One key source of confusion when reading about ADHD is that researchers on the cutting edge of ADHD have pretty much concluded that there are multiple types of "ADHD" and that the label is overused. Unfortunately no one can agree on a proper labels to use instead...

What this means is that you get a lot of studies that have conclusions such as "well this drug worked for 70% of the test subjects but danged if we know why the other 30% were unresponsive, after all, they have been diagnosed with ADHD!"

If you really want to dive deep, I suggest this talk by Russell Barkley, https://www.youtube.com/watch?v=q3d1SwUXMc0, it does an excellent job of covering why the hell the current literature on ADHD is such a mess.

He also provides lots of compelling evidence for ADHD, but also discusses various symptom (and treatment!) clusters that defy explanation if you think of just "ADHD". There is also discussion different causes of ADHD, suffice to say the symptom rate for children born to mothers who drink during pregnancy is, to say the least, way above the norm, with ADHD in those cases being co-morbid with other disorders[0]. Finally there is some slides about genetics as well.

On a related note, it is funny, the term ADHD-PI is being used by some people to basically describe an entirely different disorder with a completely different treatment mechanism. Stimulants do nothing, but there are treatment programs that do help. On ADHD support forums, those of us who are regular contributors try to do our best to educate ourselves about ways to help people who both have "ADHD" and also "ADHD-PI".

As for the entire "ADHD isn't real" crowd, well, those of us who have it get rather pissed off. I have been reduced to tears because I wanted to do something with every fiber of my being but was unable to focus on it for more than seconds at a time.

Even with a well tweaked treatment regimen, ADHD is still a daily struggle. It is a literal fight to maintain control of myself, from pushing back against a constant all consuming addiction to anything that resembles a dopamine reward cycle[1] to having a reserve of will power[2] that resembles a water pump sitting out in the dry desert heat.

One thing people without ADHD don't realize is that for those with ADHD, the drugs work very differently. Someone without ADHD can take ADHD stimulants and stay up for 2 days straight and have razor focus. Someone with ADHD who takes the same meds, well, going back to my previous metaphor, it is like priming the pump, even with drugs it still takes a slew of self control techniques learned through years of therapy to maintain control over my life.

To live my daily life, I carry an organizer around wherever I go that I write everything down in. I then have calendar events for everything I need to do. On top of that I have my life carefully arranged such that everything I need to do is within instant quick reach, because if I have to spend even a modicum of effort to achieve what I want to do, I will get distracted on my way and forget what the hell I am after.

Thankfully little details help establish some sort of normality. My keys, wallet, laptop, badge, glasses, notebook, all sit next to each other stacked in a nice pile that I can put into/on my pockets/back/face within seconds. If I get interrupted, I will forget one of those.

Side effects of this? I pretty much have to live close to work because I'll forget my laptop at home roughly 60% of the time. (I don't carry my laptop home every day so I do not have the same years of routine built up that allow me to remember the other things I need.)

Heck yesterday I went to a friend's house to make waffles and I forgot the bloody waffle mix. (Sorta the one thing required to make waffles.)

Any one set of these symptoms in of itself is not unusual. Some people have a poor memory, meh. Some people are easily distracted, sure. But when a cluster of symptoms all comes together and has a serious negative impact on the quality of someone's life, then there is a real problem that needs investigation and help.

I guess what it comes down to is, I've both struggled with ADHD myself, and seen undiagnosed, untreated, ADHD completely destroy people's lives.

Finally, there are a variety of diagnostic criteria that can be used for testing ADHD. A full suite of psychological evaluation tests will include sections on attention and focus. People with ADHD will basically fail bomb those parts of the test. (Said tests tend to take multiple days and cost thousands of dollars to administer...)

[0]ADHD's co-morbidity with bi-polar disorder is tragically high. Thankfully this is a topic which has recently gotten a lot more awareness in the medical community, so patients are finally being properly diagnosed. Of course there is a huge stigma around bi-polar disorder, much more so than ADHD, so a lot of people who are willing to admit that they have ADHD may very well turn away or refuse to believe a diagnose of bi-polar. This typically doesn't end well...

[1]I avoid MMOs and mobile gaming like the plague. I've gotten almost addicted to MMOs a couple of times, thankfully both times I was able to write a bot to play for me and then lost interest.

[2]http://www.wired.com/wiredscience/2012/10/mf-willpower/