[previously in sequence: Taxometrics, Dynamical Systems. Epistemic status: speculative. This should go without saying, but when I talk about “failures” in this post, I mean failures of biological processes, as in the term “congestive heart failure”; I don’t mean to accuse people with psychiatric conditions of being failures.]

I.

Most psychiatric disorders are at least partly genetic. Some, like schizophrenia and ADHD, are very genetic, probably 80% plus. This is strange, because having psychiatric disorders seems bad, so you would expect evolution to have eliminated those genes. Researchers looking into this question argue between two hypotheses.

First, a failure. Evolution is imperfect, so some bad genes manage to slip through. This sounds dismissive, but it's definitely true to some degree. Thousands of different genes contribute to risk for conditions like ADHD and schizophrenia, with each adding only a tiny amount of risk. When a gene is only very slightly bad, it takes evolution millennia to get rid of it, and during those millennia people are getting new very-slightly-bad mutations, so it all balances out at a certain level of bad genes per generation. Those bad genes are sufficient to explain the existing amount of ADHD and schizophrenia; they're just evolution not working as well as we'd hope.