Here is a theory of why placebos work. I don’t claim that it is original, it seems natural enough that I am surely not the first to suggest it. But I don’t think I have heard it before.
Getting better requires an investment by the body, by the immune system say. The investment is costly: it diverts resources in the body, and it is risky: it can succeed or fail. But the investment is complementary with externally induced conditions, i.e. medicine. Meaning that the probability of success is higher when the medicine is present.
Now the body has evolved to have a sense of when the risk is worth the cost, and only then does it undertake the investment. Being sick means either that the investment was tried and it failed or that the body decided it wasn’t worth taking the risk (yet! the body has evolved to understand option value.)
Giving a placebo tricks the body into thinking that conditions have changed such that the investment is now worth it. This is of course bad in that conditions have not changed and the body is tricked into taking an unfavorable gamble. Still, the gamble succeeds with positive probability (just too low a probability for it to be profitable on average) and in that case the patient gets better due to the placebo effect.
The empirical implication is that patients who receive placebos do get better with positive probability, but they also get worse with positive probability and they are worse off on average than patients who received no treatment at all (didn’t see any doctor, weren’t part of the study.) I don’t know if these types of controls are present in typical trials.
Cheap Talk 
5 comments
Comments feed for this article
August 11, 2010 at 2:45 am
Ryan
Yeah I think this is right on. Not sure how it all works out inside the immune system, but a simple example would be someone who is convinced they cannot walk being given some kind of placebo which encourages them to try walking again, then being able to walk.
However, I think the idea that a body is fully optimized with what risks to take is not always true. For example, there is the classic psychological experiment in which a dog, after being shocked dozens of times each time it tries to stand, eventually entirely gives up on trying to stand–even after the electricity is shut off. Once the electricity is shut off, the dog should be cured of not being able to stand up, but is not willing to take its perceived risk of being zapped again. Hence the dog would benefit from a well-timed placebo which would cause it to believe the electricity has gone away with sufficient probability to be worth trying to stand again.
Your argument is true for random placebos, but well-timed placebos could still offer significant benefit.
August 11, 2010 at 8:51 am
Khurram Naik
“…my idea is that nature has designed us to play safe, and never to use up everything we’ve got — because we never know what might still lie around the corner. When we reach the end of a marathon there may still be a lion waiting at the finishing post that’s going to suddenly give chase. When we’re sick with an infection and respond with an immune reaction, we may still be hit by a further infection the next day. Remember the story of the wise and foolish virgins and their lamps: it’s always wise to keep something in reserve.
I’m now thinking in terms of there being what I call a “natural health management system”, which does a kind of economic analysis of what the opportunities and the costs of self-cure will be — what resources we’ve got, how dangerous the situation is right now, and what predictions we can make of what the future holds. ”
From Nicholas Humphrey: http://www.edge.org/3rd_culture/humphrey04/humphrey04_index.html
August 11, 2010 at 11:01 am
Scott
I think the argument would make more sense if there were two inputs to survival, say health and hours awake. There is always a trade-off between the two, and the body picks optimally given the conditions.
The returns to sleep change in the presence of medicine, thus a placebo could trick the body into getting more sleep than might be optimal in the evolutionary sense. Of course, in modern times survival has little to do with how many hours one spends awake, thus it makes sense to trick the body into more sleep and better health.
In this framework the empirical prediction is that health will always improve for people that receive the placebo, but these people would be doing worse along some other dimension than those who do not get the placebo.
January 15, 2012 at 11:36 pm
Placebo Information « Cheap Talk
[…] have a theory of how placebos work. The idea is that our bodies, through conscious choices that we make or simply through […]
January 15, 2012 at 11:36 pm
Placebo Information « Cheap Talk
[…] have a theory of how placebos work. The idea is that our bodies, either through conscious choices that we make or simply through […]