Maggie Koerth-Baker Fri, Feb 10
Placebos have no repeatable physical effect that can be broadly demonstrated to exist. But, if people believe the placebo can help them, it often does—especially for inherently subjective issues like pain relief.
Nocebos are what happens when a placebo (again, something that technically has no physical effect on the body) causes a negative side-effect, simply because the person believes that such side-effects are likely to happen to them.
There is a lot we don't understand about both of these effects. After all, running really detailed tests would inherently involve unethical behavior—intentionally not treating patients or intentionally trying to induce a negative reaction in them. But that doesn't mean you can ignore these phenomena.
A great example comes in a recent column by Alexis Madrigal on The Atlantic. You're probably familiar with the idea of sleep paralysis—the experience of waking up, being mentally awake, but still physically paralyzed. This happens to people all over the world. And, all over the world, it's long been explained in folklore as the work of demons and evil spirits. (The fact that sleep paralysis is often accompanied by feelings of terror, and the sensation of something sitting on your chest doesn't hurt in that regard.) Normally, sleep paralysis brings a few minutes of terror, but no lasting harm. In the mid-1980s, however, it suddenly became capable of killing. The catch, the men it killed were all recent Hmong immigrants, living in the United States. Researcher Shelley Adler thinks it was actually a nocebo effect that killed these men—they believed themselves into an early grave.