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).
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 homeostasis effect might happen in an adversial situation. Say instead of a doctor, an opponent says something. It might cause to overcompensate in the opposite way. We could call that reverse placebo or by its more common name "spite".
As a bit of a tangent, one of things I've always wondered is "how much pain do animals feel compared to humans?" The null hypothesis here would be that they feel the same amount, of course. On the other hand, what's an animal going to do with joint pain? Humans are unique in our ability to perform interventions. Suppose two people have cancer, one shows symptoms and one does not. A tumor can be excised before it becomes malignant. One would live and the other would die. Is there a pressure on us to have pain so that we can seek treatment?
To bring this around to your comment. I wonder if we've demonstrated placebo effects on animals. That seems like an awfully hard study to conduct.
I was just today thinking about the difference between how both my children were born. There were so similar in the time they happened, how they were late, how the birth was triggered etc. But in the first case we were picked up by an emergency vehicle (there was no emergency, they just told us to do this), so it was dark, with this scary blue light as if somebody is dying, to some hectic in the hospital. And in the second we went very quietly to our appointment, rode the bus on a sunny day, took a relaxing walk outside the hospital as we were waiting etc.
When it was done with the first, my wife was pale and shivering, completely out of everything, and with the second she was just exhausted but could still smile and enjoy taking our newborn. And she looked happy.
Of course there is the case of the first being more scary because of the unknowns, but you could also make a case that it was so traumatic that it should've ruined the second one.
I also fell down on ice once and had a case of whiplash. I got worried as I was reading about the potential long term, especially in the US. And I then switched to the German Wikipedia that said doctors in Germany tell patients to relax for a few days and it will go away, and that long term issues are virtually unknown.
You could go on and on really. When you go to a doctor and they have almost no time for you, you will not feel better. Compare that with going in to somebody doing homeopathy. A friend told me they went and got a cup of tea and an hour to talk about their health issues.
So the body is obviously complicated.
One thing I haven't seen discussed is that maybe there is a subconcious social pressure element to placebos. Like: I just got a big injection from the doctor, surely it is going to help, I can't complain yet again about it. Sometimes I wonder, what if people take a placebo, report that they feel better, but continue to suffer internally? And are not even able to identify this, because their internal pain gauge is affected by the placebo. They think it helps. Maybe you could detect this by non-voluntary indicators of pain - avoidance reflexes, concentration loss, etc..
Back to the homeostatis argument, there are placebo-like effects that accidentially work. In autogenic training, you can warm up your body parts by imagining fire. What I think happens is, the body anticipates heat, widens the blood vessels to regulate against the heat, but in the cold environment this actually warms up your arms or legs.