This is not an area I’ve heard much discussion about, other than on Scott Alexander‘s blog(s):
“Codon Wheel for translating genetic code from the Wellcome Trust Sanger Institute” by dullhunk is licensed under CC BY 2.0
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.
Second, a tradeoff between two goods. The genes for psychiatric disorders are good in some way. Maybe having some schizophrenia genes (maybe not enough to give you schizophrenia) makes you more creative and raises your inclusive fitness. This keeps schizophrenia risk genes in the population, and sometimes two people with very high level of these genes will mate and their child will have schizophrenia. “Higher creativity” vs. “lower schizophrenia risk” is a tradeoff, and different people are at different points on the tradeoff, and some people will be so far to one end that they will get schizophrenia.
Recent research has pretty heavily favored the failure hypothesis. If you have enough people’s genomes, you can use some complicated math to infer how evolution is affecting different genes. And on most of the schizophrenia risk genes we know about, evolution has been gradually eliminating them in a way that looks like they’re on net harmful — not keeping them around in a way that looks like they have counterbalancing advantages. In the modern day, people with genes for psychiatric disorders tend to have fewer, rather than more children than people without those genes – except in the case of ADHD, which I’m tempted to cynically attribute to them being less likely to remember to use contraception.
Also, a lot of the theories about how psychiatric disorder genes are good suggest that different disorders are good in opposite ways. For example, schizophrenia genes are supposed to give you more artistic creativity, whereas autism genes are supposed to make you more cool-headed and rational. This makes a kind of intuitive sense looking at the symptoms of the disorders. But it turns out that many, many of the genes that cause autism also cause schizophrenia, and vice versa. They seem to be general genes for having mental disorders, with a wide variety of negative effects — which seems like a better match for the first theory where they’re just plain bad news and evolution hasn’t gotten around to eliminating them yet.
