ADHD and autism get confused for a reason that is not carelessness. They share traits, they often occur in the same person, and the language used to describe them overlaps enough that even an experienced clinician working from a short history can land on one when the fuller picture includes both. The terms have also shifted over time, with what was once called Asperger's now folded into the broader autism spectrum. Sorting out how the two relate, and where they part company, is one of the more useful things a careful assessment does.
Why are ADHD and autism so easily mixed up?
Look at the surface and the overlap is large. Difficulty with attention, restlessness, trouble reading a room, sensitivity to sensory input and a tendency to act before thinking can appear in both. A child who cannot sit still and interrupts constantly might be read as having ADHD when the driver is sensory overwhelm and difficulty with social rules. An autistic adult who masks well may be seen as merely distractible. The features are real in each case; what differs is the mechanism underneath, and mechanisms do not show up on a quick symptom checklist.
Can someone have both ADHD and autism?
Yes, and frequently. For a long time the two were treated as mutually exclusive in formal diagnosis, so people were given one label and the other was overlooked. That changed when the diagnostic manuals allowed both to be recorded together, and clinical recognition of the combination, sometimes called AuDHD, has grown since. The co-occurrence estimates vary widely depending on the population studied, but a substantial proportion of autistic people show ADHD traits and a substantial proportion of people with ADHD show autistic traits. Having both is not a contradiction, and it is not a sign that one diagnosis was wrong.
The question at assessment is rarely which one it is, but how much of each is present and how they interact.
What actually separates the two?
The clearest dividing lines sit in social drive and the shape of attention. Autistic difficulty in social situations tends to be about a different way of processing communication, social rules and reciprocity, often present from early childhood and consistent across settings. ADHD social difficulty tends to come from impulsivity and inattention: missing cues because attention wandered, interrupting because the brake came late. Repetitive behaviour, intense focused interests and a strong need for predictability point towards autism. Restlessness that is driven by a need for stimulation, and attention that varies wildly with how interesting the task is, point towards ADHD. The fuller comparison of where ADHD and autism are alike and where they differ is worth reading alongside this.
Why does misdiagnosis happen so often?
Several reasons stack up. The traits overlap. Masking, more common in girls and women and in adults, hides the autistic features that would otherwise prompt the right question. A short appointment captures the loudest presenting feature, which is often the restlessness or inattention that reads as ADHD, while the quieter autistic profile underneath goes unremarked. And because the two were historically separated, clinicians trained in one era may not look for the second once the first is found. The cost of getting it wrong is real: support built for one neurotype can miss the mark when the other is the larger part of the picture.
How does an assessment tell them apart?
By taking time and taking a history. A careful autism assessment gathers a developmental account from early childhood, looks at consistency across different settings, and weighs the social, sensory and repetitive features against the attention and impulse features. It does not rely on how a person presents in a single conversation, because that snapshot is exactly what gets people misdiagnosed. Where both neurotypes appear to be present, a good assessment says so rather than forcing a single label. Knowing both are there changes what support actually helps.
Does the diagnosis you get change the support?
It does, which is why precision matters. Strategies that suit an ADHD profile, built around stimulation, structure and momentum, can clash with what an autistic person needs, which often centres on predictability, reduced sensory load and lower social demand. When both are present, support has to hold two sometimes-competing needs at once: novelty and routine, stimulation and calm. Getting the diagnosis right is the difference between a plan that fits and a plan that quietly works against itself.
What should I do if I think I have been misdiagnosed?
Raise it specifically. If you were diagnosed with one and the other has never been properly considered, that is a fair thing to ask about. Bring concrete examples across your life, ideally going back to childhood, because both conditions are developmental and the early history is part of the evidence. You can request a fuller look through your GP, and a private route is available too. You can book an appointment to discuss which assessment fits your situation. A reassessment that finds both is not a failure of the first diagnosis; it is a more complete answer.
Why does masking complicate the picture so much?
Masking is the effort of hiding autistic or ADHD traits to fit in, and it is one of the biggest reasons the right diagnosis gets missed. A person who has spent years rehearsing eye contact, scripting conversations and suppressing the urge to fidget can present, in a short appointment, as someone with no difficulty at all. The cost is paid privately, in exhaustion and burnout after the performance ends. Masking is more common in girls and women and in adults who learned early that standing out brought trouble, which is part of why both conditions are under-recognised in those groups. A good assessment asks about the effort behind the presentation, not just the presentation itself, because what it takes to look fine is often the most telling evidence of all.
What this means in practice
- ADHD and autism share traits and often co-occur. Having both is common and is not a sign of error.
- The differences sit in social drive, the shape of attention, and the presence of repetitive behaviour and focused interests.
- Misdiagnosis is common because traits overlap, masking hides autistic features, and short appointments catch only the loudest one.
- A careful assessment uses a full developmental history rather than a single snapshot, and names both where both are present.
- If you suspect a missed second diagnosis, raise it specifically and bring examples from across your life.
If you recognise yourself in this and the label you were given has never quite fitted, that gap is worth taking seriously. The diagnostic picture for both conditions is well described in the clinical literature, including NICE guidance on autism in adults, and a proper assessment can work out how much of each is in play. Getting that right is what turns a partial answer into a plan that actually fits how your brain works.
