When people say "I don't believe in ADHD" I point out it's not like believing in Santa Claus, the reality isn't changed by our "belief". Might as well say "I don't believe in cancer", it's not going to make it disappear.
The fact is that ADHD has been shown to affect 4 to 5% of American adults and 5 to 8% of children. When the same diagnostic criteria are used, populations of other countries show comparable prevalence. This means it is a much more common disorder than usually considered.
The key issue in my estimation is the level of disability ADHD can produce in adults. Studies show devastating adverse outcomes in virtually all domains of functioning. People with ADHD fall far behind peers in educational achievement and employment, and far exceed peers in rate of divorce, motor vehicle accidents and injuries, greater rates of general health conditions and larger health care costs.
Furthermore, despite the availability and relatively low cost of effective treatments, among adults with ADHD only about 1/5 receive treatment. The total costs to the economy of untreated ADHD very likely is on the order of $100 billion annually in the US alone.
Bottom line is ADHD is a very common and disabling condition with a high cost burden to the individual, family and nation as a whole. We underestimate the impact of ADHD in all these domains at our peril.