For years, experts have known that placebos – fake injections and pills with no real medication – can improve the health of people with pain, asthma, high blood pressure and angina.

Now, they’ve learned that raising the price of a fake pill can make it work even better.

A report last week in the Journal of the American Medical Association shows that expectations – shaped by factors that include the price of a medication – play a key role in how we respond to pain relievers, as well our response to therapies for depression, cancer, strokes or heart attacks.

The latest report adds to a store of knowledge about placebos, a subject that has long fascinated scientists. Understanding placebos could unravel mysteries about the body’s ability to heal itself, they say.

Experts do know one reason placebos work: They raise expectations. Make someone believe that a tablet will make a headache disappear, and it often does – even if it turns out to be a sugar pill.

“Expectations create a different reality for us, which we don’t really appreciate,” said Dan Ariely, a behavioral economist at Duke University.

Researchers typically use placebos as controls in clinical trials of medicines and other therapies. Some volunteers get the real drug, while others get identical-looking placebos. It’s not unusual for placebos to work almost as well, and in some cases better, than the drugs being tested.