Autism and Down syndrome: similarities and differences

Autism and Down syndrome are separate conditions, but it is possible for a person to have both.

If you are reading this, you may know someone with Down syndrome or someone with autism. You may even know someone with a dual diagnosis of both.

Autism Spectrum Disorders (ASD) and Down Syndrome (DS) are both complex and lifelong. Although they share some characteristics, they have more qualities that set them apart.

ASD and DS are not the same conditions, but they can occur together. About 20% of people with Down syndrome also have autism.

There are distinct and key differences between autism and Down syndrome.

Autism Spectrum Disorders

Autism is a neurodevelopmental difference that can affect a person’s abilities in language, social interaction, and behavior. People with ASD display a wide range of traits and can look completely different.

The Centers for Disease Control and Prevention (CDC) estimate that approximately 1 of 44 children are autistic.

Doctors identify autism by observing and evaluating a child’s behavior and developmental history. It is possible to spot ASD behaviors in someone as young as 2 years old, but some autistic people are not identified until they are older or even adults.

It is usually impossible to tell that a person has autism just by looking at them.

Down syndrome

Down syndrome is a chromosomal disorder. It is the most common genetic disease in the United States, affecting approximately 1 in 700 babies born.

Medical tests called karyotypes can check for chromosomal differences associated with DS.

It is possible to diagnose DS before birth using a prenatal karyotype test. A doctor can also identify DS features at birth during an exam and confirm with a blood test.

Several factors can contribute to the cause of autism, including:

  • genetics, both genetic mutations and family history
  • environmental factors, such as exposure to pesticides during pregnancy or birth trauma that limited the baby’s oxygen supply
  • biological factors, such as infection during pregnancy and inflammation

Meanwhile, Down syndrome is a chromosomal disorder. It occurs more frequently in pregnancies with mothers aged 35 or older.

There are three types of DS:

  • Trisomy 21. Each cell has a third copy of chromosome 21 (usually each chromosome has only 2 copies).
  • Mosaic Down Syndrome. Some cells have 3 copies of chromosome 21, and other cells have only 2 copies.
  • Down translocation syndrome. It is when an extra piece or an extra whole copy of chromosome 21 is attached to a different chromosome.

Autism and Down syndrome have common characteristics. In other respects they are quite different.

For example, many people with autism prefer to keep to themselves. They may appear withdrawn or indifferent. On the other hand, people with DS are often social and friendly.

Many people with autism do not follow a usual pattern of language learning. Some never speak. Others learn to speak and then lose language development.

Meanwhile, the language development of people with DS is similar to that of typically developing children.

Other differences include:

Autism exists on a broad spectrum and has various behavioral expressions. For example, some people with autism use gestures, have common language skills, and enjoy spending time with friends.

DS is also likely to cause individuals to learn and progress more slowly. ASDs include a wide range of intellectual abilities – from severely retarded to gifted or higher levels of intelligence.

Traits shared by ASD and DS include:

  • preference for routine
  • less sensitive to the sound of their name
  • expressive language differences
  • atypical eye contact
  • sensory differences
  • repetitive play
  • challenging behaviors
  • targeted interests
  • anxiety
  • developmental differences
  • reduced reciprocal conversation

Autism does not affect a person’s appearance. However, DS causes recognizable physical changes, including:

  • eyes slanted upwards
  • short neck with extra skin on the back
  • little head
  • small ears and mouth
  • white spots on the iris of each eye
  • reduced muscle tone
  • flat facial features
  • broad, short hands, with short fingers

ASD and DS usually occur with health issues. The degree to which they affect each person can vary.


  • seizures
  • gastrointestinal problems
  • sleep disorders
  • pain threshold differences
  • metabolic differences

Down syndrome:

  • congenital heart defects
  • cataracts and poor vision
  • hearing loss
  • hip problems
  • leukemia
  • obesity
  • hypothyroidism
  • constipation
  • Sleep Apnea
  • susceptibility to infections
  • dementia
  • Alzheimer’s disease

The goal of autism treatment for children and adults is to support each individual to facilitate daily living. For some people with autism, this may be speech therapy or social skills training. Others might benefit from tutoring or physical therapy.

Sometimes doctors prescribe medications to treat co-occurring conditions like attention deficit hyperactivity disorder (ADHD) or mood disorders.

Like treatment for autism, treatment for Down syndrome is tailored to the needs of the individual. Many support services are similar to those provided for autism, but with different goals.

For example, a speech therapist can help a child with autism put words together into complete sentences. For a child with Down syndrome, the emphasis might be more on pronouncing these words clearly.

In addition to the types of healthcare and education that Down syndrome shares with autism, DS can also lead to significant medical needs, such as heart defect surgery and cancer treatment.

Many people with autism and people with Down syndrome lead busy lives. Both diagnoses are common enough that a wealth of knowledge and support is available to individuals and families who need it.

The Autism Society is a starting point for information and education. The National Down Syndrome Society offers helpful resources and programs. The Disability Advocacy and Education Fund can help with civil and human rights advocacy, as well as education and public policy.

Autism and Down syndrome can occur together, but because autism is diagnosed by observing behavioral differences, it may not be as noticeable in a young child with Down syndrome.

If your child with DS seems less social than others, a doctor can advise you on the possibility of a dual diagnosis ASD and DS.

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