The New York Times has an interesting post on how even experts can get caught in groupthink and bad advice can become the “consensus” of experts. The reason this happens is as follows:
We like to think that people improve their judgment by putting their minds together, and sometimes they do. The studio audience at “Who Wants to Be a Millionaire” usually votes for the right answer. But suppose, instead of the audience members voting silently in unison, they voted out loud one after another. And suppose the first person gets it wrong.
If the second person isn’t sure of the answer, he’s liable to go along with the first person’s guess. By then, even if the third person suspects another answer is right, she’s more liable to go along just because she assumes the first two together know more than she does. Thus begins an “informational cascade” as one person after another assumes that the rest can’t all be wrong.
See full article. It argues that the whole “fatty food is bad for your heart” is a misconception that got the status of “consensus” because of an informational cascade.