Why the symptoms were picked out and given a name
Professionals used to believe ADHD was something children grew out of, but not anymore.
ADHD has always been strongly related to school performance. If a child is not focused on school, and seems unwilling or unable to concentrate, everyone tends to think of ADHD as the cause.
This makes me think that ADHD was "discovered" as an explanation to why some kids are not so good at school.
If you look at the symptoms for ADHD, you'll notice that each of them is completely normal behavior, but every one of them reduces performance at school.
Do the symptoms for ADHD cluster?
Yes and no. They cluster in more than 1 cluster.
Studying personality psychology, I have noticed a growing number of completely normal personality traits that remind me alot of ADHD.
A stereotypical boy with ADHD (restless, fidgety, takative, forgetful, unorganized) would easily remind you of the MBTI personality type ESxP.
(ESFP: http://www.16personalities.com/esfp-personality ESTP: http://www.16personalities.com/estp-personality )
A stereotypical girl with ADHD (silent, daydreaming, inattentive) would remind you of the MBTI personality trait INxP. (INTP: http://www.16personalities.com/intp-personality INFP: http://www.16personalities.com/infp-personality )
Girl's ADHD: http://www.addvance.com/help/women/daydreamer.html
Also note how the "boy's ADHD" and "girl's ADHD" are pretty different. The only common traits are poor attention and organization at school.
The typical ADHD behavior also matches well with several parts of the Big 5:
Substantial effects emerged that were replicated across samples.
First, the ADHD symptom cluster of inattention-disorganization was
substantially related to low Conscientiousness and, to a lesser
extent, Neuroticism. Second, ADHD symptom clusters of
hyperactivity-impulsivity and oppositional childhood and adult
behaviors were associated with low Agreeableness.
Although previous research on personality and ADHD has focused
primarily on extraversion and neuroticism, the present study found
that agreeableness and conscientiousness were stronger predictors.
This pattern of results is consistent with the clinical literature on
adults with ADHD.
Conscientiousness is what measures your ability to stay focused (and more), and neuroticism measures your patience (and more). And those are exactly what primarily defines the J/P factor in MBTI - and ADHD.
You're a P? Congratulations, you may have ADHD. You're a J? Nope, no ADHD there.
Additionally, several of the required symptoms for ADHD do not have any fixed requirement for frequency - they are relative.
When each of these symptoms are completely normal at the same time, the whole diagnose becomes relative - Are you too unfocused to function at school, or do you manage to get your homework done despite of poor attention?
Because nobody will diagnose you with ADHD as long as you are able to cope and function with school and everyday life.
A bit of the problem with the ADHD diagnosis is how there are no exact measurements that can tell whether or not an individual is "suffering".
Having a hard time in everyday life is also one of the requirements to have a diagnosis, but this is more relative than all the other symptoms.
Considering how modern societies are increasingly demanding for the individual, and Conscientiousness is -the- strongest predictor for academic success, there is bound to be a growing gap between the most and least successful individuals.
The least successful individuals are those who struggle to stay focused, and some of these seek psychological help for it. "You struggle at school because you have trouble staying focused? Here, take this test. Yes, I can see that you have poor attention. You have ADHD"
To put it short: ADHD symptoms are all about academic capability