At its core, ADHD's main mechanistic difference is undeniably an executive function deficit -- the frontal lobe not being activated enough to properly regulate functions of the rest of the brain or exert executive planning and follow-through.

The "paradoxical" effects of ADHD stimulant-type medications is the most direct indicator that this is indeed the case: taking a CNS stimulant normally causes more arousal, agitation, and activity in NT individuals.

But in ADHD individuals, CNS stimulants up-regulate frontal lobe activity, which manifests as a relaxant, since the frontal lobe is doing a better job of regulating and stabilizing the rest of the brain. Executive function also improves considerably.

So the most objectively measurable symptom of ADHD is whether this paradoxical behavioral alteration occurs from taking a CNS stimulant.

You can argue all you like about whether ADHD is within this or that percentile threshold -- but you can't argue with a proven, consistent, clinical result that points strongly to under-functioning of the frontal lobe in individuals with ADHD.

I've experienced this myself. My quality of life is profoundly different after taking Ritalin. I feel more relaxed; the buzzing and noise in my head is significantly reduced. I stop feeling the profound urge to get distracted like Doug the dog from the movie Up: ... "squirrel!"