Personally, I suspect it's autism rather than ADHD (and Inattentive-type ADHD, AKA: ADD). In my experience the people I've known who have just ADHD tend to be better at filtering out external stimuli than those I've known with Autism. And likewise the folks I've known who have both ADHD and Autism tend to have a harder time filtering out stimuli, or getting distracted by particular stimuli, than those with just ADHD.

That said, my own personal (and very unscientific) theory is that ADHD and Autism are merely differing manifestations of the same neurological phenomenon. So I could see that somebody who has ADHD might be more sensitive than a neurotypical person.

Doesn't autism have a whole set of extra social issues like not being able to understand emotions of others?

One of the things I've picked up reading HSP literature is that the human brain regresses when pushed past its limits. You go into fight, flight, or freeze, and from there everyone starts to look the same - like a petulant toddler. Part of self care is giving yourself permission not to torture yourself. To wear earplugs, prefer quiet restaurants, to ask the host of a party to please stop burning incense like it's Christmas Mass, or to just give yourself permission to pop in and then leave after an hour.

There's overlap between HSP and alcoholism as well. Needing to get smashed at a party in order to 'relax' is more than just decompressing after a hard workweek. Alcohol dulls the senses.

ADHD types can tune things out to pathological levels when hyperfocusing on whatever dopaminergic activity they want to do, the problem is it's about the opposite when when it is something without the interest or reward present. In both cases it can be argued a failure of executive functioning is responsible.

> ADHD and Autism are merely differing manifestations of the same neurological phenomenon.

I struggle to imagine how you connected those dots (at least without omitting many other dots that we know connect to one condition but not the other)

It isn't though. Sensory processing also shows up on the Autism spectrum (famously, even), but also in people who have neither. The latest numbers have upward of 30% of people classed as Highly Sensitive (HSP), and 20% as low sensitivity. Most HSPs self-identify as 'introverts' unless some other label sticks first.

So the dynamic I propose is that an HSP is going to out themselves for any other ways they're neurodivergent. ADHD people getting distracted by every background noise, every smell. Asperger's people hitting their limits and flaming out, and ADHD people hitting their limits and flaming out, (in roughly chronological order).

I suspect we will find the Venn diagram of HSP and adult diagnosis of ADHD/autism is going to be thin compared to child diagnosis (and unfortunately mostly female, because there's still a lot of internalized sexism around well behaved girls). Few have the coping mechanisms to escape notice.