What is the evolutionary advantage of the placebo effect?

Why would an organism’s immune system not always do its best to save the organism. Why should the immune system do a better job when the host “believes” they’ll get better?

How could this bizarre behavior on the part of the immune system have evolved?

Whether the placebo effect actually works to bolster the immune system or not is debatable. But for the sake of argument let’s assume it does. The theory runs as follows:

Prolonged stress in mammals is a bad thing. Stress is a prolongation of the flight or fight response. Under conditions of stress your body releases all sorts of hormones that divert blood away from the vital organs towards the muscle, slow digestion, increase blood pressure, prevent rest and inhibit the immune system. Those reposnses are all very sensible reactions if you are about to be burned to death by a forest fire since they tend to take you out of danger faster. They are terrible reactions when they are prolonged over a period of weeks rather than days or seconds.

The trouble is that humans are just mammals. We evolved from a stock which couldn’t anticipate the future more than a few days ahead and that live din an environment where something was either immediately fatal or it went away after a few days. For other mammals in the wild stress is a non issue because it only lasts a few days and then vanishes.

Humans unfortunately can anticipate the future and when we think we are likely to die or be permanantly maimed we experience exactly the same reactions that a deer might feel at the approach of a forest fire. That is bad because unlike a fire the stress of chronic disease is prolonged.

If you believe that you won’t die or be permananently maimed then you naturally stress less. That means you sleep better, you eat better, your blood pressure is lower and your immune system isn’t compromised. It shouldnt; suproise anyone that those things will assist in reovery form illness and injury.

Basically the bizarre behaviour isn’t bizarre for animals with limited self awareness faced with simple problems that can be run away from. Humans are self aware, our problems are complex and we ofteh can’t run away from them. IOW we have both socially and mentally out-evolved our own endocrine system. Given a few hundred thousand years the two systems will doubtless run in synch again, but at the moment we are at the point where one system has evolved faster than the other.

A more succinct and effective explanation I’ve not lately seen. Well done, Blake.

It could be that the placebo effect is a byproduct of some other system – a little bug in our programming that hasn’t been bred out because it has no net effect on our ability to reproduce successfully. Not every little quirk of humanity can be explained in terms of evolutionary advantage.

Wouldn’t there have to be a selection pressure for this to occur? It seems to me like even the most stressful among us have no problem making babies.

Tell that to the 40 year old man who died of a heart attack last week. I imagine he will have a great deal of difficulty just caring for the children he has already produced, much less siring any new offspring.

Polerius’ point was precisely the opposite: the placebo effect isn’t a bug, it’s a massive boon.

The question was then why such a massively beneficial system only runs when we are deluded. Regardless of its cause it should run all the time. IOW the question is not why the placebo effect exists but why the ground state is less efficient than the state induced by placebo. Why would any organism evolve an immune and healing state that operates sub-optimally except in extraordinary circumstances?

The trick is that the question is actually approaching the problem backwards. The placebo effect isn’t a heightened healing state, it is the ground state. It is the people not under the influence of the placebo effect who are operating sub-optimally due to stress. All the placebo does is reduce the stress and bring the system back to its normal operating state.

Edit: Much as I like my explanation its almost word for word what was said by the second poster above. So uh ‘what he said’ probably is a better summary, must read thread more carefully in future.


I’d think 40 is pretty old, in terms of reproduction, and the vast majority of people already have most of their kids by that age. In the modern age, I’m not aware of any facts that show children who lose one or both parents have less children themselves…but I could be wrong. And I suppose it could have a very small effect over a very extended time period. Hey, maybe someday we will be immune to the effects of Mcdonald’s (if we don’t concoct our own solution first).

This is an interesting theory, but do you have any data/cites to back it up?

For example, if someone is very stressed out due to work, and he has a medical problem for which he is given a placebo, the medical problem may go away due to the placebo effect, despite the high levels of stress at work.

Are you claiming that unless one’s stress levels are reduced, there is no way the placebo will have any effect? Are there any studies that show this?

Maybe taking the placebo will slightly reduce the stress associated with the medical problem you are having, but I can easily a case where there are two individuals with the same medical problem and which the following happens:

  • one highly stressed out, given a placebo, and getting better
  • one not so stressed out, not taking any pill, placebo or otherwise, and not getting better.

Overall, I agree that stress reduces your immune system’s ability to function, but whether this mechanism has anything to do with the placebo effect, I’m not so sure.

Any cites?

The highly stressed person may have an inherently better immune system, so the situations you cite can easily occur within the theory.

The question is inherently difficult to answer because the placebo effect in itself is open to question, so its just fun to have a go at answering it.

As per the Wiki link which cites a 2001 study, it can be viewed as mostly subjective rather than really having a particularly large effect on recovery. Ie you tell a person the pill will work, they feel a bit better, but it doesnt really speed up things in any significant way as far as objective measures go.

So any evolutionary argument has to be theoretical because the original position is problematic.


That may be the case. However, even for subjective things, like pain:

  1. Why is the body set up to behave this way?
    i.e. why does my headache go away if I take a placebo? why doesn’t my body make the headache go away without the need to be fooled into believing it will go away?

  2. Regarding the stress theory above, is there any indication that, for the patients whose subjective experience of pain was reduced, it was accompanied, or preceded by, a reduction in stress?
    Have stress levels (e.g. via stress-related hormones) been measured before and after the placebo has been taken to correlate reduction in stress with improvement in the subjective experience of pain?

The wikipedia article does not mention this, and I haven’t seen this elsewhere either.

Do you have any cites?

Sorry, the point is basically the whole idea is open to question. As in:

“The authors concluded that the placebo effect does not have “powerful clinical effects,” (objective effects) and that patient-reported improvements (subjective effects) in pain were small and could not be clearly distinguished from bias.”


So the body may not be even ‘set up’ to act in this way in the first place. Your headache doesnt go away as such, its more the case that if an authority says ‘take this and your headache will get better’ that you’ll say ‘yes its not as bad now’ when asked later, because we’re not particularly good at accurately comparing things like pain with pain that has occurred in the past, presumably because evolutionarily speaking we didnt really need that kind of accuracy in order to survive.

Ie its more about compliance with authority than any measurable effect.


What you cited was one study (or meta-study, to be more accurate)

In another part of the same wikipedia article, it says

So, something does seem to be happening at the chemical level (release of natural opiates) that causes the experience of pain to go away. The placebo may not cure the problem, but it does seem to be able to reduce the experience of pain (and not in a “compliance with authority” kind of way)
And, how come you’ve gone from

“Much as I like my explanation its almost word for word what was said by the second poster above” (who was arguing that the placebo effect was due to the reduction in stress and the resulting improvement in the immune system, back to its “ground” state)


“its more about compliance with authority than any measurable effect”


Im starting to wonder what you’re after here. If you dont find my reply useful, feel free to ignore it.

The original reply I wrote talked about reduction in anxiety, and perception of pain levels related to anxiety. I didnt talk about physiological improvements in healing but overall the reply was similar in regards to talking about it as a cognitive process and viewing it as an advantage granted by our ability to communicate.

The study I quote is qujite recent, and basically challenges the general concept of placebo. As I said its open to question what the placebo effect actually is, making it difficult to give a single answer to your question. My answer is in relation to the study quoted.


It is an example of mind over matter. Belief systems are very important, they produce thoughts, then choices that can help us or hurt us. It has nothing to do with evolution, it has to do with the spirituality of mankind.