I mean ADHD, Autism, Dyspraxia, etc. have a specific set of symptoms and specific treatments, and a large part of the population has those (as well as other mental disabilities like MDD, Bipolar, etc.). Many psychology researchers tackling the subject find that ADHD is severely underdiagnosed in the population (despite popular uneducated belief being that ADHD is overdiagnosed due to misinformation being widely spread on TV shows in the 2000s), with around 20% of people likely meeting the criteria for an ADHD diagnosis (diagnosis rates are usually in between 3% and 10%).

It’s postulated that the high occurence of ADHD compared to other disorders comes from our days as hunter-gatherers – then, many of the behaviours of ADHD would have been extremely helpful, such as high alertness/awareness of changes in the environment such as sound cues and slight visual changes, and impulsiveness/drive to be active to seek out berries and prey and such, making a decent portion of members of a group having ADHD be a huge benefit and boost to survival rates. But most of those useful effects have become quite useless in modern society, and many of the symptoms (like dysfunctional working memory & inattentiveness) have become a massive detriment under industrialism. It is likely that in pre-industrial/medieval society ADHD was still a net benefit, at least according to what little we can ascertain from it there.

Genes contributing to Autism Spectrum Disorder have also been positively selected for even since before humans came about, since they also brought benefits throughout primate evolution.

You can really take a lot of common mental disorders and find some sort of evolutionary reason for it; even mood disorders, anxiety disorders, insomnia disorders, schizophrenia, etc. would have had some potential benefits to prehistorical human groups.

Fun fact, there was a study done on prison populations in Estonia and it was determined that 40% of prisoners had undiagnosed ADHD – the symptoms of ADHD are kind of contrary to the core principles of being a capitalist worker, often with it being mistaken for “laziness” or “lack of motivation” (qualities which bring shame and have, for most of modern history, gotten you shunned from society), so they have a much higher likelihood of falling behind in life without proper treatment.