There is a range of psyches that has been favored by natural selection, maybe somewhat different in different parts of the world. Some people are outside that set: their minds are different, different in ways that were never favored by Darwinian selection*. Now some traits we don’t like are probably in the set favored by natural selection: some kinds of unpleasantness probably work, at least at low frequencies. Sociopaths might fall in this category. There certainly are species with adaptive genetic behavioral variation – alternate male morphs, for example.
Moreover, standard psyches have their limitations. There are tasks that normal humans are bad at – think of optical illusions, or the Monte Hall problem. Or making sense.
Then there are special cases where people with psyches that clearly are out of whack are particularly good at some task, like bipolar poets.
Does this mean that we should say that having a Darwinian mental disease is hunky-dory? Should we refrain from trying to develop treatments or cures? Should we consider all possible mental states equally ‘valid’ ( whatever that means) ?
Of course not: only a loon would believe that. Being a normal human being isn’t a panacea, but adding gobs of genetic noise (or environmental insults like prenatal rubella or cytomegalovirus) isn’t going to make things better. That pattern at least survived all the tests of the past.
Having an accurate evaluation of a syndrome as a generally bad thing isn’t equivalent to attacking those with that syndrome. Being a leper is a bad thing, not just another wonderful flavor of humanity [insert hot tub joke] , but that doesn’t mean that we have to spend our spare time playing practical jokes on lepers, tempting though that is.. Leper hockey. We can cure leprosy, and we are right to do so. Preventing deafness through rubella vaccination was the right thing too – deafness sucks. And so on. As we get better at treating and preventing, humans are going to get more uniform – and that’s a good thing. Back to normalcy!
* Their minds ain’t right.