Having an agreeable personality might make you popular at work and lucky in love. It may also enhance your brain's built-in painkilling powers, boosting the placebo effect.

Researchers at the University of Michigan, the University of North Carolina and the University of Maryland administered standard personality tests to 50 healthy volunteers, identifying general traits such as resiliency, straightforwardness, altruism and hostility. Each volunteer then received a painful injection, followed by a placebo—a sham painkiller. The volunteers who were resilient, straightforward or altruistic experienced a greater reduction in pain from the placebo compared with volunteers who had a so-called angry hostility personality trait.

The difference was not just psychological. The researchers, led by Jon-Kar Zubieta of the University of Michigan, used PET (positron-emission tomography) scans to measure levels of mu-opioids—the brain's own painkilling chemicals—in the volunteers’ brains during the placebo procedure. The brains of volunteers with “more agreeable” personalities, according to Zubieta, released more of these natural painkillers, thus enhancing the placebo effect.

“The regions where we see these changes are all engaged in taking sensory information from outside, integrating it and giving it an emotional content,” Zubieta says. “Personality traits like straightforwardness and altruism are part of an overall capacity to be open to new experiences and integrate that information in a positive fashion. That's probably what drives the placebo effect.”

The findings could help make clinical trials for new drugs, which depend heavily on placebo testing, more accurate.