I recently read about the Mississippi bubble of 1717-19 in Charles Mckay's Extraordinary Popular Delusions and the Madness of Crowds.
That bubble along with the others detailed in the book are a clear example of the madness of crowds. But we also hear about the wisdom of crowds. The most famous example is the "Ask the Audience" lifeline in "Who Wants to be a Millionaire?" type shows, which ends up being far more accurate than the other lifelines, with the audience getting the answer right 91% of the time.
So at times populations can extremely wise, and the consensus view is trustworthy. At other times they can be absolutely mad, their opinions feeding off each other and growing more and more delusional until reality brings them crashing back into the ground.