The phrase "popular delusion" refers to a widely accepted belief or idea that is not based on sound reasoning or evidence. It implies that many people believe in something that is false or irrational. Full definition
He has positioned himself as a bubble about a bubble, madness about madness, as much of a statement on popular delusion as a product of it. (newcriterion.com)
If you haven't read it, pick up a copy of Extraordinary Popular Delusions and the Madness of Crowds. (russellblake.com)
Unfortunately, as Charles Mackay has so well illustrated, such scientific evidence is typically ignored in the hysterical contagion of popular delusions. (firstthings.com)