> In the general case, having delusions and other psychotic issues are not selected for traits with positive outcomes for the individuals having them. Your life is hard if you have them and conventional success is rare.

It varies per delusion - for example, most successful people suffer from the delusion that they have ~omniscient level knowledge of what's going on - if it were not for this misplaced confidence within human consciousness, I suspect it would be much more difficult to get things done (though, a lot of the dumb stuff that's done would get avoided as well, so it's a tough call what's net best).

Here's a lecture by Robert Sapolsky where he goes into some theories involving schizotypal individuals (and other mental disorders) and culture: https://www.youtube.com/watch?v=4WwAQqWUkpI

Long story short, the theory goes that being schizotypal (but not full-blown schizophrenic) could be an advantageous trait in human society, usually by filling some spiritual, ritual, or religious role in the group. These people are high status in their societies, and have many chances for passing on their genes.

Yep, that's exactly the lecture I had in mind for my original comment. I decided to shorten down to "no one is sure what the advantage is" rather than try to both explain Sapolsky's conjecture and not mislead people into thinking its an established scientific fact.

> We have lots of examples of this in other heritable disease types

You're saying that diseases persist in humans because they're selected for, because they're beneficial to others? That's interesting, do you have some examples?

I would phrase it as the genes are selected for, not that the diseases are selected for, but yeah you understand correctly. Others have already filled in examples like you asked for.

sickle cell anemia is the classic example; one copy of the gene gives you malaria resistance, two copies gives you the disease