Though some people respond more strongly to placebos than others do, it seems that everyone responds at some time or other. And doctors seem to play a large role in the degree of that response.

"The thing that trumps everything is the enthusiastic physician," said Dr. Dan Molerman of the University of Michigan.

For example, one study offered the same drug to patients with identical symptoms, with one difference. Some were told by their physicians, "This drug has been shown to work," while others were told, "I am not sure if this treatment will work -- let's just try it." The first group of patients did much better, Molerman said. "The physician is an agent for optimism and hope and a great inducer of beliefs."

Physicians can even fool themselves. Years ago, researchers carried out controlled studies of a drug for angina or heart pain and found it was no better than a placebo, Molerman said. Once doctors knew that, its effectiveness fell.

While doctors and patients affect one another's expectations, both are swept up into a wider context of culture and biology, said Dr. David Morris, an adjunct professor of medicine at the University of New Mexico in Albuquerque. The brain circuits through which placebos act, he said, are activated through the experience of living in a particular culture.

To explore the importance of cultural context, Molerman, in an analysis forthcoming in the journal Medical Anthropology Quarterly, compared 122 double-blind placebo-controlled ulcer studies from all over the world. Doctors used the same techniques, the same drugs and the same placebo pills and studied an image of the stomach lining before and after treatment to see what worked. The drugs worked 75 to 80 percent of the time, Molerman said, whereas the placebos worked from zero to 100 percent of the time, depending on the country. The placebo healing rate for ulcers in Germany was 60 percent, almost double the world average of 36 percent, which is about where the United States fell. But in Brazil, the mean placebo healing rate was a startling 6 percent.

"I don't have hint of what is going on here," Molerman said. "I can only say that cultural differences affect ulcer treatments, even though ulcers are the same the world over."