The growing potency of placebos is not limited to psychiatric drugs. When researchers started looking closely at pain-drug clinical trials, they found that an average of 27 percent of patients in 1996 reported pain reduction from new pain medications being developed relative to placebo pills. By 2013 that difference had shrunk to just nine percent. In the last decade, more than 90 percent of painkillers developed in the United States have failed to show a significant improvement over placebos in the final stages of clinical trials.

At this point we can only speculate about the factors contributing to the growing potency of placebos. While research into the mechanisms underlying the effects of placebo treatments is in its infancy, we do know that increasing people’s expectations regarding the occurrence of a non-volitional response increases its likelihood of occurring. This type of expectancy effect plays a key role in hypnotic suggestion. For example, in hypnotizable subjects a hand levitation induction can lead subjects to experience their hands as spontaneously rising. Expectancy effects of this type can activate psychological and autonomic nervous system changes as well.