Comment by jjk166
> you would expect protective social anxiety to arise in people whose situation hasn't much changed except that they've encountered people whose has (as with sedentary villagers encountering nomads who may or may not be about to ransack their settlement).
No, you would not expect that. If rats that self-isolate when their gut micro-biomes change avoid diseases that other rats get afflicted with, then evolution will lead to the mechanism spreading through the rats over time. The rats do not need to know why self-isolating helps, they do not need to even know that self-isolating helps, and they do not need to be in the position where it would help the most.
GP's hypothesis is that diet/environment microbiome changes induce a protective social anxiety. However, a nomadic population - the most likely to exhibit those changes - are successful when exhibiting prosocial or confrontational characteristics, the opposite of social anxiety. Social anxiety is most protective for sedentary groups, who probably should be suspicious of outsiders.
Nothing you said contradicts this, and I'm not sure how to square the circle, even though microbiome-mediated behavior as an explanation for broad, otherwise irrational behavior patterns is attractive.