People in control groups are usually given a sugar pill and are observed to assess if any changes occur to their health during the experiments. How do we know that these changes, if any, were the result of taking the sugar pill and not just random changes that might have happened even without taking the placebo? Are there any studies that prove that the placebo effect really exist?
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I’m actually not convinced that the placebo effect, as most people think about it, exists. That is to say, the idea that expectations of benefits from treatment leads to actual physiologically relevant changes. There are many effects that you see in the placebo group of a clinical trial, but I don’t think any are appropriately categorized as the sort of “mind over matter” placebo effect that most people think of.
Regression to the mean explains some of it. People in clinical trials are selected because they are in some way abnormal, when assessed at particular point in time. If you then reassess them later, in most cases the group will more closely resemble the average population, simply for statistical reasons.
Biases explain some of it too. For instance, the placebo effect is strongest for self-reported outcomes and tends to be minimal for more objective outcomes. There was a study looking at placebo inhalers in asthma. The placebo and the active bronchodilator produced similar subjective effects (patients reported feeling less breathless in both groups), but only the active bronchodilator actually improved objective measures of lung function. There is a placebo effect at play, but it’s to do with what the patient is reporting, rather than a change in physiology due to expectations of treatment.
These are just two effects that partially explain apparent benefits in the placebo group of trials. I suspect that the more statistical effects you control for, the smaller the placebo effect will be, and that there is no underlying relevant physiological change in most cases. There are several placebo effects (i.e. factors that explain benefits seen in control groups), but the placebo effect that most people think of isn’t really a thing.
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