Note: I am aware there is a great deal of literature in the field of placebo studies and this is intended only as a “pop primer”. See Price, Finniss and Benedetti (2008) for a review.
I have come to realise that there is a great degree of misunderstanding in the general public of what the placebo effect actually is. Furthermore, I argue that the term is grossly misused not just by the public, but by some scientific literature. I have heard people say that the placebo effect is “mind over matter” or “the power of the mind”; alternatively many people use it as a general term to explain any health improvements that are not associated with medication. This is not correct.
A placebo is a totally inert substance given to a patient. The placebo effect is the observed positive effect a placebo has in a clinical trial. It is simpler to think of it as a “placebo response” – this is the change in a symptomatology of an individual that occurs as a result of placebo. Clearly note – as a result of the placebo.
I think a great deal of confusion surrounding the placebo effect is brought about by the ease with people confuse separate phenomena for a placebo response. To explain, I will give an example – take the following graph. [Data for illustrative purposes]
Assuming that there was a statistically significant change between month one and four in the placebo condition, it is likely someone would claim that this was due to the placebo effect (i.e. the effect of taking the placebo causing a psycho-biological response). However, this is not the case and we can’t actually assume that. The reason for this is because we don’t have a “no-treatment” group. There are a wide variety of reasons why people with depression could improve over four months which have nothing to do with taking a placebo. Take a look at the following graph for instance [Data for illustrative purposes].
Now let us assume that there were no statistically significant group differences between the “no treatment group” and the “placebo group”. This is telling us that people improve over time regardless of whether they are given a placebo or not. This could be due to pure statistical/psychometrical reasons – for instance “regression towards the mean“. (This is the phenomenon that if a variable is extreme on its first measurement, it will tend to be closer to the average on a second measurement). Such a reduction could also be explained by “natural history” – the magnitude of symptomatology change over time in absence of treatment. Thus, it would be incorrect to say that there was statistically significant improvements in the placebo group due to a placebo effect because we are aware other variables have caused the improvements.
So in order to be able to say that the placebo effect is present, we first have to be able to rule out other phenomena such as regression towards the mean and natural history by conducting studies in which carefully designed “no treatment” or “natural history” control conditions are present to compare the placebo condition against. However, such studies still don’t tell us what is causing the placebo effect; in my opinion yet another cause of significant confusion.
The placebo effect is a result of many (if not, innumerable) variables (see Price, Finniss and Benedetti (2008) for a review). However, remember that these variables are not the placebo effect in and of themselves. For instance, expectancy, which is the subjectively perceived likelihood of an outcome or effect, is considered to be one of the driving factors behind the placebo effect. Montgomery & Kirsch in 1997 showed “expectancy accounted for 49% of the variance in postmanipulation pain ratings”. In other words this study demonstrated that one’s expectations of a drug and recovery process is a significant variable accounting for the amount of pain reduction reported. Through a well planned study, identified the placebo effect and the documented a causal agent.
However, just because there are expectancy effects present doesn’t imply a placebo effect. Likewise, just because a causal factor of the placebo effect is present, doesn’t imply the placebo effect. For example, a 2005 study conducted by Bausell, Lao, Bergman, Lee and Berman found three important things. Firstly, that there was no statistically significant differences in pain perceptions between a real acupuncture group and two forms of placebo acupuncture groups. Secondly, the study found that patients who believed that they received real acupuncture reported significantly less pain than patients who believed that they received a placebo (regardless of what they actually received). Finally, they found that acupuncture does nothing at all to help dental pain.
From these findings, the only conclusion is that the effects of acupuncture analgesia are due to an expectancy effect. This is because the effect of pain reduction (or lack there of) was not due to the placebo group – it was due to expectations of the treatment and its efficacy. There was only an expectancy effect. Thus, for the people who did report a reduction in pain from acupuncture we cannot say that it was a placebo effect, we can only say it was due to an expectancy effect. Thus, the causal factors behind the placebo effect can be present in isolation of the placebo effect. Furthermore, I would argue that without a no-treatment control – they couldn’t have demonstrated a placebo effect anyway.
So where does this leave us? Well, the placebo effect is present when all other possible causes have been ruled out; a well designed no-treatment control group or natural history group has been incorporated into the study; and it can be shown that there is even a placebo response. There are a wide variety of known causes of a psycho-biological placebo response, with one’s expectations being only one of many. However, it is important to note that the causal factors behind a placebo response are not in themselves indicative of a placebo effect.
So for this reason, next time you hear someone talking about “a placebo effect” you can be a jerk and point out the faults in their layman’s use of the the term. This now raises the interesting questions -when something is medically inert, yet it produces a result better than nothing at all – why does this happen? For answers to this, you need to delve into the research yourselves!