Just believing that you're blood doping is enough to help you run faster, recent research found.

When the runners found out that they'd been given nothing more powerful than an injection of salt water, many were red-faced and a little embarrassed, lead author Ramzy Ross said. While the subjects may have felt duped, however, it's actually another testament to the power of expectation on our health, placebo experts said. Other work has shown that placebos can soothe a baby's cough, relieve migraine pain, and even mend torn knee cartilage as well as surgery.

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One of the key issues in placebo research is parsing out what makes them work, said John Kelley, deputy director of the Program in Placebo Studies at Harvard Medical School and an associate professor of psychology at Endicott College.

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The two most common components, he said, are conscious expectations and classical conditioning. Think of the first as what you could verbalize: In the case of the runners, for example, most would say that taking erythropoietin (EPO) would improve their aerobic capacity and delay fatigue. As for classical conditioning, just as Pavlov's dogs started salivating when the physiologist entered the room in anticipation of being fed, people experience physical responses to certain associations.

In fact, even when researchers tell subjects they're taking a placebo, so-called open label studies still show a positive placebo effect.

Researchers attribute it to the conditioning response, Kelley said. It works best in people who have been treated previously for a condition. People with migraine headaches who experienced pain relief by taking a certain drug in the past also felt better when taking a pill that looked similar -- even when they knew it was not the actual drug.

By that rationale, athletes who have previously blood doped for real might fare even better on sham EPO, Kelley said. Previous research on cyclists showed that the benefits of taking morphine during workouts were maximized when the cyclists were preconditioned to using it first. Ramzy's team also delivered the placebo to the runners via injections instead of pills, knowing that most people have a stronger reaction to more invasive procedures.

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The runners who were "really into what we were doing and understood how EPO could work" had the best results, Ramzy said. The average improvement over the 3,000-meter races was 1.2 percent, or about 10 seconds. The study will be published in Medicine Science in Sports Exercise.

Because placebos tend to work best in situations that contain some type of psychological component, such as pain, the results of the running study make sense, Kelley said.

There could be a bonus after-effect, too: After the subjects in the running study recovered from the initial shock of realizing they had not taken a performance enhancement, most took it as a confidence boost.

"They said, I know that I can do it now," Ramzy said. "There's this belief system that wasn't there before."