I self-diagnosed Aspergers in the mid 2000s. The primarily inattentive subtype of ADHD is a better match for my symptoms, particularly the social ones, but both my psychiatrist and neuropsychologist agreed that my self-diagnosis made sense and I have traits of both disorders. They are very often comorbid. I just wasn't familiar enough with the diagnostic criteria of ADHD to realize I had it. (A roommate with ADHD that I had at the time suggested it, though.)
Also, ADHD is in fact under-diagnosed, particularly in adults and especially in women.
I don't mean to dismiss everyone who self-diagnoses. I was pointing out a known social phenomenon or trend of people doing so, possibly because of greater public awareness of the condition. Also see gluten intolerance for a non-psychological example.
Funnily enough, I chanced upon a random YouTube video yesterday and the very top comment was someone mentioning ADHD and others chiming in, while skeptics criticizing the self-diagnosing and likening it to those who self-diagnose other conditions such as depression:
Whether right or wrong, it definitely is a known internet social phenomenon. That's what the ancestral post in this current subthread was referring to by referring to ADHD as the new Asperger's.
Also, ADHD is in fact under-diagnosed, particularly in adults and especially in women.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4195639/
Some children are overmedicated, but there's evidence it's also underdiagnosed in children.