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We have another report this morning on the difference between what the evidence proves and what people believe. Three different studies have now found that an herbal remedy does not fight colds. Yet those findings have not prevented echinacea from remaining one of the most popular supplements on the market. NPR's Allison Aubrey reports on why health-care consumers often ignore the evidence.

ALLISON AUBREY reporting:

Common cold researcher Ronald Turner got a lot of press last week. Dozens of newspapers and TV newscasts reported on his echinacea study. The herb doesn't work, he told all the interviewers. And now he has another hunch, that is, that people will keep right on using it.

Mr. RONALD TURNER (Researcher): I've given up a long time ago with trying to convince people that science should dictate how they behave. And so I don't imagine that the people who are convinced that echinacea works for them are going to be swayed one way or another by the results of our study.

AUBREY: Echinacea enjoys lots of those true believers. People get a nasty cold, they start taking daily doses of the herbal remedy, and in a few days, they begin to feel better.

Dr. DON HANTULA (Psychologist): So the next time you get a cold, you're a little more likely to take the echinacea.

AUBREY: Believing that the herb is what made you feel better, says Don Hantula. He's a psychologist at Temple University. He says what's really at play here is something he calls an illusory correlation or an illusion based on a flawed assumption.

Dr. HANTULA: Colds being what they are, it's going to cure itself anyway, so if the next time you get a cold, you take the echinacea, it gets better, it just reinforces itself over and over again. You build that experience up over a while and it's very hard to convince people that something like echinacea does not work.

AUBREY: When your personal experiences is telling you that something works and a scientific study comes along or two or three of them and says it doesn't, you're not likely to accept that.

Professor SCOTT PLOUS (Wesleyan University): Scientists might hope that the results of a particular study, especially a large-scale, well-controlled study, would trump anecdotal evidence or personal experience. That's not always going to be the case. Often what happens is the most vivid or personal information is weighed more heavily than statistical or abstract information.

AUBREY: Scott Plous is a professor of psychology at Wesleyan University. In his research on decision-making, he finds people will use reason and evidence but not right away. Particularly when they get information that clashes with their assumptions, they're slow to change.

Prof. PLOUS: It's not the case that they will ignore science wholesale but they'll integrate what they learn about science with other factors.

AUBREY: Other factors such as needing to hear it from someone you know and trust as well as from news accounts, but there's still one more thing that's influencing us. In a consumer society, advertising and marketing loom large.

Professor WALLACE SAMPSON (Emeritus Clinical Professor, Stanford): It's a dance between the producer and the consumer.

AUBREY: Wallace Sampson is emeritus clinical professor at Stanford and the editor of the Scientific Review of Alternative Medicine. He says echinacea is a good example of a product that's marketed with many claims.

Prof. SAMPSON: The producer or marketer will put it on the market and make certain claims about it, even make the wording very vague but very suggestive, and the consumer, of course, is at fault for falling for the ad and for misinterpreting the words.

AUBREY: Then manufacturers fund their own scientific studies. Sampson says they're usually not designed as well and...

Prof. SAMPSON: When you add on top of that the profit motive and the belief that people have in the business, that what's really going to come out of the trial has got to be positive, then you add on another layer of biases that render the trials unduly positive.

AUBREY: So when it comes to health and decision-making, it's pretty clear why so many consumers are confused.

Allison Aubrey, NPR News, Washington.

INSKEEP: You're listening to MORNING EDITION from NPR News.

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