How well defined is ADHD?

I’ve long had at­ten­tion and fo­cus prob­lems, but never ex­plored the pos­si­bil­ity that I have ADHD till re­cently. I un­der­stand that it’s a stan­dard term, but I’m still a bit sus­pi­cious; Psy­chi­a­try doesn’t seem like the most re­li­able field.

Are there good rea­sons for pick­ing out the be­hav­iors as­so­ci­ated with ADHD and giv­ing them a name? Ob­vi­ously this is a differ­ent ques­tion than whether ADHD is a ‘di­s­or­der’ or ‘dis­ease’, and whether ADHD med­i­ca­tion is good or bad for peo­ple.

An­swers that would satisfy me

  • Be­hav­iors as­so­ci­ated with ADHD strongly cluster

  • An­a­lyz­ing ques­tion­naires of at­ten­tion and fo­cus be­hav­iors with fac­tion anal­y­sis nat­u­rally pro­duces an ‘ADHD di­men­sion’ that ex­plains a lot of var­i­ance (similar method­ol­ogy to iden­ti­fy­ing Big 5 per­son­al­ity traits).

  • ADHD di­ag­no­sis a strong pre­dic­tor of any­thing in­ter­est­ing (in­come or grades or some con­trived but in­ter­est­ing lab test)?

  • Some­thing else along these lines.