Autism, ADHD, and Down Syndrome: What the Research Shows
Down syndrome, autism, and ADHD can each occur on their own, but they also co-occur more often than many families realize. Understanding this overlap helps ensure a child or adult gets support for everything actually going on, not just the diagnosis that was identified first.
Down syndrome and autism together
Autism is recognized as one of the conditions that can co-occur with Down syndrome, and research suggests the overlap is meaningful enough that clinicians working with Down syndrome should actively screen for autism rather than assuming behavioral or communication differences are fully explained by Down syndrome alone. Some research has found that autism symptoms in people with Down syndrome are closely associated with co-occurring intellectual disability specifically, though scientific understanding of exactly how these three things interact is still developing, and more research is genuinely needed.
Why autism can be harder to identify alongside Down syndrome
Some traits associated with Down syndrome, certain social and communication patterns, repetitive behaviors, can overlap with traits used to identify autism, making it harder for clinicians to separate the two without specific training in both conditions. This is part of why a missed or delayed autism diagnosis is a real risk for children who already have a Down syndrome diagnosis; professionals and families alike may attribute every difference to Down syndrome alone, when autism may also be present and would benefit from its own specific supports.
Signs that may indicate autism alongside Down syndrome
- A significant loss of previously gained skills (regression), rather than a gradual, expected developmental pace
- Repetitive behaviors or intense, narrow interests that go beyond what is typical even accounting for Down syndrome itself
- Social communication differences that seem more pronounced than what’s typically expected with Down syndrome alone
- Significant sensory sensitivities or sensory-seeking behavior
None of these signs alone confirms autism, but if you notice a pattern, it’s worth specifically requesting an autism-focused evaluation, separate from Down syndrome’s own developmental monitoring.
Getting an accurate evaluation
Look for an evaluator with specific experience assessing autism in children who also have Down syndrome or another intellectual or developmental disability, since standard autism assessment tools were not always developed or validated with this specific population in mind. A general developmental pediatrician with broad experience across both conditions is often a strong starting point if a specialist combining both isn’t immediately available.
Why getting both diagnoses right matters
A child with both Down syndrome and autism may benefit from supports specifically designed for autism, structured communication strategies, sensory accommodations, predictable routines, in addition to supports already in place for Down syndrome. Missing the autism piece means missing an entire category of support that could genuinely help.
This is true for ADHD too
ADHD is also recognized as occurring at meaningfully higher rates in people with Down syndrome than in the general population. The same principle applies: attention, impulsivity, or activity-level differences shouldn’t be automatically attributed to Down syndrome alone without considering whether a separate, treatable ADHD diagnosis might also be present.
Key words to know
Down syndrome (Trisomy 21): A genetic condition caused by an extra copy of chromosome 21, associated with a distinct set of physical and developmental characteristics.
Regression: A meaningful loss of previously gained skills, which can be a signal worth evaluating further rather than simply expected developmental variation.