4) The human brain has many millions of idiosyncratic failure modes. We all display hundreds of them. The psychological disorders that we know of are all extremely rare and extremely precise, so if you ever met two people with the same disorder it would be obvious. Named psychological disorders are the result of people with degrees noticing two people who actually have the same disorder and other people reading their descriptions and pattern-matching noise against it. There are, for instance, 1300 bipolar people (based on the actual precise pattern which inspired the invention of the term) in the world but hundreds of thousands of people have disorders which if you squint hard look slightly like bipolar.
This seems mostly believable, except often (not always, I suspect) people name disorders less precisely than this.
This seems mostly believable, except often (not always, I suspect) people name disorders less precisely than this.