Now that sums of much of ADHD experience. More seriously, there are indications that aspects of ADHD affect information processing in potentially beneficial ways. I have a hunch that given the overall detriment ADHD has to many life skills that ADHD genes would be selected along with genes for higher IQ as a counterbalance, with both combined together conferring unique advantages.
People with adhd are more likely than not-adhd to feel that they could have achieved better academic results than they did. Mensa is popular among people who value external metrics of intelligence, like academic results. So it seems plausible that people with adhd are more likely to feel they have “something to prove” which can be gained by joining Mensa.
Exactly. If they're intelligent but have ADHD they could feel insistently valued by the academic system and required additional validation via Mensa. ADHD by itself, as far as I know, isn't positively correlated with IQ.