A study recently published in PLOS one (Catechol-O-Methyltransferase val158met Polymorphism Predicts Placebo Effect in Irritable Bowel Syndrome) purports to have found a gene variant that correlates strongly with a placebo response in irritable bowel syndrome (IBS). The study is small and preliminary, but the results are interesting and do raise important questions about placebo responses.
Researchers are increasingly trying to tease apart the various components of “the placebo effect.” In reality we should use the term “placebo effects” as it is demonstrably multifactorial. “The placebo effect” really refers to whatever is measured in the placebo arm of a clinical trial – everything other than a physiological response to an active intervention. Within that measured response there are many potential factors that would cause an outcome from a fake treatment to be different from no treatment at all. These include statistical effects like regression to the mean and the natural course of symptoms and illness, reporting bias on the part of the subject, and a non-specific response to the therapeutic interaction with the practitioner.
It is also critical to realize that placebo responses vary greatly depending on the disease or symptom that is being treated and the outcome that is being measured. Placebo response is greatest for subjective symptoms of conditions that are known to be modified by things like mood and attention, while it is virtually non-existent for objective outcomes in pathological conditions. So there is a substantial placebo response for pain and nausea, but nothing significant for cancer survival.