Do you have a source on this? I don't understand how evolution could favor such a negative trait on the individual level, and as far as I understand group selection has been mostly rejected by modern evolutionary biologists
IMO there are too many phenomena that are very difficult to explain only with individual selection, but trivial to explain with group selection.
Not sure about the claim that evolution favors groups whose rejected members die (not sure what rejected even means in this context), seems to me like that would depend a lot on environmental context and vary across species and across time.
Homosexuality is an obvious example that comes to mind. If you know others please do post them.
Not all things rejected by modern science is rejected for scientific reasons, unfortunately.
Group selection looks like the most logical explanation to me.
Member gets sick
Member gets rejected from the group (because for each individual in the group, it's better to avoid the sick member - no group selection necessary)
Member dies
So the member doesn't get sick because they're rejected - they get rejected because they are sick. This would have obvious benefits for each individual that rejects the sick member, regardless of the behavior of the rest of the group (https://esajournals.onlinelibrary.wiley.com/doi/abs/10.2307/...)
I vaguely remember reading adverse health outcomes in lower status individuals may be linked to the chronic stress attributed to being low status, but can't remember the exact source but this one came up in a quick search:
https://www.sciencedirect.com/science/article/pii/S235228951...
The placebo effect comes from social support and goes beyond pain.
As for the implementation details, expectations play a role. A drug that modulates the cholecystokinin neurotransmission in the brain (involved in expectation processing) was found to enhance the placebo effect of a placebo pill (on mobile, can’t search right now, but with “ cholecystokinin” and “placebo” you should find the paper on google scholar).