Sickle-cell anemia has been a favorite example of biologists for years, because it’s an obviously harmful trait which is strongly selected for by evolutionary pressures. The gimmick is simple; it’s a recessive trait, so you only get the anemia if you have two genes for it. If you only have one gene for it, you’re effectively immune to malaria. In circumstances where lots of people die from malaria, this can be a marvelously useful trait to have – useful enough to make up for losing a few kids to anemia.
One of the things I’ve thought a lot about recently is the ways in which autistic people can be resistant to cognitive dysfunctions that are taken for granted in non-autistic people. A friend passed on a truly horrifying thing that a friend’s mother was falling for. It’s a generic scammer claiming to treat autism with homeopathic medicine, complete with rants about mercury and vaccination and so on. There’s some very well-tuned persuasive writing there, designed to hit all the “coverups by big medicine” and conspiracy theory buttons, complete with references to how you may have stockpiled for Y2K. (Amazingly, people who did that still don’t realize why it was stupid.)
The thing is… Those social triggers don’t work very well on me, or on my friend (who’s also autistic). We can see the technique, but there’s no impact. And that means that people who talk to us, who might otherwise get suckered, get some resistance.
So this leads me to a theory. “Severe” autism is clearly pretty disabling, but we find a lot of benefits to “mild” autism. What if this is a cognitive trait with similar evolutionary pressures? Having a little of it can produce benefits, and even if you don’t have the trait yourself, being near people who do (and maybe related to them) can still be an advantage.