5 did away with the term “mental retardation” by renaming it “intellectual disability” and providing three criteria for a diagnosis (American Psychiatric Association, 2013):

• Deficits in intellectual functioning. This criteria includes impairments in areas like reasoning, problem solving, planning, and learning. Such deficits should be confirmed by both clinical assessment and individualized, standardized testing
• Significant adaptive living skill deficits. Three areas are involved, determining how well the individual copes with everyday tasks: these areas are conceptual, practical and social domains. Deficits in these domains require ongoing support, because without support such adaptive impairments limit functioning in daily life
• The onset of intellectual and adaptive deficits should occur during the developmental period

Another change occurring as the DSM-4 gave way to the current edition, is the replacement of absolute cut-off IQ scores to classify disability into categories of severity. The current version of the DSM classifies intellectual disability according to adaptive functioning—within the domain of intelligence quotient scores. For a detailed explanation of the diagnostic criteria the DSM-5 should be consulted.

DSM-5 diagnostic criteria for autism spectrum disorder

The DSM-5 criteria for an autism diagnosis specifies two general areas in which symptoms are found. These two core areas include deficits or impairments in social-communication and “restricted, repetitive and/or sensory behaviors or interests” (American Psychiatric Association, 2013).

When looking at the diagnostic criteria for these two conditions, parents who identify deficits or challenges in all adaptive functioning domains should consult with a medical expert to determine whether their child may have an intellectual disability. For an autism diagnosis, on the other hand, deficits or impairments in specific adaptive functioning domains may suffice.

An autistic child lacking empathy, or empathy according to neurotypical standards, may have deficits in the social domain, but in the conceptual domain (math or knowledge) they may be on par with peers. Children on the spectrum with high intellect may, for example, fare well in the conceptual domain of language and reading. But they may have deficits in the social and practical domain, struggling with making friends and time management.

A simplistic way of differentiating the two conditions would be to ascribe deficits in more narrowly defined or core areas to autism, while intellectual disability entails deficits across functional domains. Of course autism is found on a spectrum which means every individual will present differently—further complicating the matter are the many different co-occuring conditions which are often diagnosed along with autism.

Intellectual disability is frequently mentioned in studies as the most common co-occurring condition associated with autism. A study (Mpaka et al., 2016) found intellectual disability (75.83%) and epilepsy (72.50%) as the main comorbidities associated with autism.

ASD and intellectual disability: not synonymous but sometimes similar

When looking at research—where the co-occurence of autism and intellectual disability is examined—it illuminates why even doctors sometimes get the diagnosis wrong. It seems almost impossible to find a definite divide separating autism and intellectual disability; and when the two co-occur the boundaries are even blurrier. Medical professionals should not be blamed too harshly for this diagnostic struggle; research shows that there may be shared or overlapping genes linking these two complicated conditions.

Researching genes involved in ASD and intellectual disability

Researchers have made interesting discoveries about the genes involved in neurodevelopmental disorders. A recent study (Frega et al., 2020) titled Distinct Pathogenic Genes Causing Intellectual Disability and Autism Exhibit a Common Neuronal Network Hyperactivity Phenotype illuminates the nature of biological convergence on neuronal activity.

Other studies have also found a genetic overlap between autism and intellectual disability. Research focusing on the genes involved in autism, and differentiating those from genes associated with intellectual disability, become important for more accurate diagnosis and subsequent appropriate interventions. A link between the increase in prevalence of ASD and decrease in prevalence of intellectual disability may also be important to examine.

Communication, the common ground

In addition to genetic similarities, deficits in communication is another area where intellectual disability and autism may converge. As mentioned above, (social) communication impairment is one of the core symptoms and diagnostic criteria of autism. A study examining communication difficulties in adults with intellectual disability (Smith et al., 2020) found over 57% of participants experienced communication difficulties. Of these, 23.5% reported severe difficulties.

In the study (Smith et al., 2020) researchers found that over half of participants find it difficult to communicate with professionals. In this study the participants were adults, one can only imagine the communication difficulties of children with intellectual disabilities when interacting with doctors for diagnostic purposes.

The communication difficulties associated with autism and intellectual disability complicate diagnosis. Consider a child on the spectrum who has been sheltered extensively due to sensory issues. Their lack of exposure to language, and learning opportunities may affect intelligence scores—testing that does not take such circ*mstances into consideration may not reflect a true measure of intelligence.

A review article looking at the diagnostic and treatment issues relating to autism and intellectual disability speaks of the difficulty of discerning whether autism is present for those with profound intellectual disability. Whether social-communication symptoms and repetitive behaviors are due to autism or just part of the cognitive impairment is almost impossible to establish in such individuals (Howlin, 2000).

