The volunteers then had infusions of two forms of saline solution into their jaw muscles: one that was expected to cause pain and another that should not. They sometimes got a real pain reliever, and at others got a placebo, never knowing what combination of conditions they were getting. Not only did participants rate their pain and their pain relief regularly; their levels of the stress hormone cortisol — a good gauge of discomfort — were tested and the activity levels of their body's own painkilling response in the brain were measured.
Those who experienced pain relief from the fake analgesic didn't just report it; their brain showed that their body's pain suppression mechanisms — the natural release of opioid-like chemicals in the brain — snapped into high gear. And when researchers looked to see which subjects responded most strongly to the placebos, they saw people who rated highly on measures of altruism and the capacity to withstand and overcome stressors. They also tended to be highly straightforward: more direct and frank in their approach to others, less guarded and not manipulative.
“People with those factors had the greatest ability to take environmental information — the placebo — and convert it to a change in biology,” said University of Michigan psychiatrist Dr. Jon-Kar Zubieta, the paper's senior author and an expert on the placebo effect.
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