Understanding Down Syndrome and Autism Spectrum: A Complete Guide

It is common for families and caregivers to wonder about their children’s development, and this may be especially true when a child is born with Down syndrome or receives a diagnosis of autism spectrum disorder. It’s not unusual to wonder if the two are the same, and if not, how they differ. In this article, we set out to explain what Down syndrome is, how it differs from autism spectrum disorder, and what it might look like when a child has a diagnosis of Down syndrome with co-occurring autism spectrum disorder.

What is Down Syndrome?

Down syndrome, also referred to as trisomy 21, is a genetic condition in which a person has an extra copy of chromosome 21. Down syndrome is the most common genetic chromosomal condition and occurs in roughly 1/775 live births in the United States (De Graaf, 2024), and 1/1000 live births worldwide. 

Clinical Characteristics

Down syndrome is the most common cause of intellectual disability, which is characterized by “significant limitations both in intellectual functioning and adaptive behavior as expressed in conceptual, social, and practical adaptive skills” and must occur originally during the individual’s developmental period (i.e., prior to age 22; Schalock et al., 2021). Limitations in intellectual functioning and adaptive skills has been further defined by the DSM-5:

1. Significant limitations in intellectual functioning

Individuals with intellectual disability have lower general mental abilities, which includes their abilities to problem-solve, learn from experience and from academic contexts, engaging in reasoning and abstract thought, as well as engaging in purposeful planning. Intelligence quotient (IQ) scores fall below a standard score of 70 (note that an “average” IQ score falls between 85-115).

• Mild intellectual disability: IQ score of 50-69
• Moderate intellectual disability: IQ score of 35-49
• Severe intellectual disability: IQ score of 20-34
• Profound intellectual disability: IQ score below 20

• Significant limitations in adaptive functioning

Individuals with intellectual disability have significant challenges with their adaptive functioning skills that limit their ability to be independent in society. To meet the diagnostic criteria for an intellectual disability, a person must have a deficit in at least one of the three areas, and challenges must be present across multiple environments (note that these adaptive functioning skills are mediated by a person’s culture): 

• Conceptual skills (such as language, literacy, math)
• Social skills (e.g., social interaction and participation, social communication)
• Practical skills (e.g., tasks of daily living)

 

Individuals with Down syndrome tend to have an intellectual disability in the mild to moderate range, and they tend to demonstrate variable cognitive abilities. Of note, those with Down syndrome tend to have challenges with verbal working memory, as well as a host of executive functioning skills like problem-solving, planning, inhibition/impulse control.

 

Physical Characteristics

Individuals with Down syndrome tend to have low muscle tone (i.e., hypotonia), hyperflexible joints, shorter limbs, and some distinct facial features. These facial features include a small chin, small mouth (from which the tongue seems to protrude), almond shaped eyes, and a fold of skin on the eyelid.

 

Social and Behavioral Characteristics

Children with Down syndrome tend to have relative strengths in their social interaction abilities compared to cognitive and communicative skills. They tend to be motivated to engage in social interactions and this is often evidenced by the frequency with which they approach others. With that said, however, research suggests that this may be related to their difficulty with impulse control. Additionally, it is suggested that those with Down syndrome demonstrate lower levels of emotional regulation and persistence to complete tasks that are challenging (Guralnick et al., 2014).

 

Communication Challenges

Individuals with Down syndrome tend to have more difficulties with expressive language compared to receptive language; however, challenges in receptive language are also noted. Sometimes individuals with Down syndrome are non-speaking, and others have reduced intelligibility, which may be in part due to the structural differences of the articulatory system or recurrent ear infections that are common in those with Down syndrome. Vocabulary development is delayed in children with Down syndrome; however, some evidence suggests that it follows the normal trajectory. Grammar, on the other hand, is an area of language that is primarily impacted in Down syndrome, in both expressive and receptive modalities. Comprehension of grammar is delayed, and individuals with Down syndrome produce sentences that are shorter and less complex compared to those without Down syndrome.