For individuals with autism and intellectual disability, an accurate diagnosis may be important. They may need support that is “far more individualized, specialized and structured than do non-autistic individuals of the same IQ level”. (Howlin et al., 2000)

A diagnosis of autism and intellectual disability

It’s estimated that around 30% of children on the spectrum in the US also have an intellectual disability. Furthermore, research (Jensen et al., 2020) suggests that individuals with both autism and intellectual disability have more rare, harmful genetic mutations than those with only a diagnosis of autism. Those on the spectrum with rare mutations in autism genes seem to have lower IQ scores compared to autistic individuals with common mutations in autism genes.

A study (Kurzius-Spencer et al., 2018) found children with autism and intellectual disability to be at greater risk of some behavioral problems and lower risk of others, when compared to children with autism only. The study concluded that successful intervention hinges on a comprehension of the complex interaction between co-occurring behavioral and neurodevelopmental problems.

If a child is diagnosed with both autism and intellectual disability parents may wonder which symptoms to address and how one condition will influence the other. Therapists believe that working on functional communication skills may be a useful early intervention strategy for both autism and intellectual disability. If a child can communicate wants, needs, and emotions behavior and quality of life often improves.

It’s not always autism

The hazy line between autism and intellectual disability is challenging for parents and clinicians, especially when it comes to accurate diagnosis of developmental disorders.

Some parents admit that they don’t care if their child with an intellectual disability receives a wrongful, additional autism diagnosis. The communication and behavioral challenges addressed in autism interventions are often appropriate for children with intellectual disabilities too. For these parents an autism diagnosis may not be accurate, but their child’s condition receives treatment and their chance of stigma with an autism diagnosis is less than with an intellectual disability diagnosis. The idea that those with autism are entitled to better services than individuals diagnosed with other mental conditions seems quite pervasive.

The work of autism advocates is creating awareness, promoting education about the condition, and destroying the stigma surrounding neurodiversity. Elevating those on the spectrum should not, however, cast a shadow of stigma on other mental conditions like intellectual disabilities.

Perhaps this was the real intention of the person-first movement: whatever the condition, treatment and resources should be specialized to individuals, rather than catering only to diagnostic classification. Whether a child has autism, an intellectual disability, or both, they should receive the support best suited to enhancing their quality of life.

References:

American Psychiatric Association. (2013). Diagnostic and statistical manual of mental disorders (5th ed.). https://doi.org/10.1176/appi.books.9780890425596.

Frega, M., Selten, M., Mossink, B., Keller, J. M., Linda, K., Moerschen, R., Qu, J., Koerner, P., Jansen, S., Oudakker, A., Kleefstra, T., van Bokhoven, H., Zhou, H., Schubert, D., & Nadif Kasri, N. (2020). Distinct Pathogenic Genes Causing Intellectual Disability and Autism Exhibit a Common Neuronal Network Hyperactivity Phenotype. Cell reports, 30(1), 173–186.e6. https://doi.org/10.1016/j.celrep.2019.12.002.

Howlin, P. (2000). Autism and intellectual disability: Diagnostic and treatment issues. Journal of the Royal Society of Medicine, 93(7), 351–355. https://doi.org/10.1177/014107680009300704.

Howlin, P., Mawhood, L., & Rutter, M. (2000). Autism and developmental receptive language disorder–a follow-up comparison in early adult life. II: Social, behavioural, and psychiatric outcomes. Journal of child psychology and psychiatry, and allied disciplines, 41(5), 561–578. https://doi.org/10.1111/1469-7610.00643.

Jensen, M., Smolen, C., & Girirajan, S. (2020). Gene discoveries in autism are biased towards comorbidity with intellectual disability. Journal of medical genetics, 57(9), 647–652. https://doi.org/10.1136/jmedgenet-2019-106476.

Kurzius-Spencer, Margaret & Pettygrove, Sydney & Christensen, Deborah & Pedersen, Anita & Cunniff, Christopher & Meaney, Francis & Soke, Gnakub & Harrington, R. & Durkin, Maureen & Rice, Sydney. (2018). Behavioral problems in children with autism spectrum disorder with and without co-occurring intellectual disability. Research in Autism Spectrum Disorders. 56. 61-71. 10.1016/j.rasd.2018.09.002.

Mpaka, D. M., Okitundu, D. L., Ndjukendi, A. O., N’situ, A. M., Kinsala, S. Y., Mukau, J. E., Ngoma, V. M., Kashala-Abotnes, E., Ma-Miezi-Mampunza, S., Vogels, A., & Steyaert, J. (2016). Prevalence and comorbidities of autism among children referred to the outpatient clinics for neurodevelopmental disorders. The Pan African medical journal, 25, 82. https://doi.org/10.11604/pamj.2016.25.82.4151.

Smith, M., Manduchi, B., Burke, É., Carroll, R., McCallion, P., & McCarron, M. (2020). Communication difficulties in adults with Intellectual Disability: Results from a national cross-sectional study. Research in developmental disabilities, 97, 103557. https://doi.org/10.1016/j.ridd.2019.103557.