People who respond positively to “dummy” pills are no dummies. In fact, a new study finds they tend to be people you would describe as resilient, altruistic and straightforward.

Maybe the people who don’t respond well to placebos are the dummies: Angry, hostile and prone to negativity, they seem less capable of harnessing their minds to the task of healing their bodies, according to new research.

In clinical trials, a placebo gives researchers a basis for comparison. If an experimental treatment works far better than the placebo, its effect is presumed to be “real.” The “placebo effect” was long dismissed as an improvement that is “all in your head.”

In fact, the placebo effect is often a powerful testament to the mind’s influence over physical pain, infection and disease. The belief that a treatment will work can help mobilize the immune system, blunt pain and promote healing.

For doctors, knowing who is most, and least, responsive to the placebo effect can be a useful clue to determining treatment. And for researchers, it would be helpful to know which subjects would probably respond irrespective of whether they get the real thing or the sham.

Now, both have an answer, published this month in the journal Neuropsychopharmacology. About 50 volunteers at the University of Michigan completed assessments of personality traits known to stay stable across most people’s life spans: altruism, empathy, extroversion, openness to experience, agreeableness and conscientiousness, among others.

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. The participants rate their pain and pain relief regularly. In addition, their levels of the stress hormone cortisol (a good gauge of discomfort) and the activity levels of their brain’s own painkilling response were measured.

When researchers examined 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 more direct 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.