 

What is Autism Spectrum Disorder?

Recall from this blog post that autism spectrum disorder is a neurodevelopmental condition that is defined in the DSM-5 as a disorder that results in social communication and social interaction challenges across contexts. Specifically, according to the DSM-5, autism is diagnosed based on two separate, and often related, areas:

1. Differences in social communication and social interaction
   Individuals with autism have challenges using social communication skills to engage in social interactions. Oftentimes this can look like difficulties taking conversational turns, using and understanding body language or tone of voice, and making and maintaining friendships.
2. Restricted and repetitive behaviors and interests (RRBIs)
   Individuals with autism must also have RRBIs. This can include behaviors (such as movements like rocking or handflapping) and/or focused interests (dinosaurs, trains, Minecraft, Bluey, creeks, you name it!). Additionally, individuals with autism tend to prefer routines, and may also experience sights, sounds, and textures more intensely than others.

To receive an autism diagnosis per the DSM-5, an individual must meet both of the criteria outlined above, and these characteristics must be present in early childhood. However, it is also the case that characteristics of autism may not fully come online until the child gets older and experiences social challenges. Moreover, these challenges must impact the person’s ability to function in social or occupational settings, or other areas that are important, and must not be explained by an intellectual disability. Note, however, that a person can have autism and co-occurring intellectual disability.

Differences and Similarities Between Down Syndrome and Autism Spectrum Disorder?

There are many differences between a person diagnosed with Down syndrome compared to one diagnosed with autism spectrum disorder. The primary difference is that Down syndrome is caused by an extra chromosome and diagnosed using a medical test; whereas autism spectrum disorder is a neurodevelopmental condition that is diagnosed through a series of behavioral tests. Additionally, Down syndrome is associated with intellectual disability, and intellectual disability is not a diagnostic feature of autism; in fact, many people with autism do not have a co-occurring intellectual disability.

Feature	Down Syndrome	Autism Spectrum Disorder
Cause	Genetic (extra chromosome 21)	Neurological/developmental differences
Primary Features	Global developmental delay, intellectual disability, characteristic facial features, strong social interest	Differences in social communication, repetitive behaviors, sensory processing differences
Language Profile	Often delayed expressive speech and language but relatively good social interaction skills	Varies. Some highly verbal, others non-speaking; language differences often linked to social communication
Social Interaction	Typically seeks connection and social engagement	May prefer solitary play or connect differently
Medical Diagnosis	Chromosomal test (karyotype)	Behavioral and developmental evaluation

Can Someone Have Down Syndrome and Autism Spectrum Disorder?

Yes, it is possible to have Down syndrome and co-occurring autism spectrum disorder. Research suggests that roughly 16-18% of people with Down syndrome also have a diagnosis of autism spectrum disorder (Richards et al., 2015).

FAQs

Is Down syndrome the same as autism?
No, Down syndrome and autism spectrum disorder are not the same thing. Down syndrome is a genetic condition in which there is an extra copy of chromosome 21 and mild to moderate intellectual disability, and autism spectrum disorder is a neurodevelopmental condition characterized by challenges with social interaction/social communication and the presence of restricted and repetitive behaviors and interests.

Can someone have both Down syndrome and autism?
Yes, about 16-18% of people with Down syndrome also have a diagnosis of autism.

How can I support my child with Down syndrome and co-occurring autism spectrum disorder?

Children with Down syndrome and autism benefit from a team approach that includes the family, the child’s teacher, a speech language pathologist, and sometimes a psychologist, physical therapist, and/or occupational therapist. A speech language therapist will work on supporting language and communication skills, social interaction skills, and narrative discourse skills.

Will my child with Down syndrome and autism who is non-speaking learn to talk?

It depends–sometimes children who are non-speaking develop the skills to use verbal speech; other times they rely on augmentative and alternative communication (AAC) methods to communicate their wants, needs, thoughts, feelings, and stories. All types of communication modalities are valued and honored. Talk to a speech pathologist today to see how best to support your unique child in their communication needs.