Autism and Down syndrome are different developmental conditions. Down syndrome is a chromosomal condition, most commonly caused by an extra copy of chromosome 21. Autism spectrum disorder (ASD) is a neurodevelopmental condition defined by patterns of social-communication differences and restricted or repetitive behaviours/interests.
A person can have one, the other, or both. Comparing them should focus on understanding support needs rather than ranking abilities or predicting an individual’s future from a diagnosis.
At a glance
| Point | Autism | Down syndrome |
|---|---|---|
| Primary basis | Neurodevelopmental diagnosis | Chromosomal/genetic condition |
| Diagnosis | Developmental history and behavioural criteria | Chromosome testing can confirm diagnosis |
| Physical appearance | No single characteristic appearance | Often associated with recognizable physical features, though individuals vary |
| Medical associations | Can include epilepsy, sleep, GI, mental-health and other co-occurring conditions | Higher rates of certain heart, hearing, thyroid, vision and other medical conditions |
| Communication profile | Highly variable; spoken language ranges widely | Speech/language development may be delayed; receptive/expressive profiles vary |
| Can they co-occur? | Yes | Yes; a person with Down syndrome can also be autistic |
Autism
A neurodevelopmental condition characterized by persistent social-communication differences and restricted/repetitive patterns of behaviour, interests or sensory responses, with wide variation among individuals.
Down syndrome
A genetic/chromosomal condition caused by extra chromosome 21 material, most commonly full trisomy 21.
Why autism can be missed in Down syndrome
Developmental delays may initially be attributed entirely to Down syndrome. Autism evaluation becomes important when the person’s social communication, sensory profile, repetitive behaviours or developmental trajectory suggest needs beyond what is expected from their existing diagnosis.
Assessment should be performed by clinicians familiar with both conditions.
Individual variation is large
Neither diagnosis tells you exactly how a child or adult will communicate, learn, live independently or participate socially. Support needs can range from relatively limited accommodations to substantial lifelong assistance.
Strengths are equally individual: people may have strong visual learning, memory, social motivation, focused interests, practical skills, humour, creativity or other abilities.
Support should follow needs
Useful supports can include speech-language therapy, occupational therapy, educational accommodations, augmentative communication, behavioural/mental-health support and appropriate medical monitoring.
The best plan starts from the person’s communication, sensory, health, learning and daily-living needs rather than from stereotypes.
Important note
Use person-preferred identity language. This page is educational and not a substitute for developmental or genetic assessment.
Frequently asked questions
Can a person have both autism and Down syndrome?
Yes.
Is Down syndrome a type of autism?
No. They are different conditions.
Can autism be diagnosed with a blood test?
No. Autism is diagnosed clinically from developmental and behavioural information.
Does Down syndrome always cause intellectual disability?
Most people have some degree of intellectual disability, but abilities and support needs vary widely.
Sources and further reading
KnowDifferences Editorial Team
Independent explanations with definitions, practical examples and references. Read our editorial approach.