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.
[In America] some Hmong felt that they had not properly honored the memories of their ancestors, which was a known risk factor among the Hmong for being visited by the tsog tsuam. Once the night-mare visitations began, a shaman was often needed to set things right. And in the scattered communities of Hmong across the country, they might not have access to the right person. Without access to traditional rituals, shamans, and geographies, the Hmong were unable to provide themselves psychic protection from the spirits of their sleep.
Drawing on all this evidence, Adler makes the provocative claim that the Laotian immigrants of the 1980s were in some sense killed by their powerful cultural belief in night spirits. It was not a simple process.
"It is my contention that in the context of severe and ongoing stress related to cultural disruption and national resettlement (exacerbated by intense feelings of powerlessness about existence in the United States), and from the perspective of a belief system in which evil spirits have the power to kill men who do not fulfill their religious obligations," Adler writes, "the solitary Hmong man confronted by the numinous terror of the night-mare (and aware of its murderous intent) can die of SUNDS."
Via Christopher Ryan