Executive function
Starting, switching, sequencing and finishing all wobble in both conditions, for different reasons: interest-led attention on the ADHD side, transition cost and overwhelm on the autistic side.
Part 2 of 4
The genuine difference: the engine
Strip the shared symptoms away and the engines run opposite. ADHD is stimulation seeking: novelty, urgency and interest decide what is possible today. Autism is predictability seeking: sameness, depth and known patterns make the world tractable. One system asks what is new; the other asks what is safe.
That is why the same behaviour can mean opposite things. Two people decline the party: one because the group is unbearably understimulating, one because it is unbearably overstimulating. Two people build a morning routine: one abandons it in nine days from boredom, one defends it like infrastructure. Diagnosis is about the why, which is why questionnaires alone mislead.
When both engines share one nervous system, they argue, and the argument is the AuDHD experience: needing the routine and starving inside it, chasing the new thing and grieving the sameness it broke. The AuDHD explainer and the signs guide describe that collision in detail.
Part 3 of 4
Why the system split them, and what it cost
Until 2013 the diagnostic manual barred clinicians from diagnosing ADHD in an autistic person, treating attention problems as a feature of autism. The fifth edition removed the exclusion, and dual diagnosis went from forbidden to routine inside a decade.
The cost of the ban is still walking around: adults assessed before 2013 were stopped at one label, usually whichever presented loudest that year, and many were then treated for half a picture. Stimulants steadied attention while sensory life stayed unexplained; autism support built routines the ADHD side then burned down.
If you hold one diagnosis and the other half keeps not adding up, that is a legitimate reason to reopen the question, and it is worth saying exactly that to the assessing clinician. The assessment guide covers who can do this in your state, and the cost guide what it costs.
Part 4 of 4
A note on language, and what to do next
Some people say autistic person, some person with autism; some claim ADHD as identity, some treat it as a diagnosis they carry. This site uses the identity-first forms most Australian community organisations use, and none of it is a rule. People get to name themselves.
What to do with all this: if the shared ground rang true but you cannot place the engine, take the recognition tool, which sorts thirty-two experiences into ADHD flavoured, autistic flavoured and common to both. Free validated screeners, one step firmer, are hosted at Embrace Autism and linked from the resources page.
And if both engines sound like yours, say so before booking anything: an assessment that considers both at once costs less and misses less than two assessments a year apart.
Does this page sound like you?
The recognition tool sorts thirty-two of these experiences into ADHD flavoured, autistic flavoured and common to both, and shows where your ticks sit. Two minutes, nothing leaves your device, and it is a mirror, not a test.
The short version
Quick answers
How can I tell if I have ADHD, autism or both?
You mostly cannot from the inside, because the overlap is large and the differences are about why a behaviour happens, not what it looks like. Screeners help sort signal, and an assessment that considers both conditions at once is the reliable answer. Telling the assessor you want both considered is the single most useful sentence.
What do ADHD and autism have in common?
Executive function struggles, sensory sensitivity, social exhaustion, masking, time blindness in practice, and burnout. This shared territory is why the two get mistaken for each other and why one diagnosis so often arrives years before the other.
What is genuinely different between them?
The engine. ADHD chases stimulation: novelty, urgency and interest set the agenda. Autism seeks predictability: routine, depth and known patterns make the world manageable. Where one person abandons the routine from boredom and another defends it from chaos, the outward behaviour can even look the same while the reason is opposite.
Why was a dual diagnosis impossible before 2013?
The diagnostic manual explicitly barred diagnosing ADHD alongside autism, treating attention problems as part of autism. The fifth edition removed the exclusion in 2013. Anyone assessed before then was forced into one box, which is a large part of why so many adults are now collecting the second half of their answer.