Here’s Peter Orszag, director of the Office of Management and Budget, on the placebo effect, from a Slate Q&A:
One of the more fascinating studies I’ve come across in this vein was done by two Harvard psychologists who studied hotel cleaning staff and the effect of “perceived” exercise. At the outset, they found that most hotel workers didn’t believe their work was exercise—two-thirds reported that they didn’t exercise regularly, and more than one-third reported not exercising at all. But when the researchers informed a group of these workers that their eight-hour-a-day activity of cleaning rooms counted as “exercise,” the effects were similar to an increase in actual exercise. The workers lost weight and lowered their blood pressure, apparently just by thinking that the same activity they were already doing was exercise! That’s just one example that shows the power of human psychology.
Here’s the study.
Here’s a book the authors cite, about stress and health.